How to Plan a MELSEC-Q Rack for Expansion

By Abdullah Zahid
23 min read
Mitsubishi Electric MELSEC System Q modular PLC rack with extension base units and QCPU modules for industrial automation expansion

MELSEC-Q Rack Expansion Selection Guide: How to Plan a MELSEC System Q Rack for Future Growth

Controls engineers specifying a new MELSEC System Q panel — or expanding an existing one — face the same critical decision: how much rack capacity to build in now so the system can grow without forcing a panel redesign in three years. The answer depends on which QCPU tier you select, how many extension base units it supports, and whether your total extension cable length stays within the documented 13.2 m limit. Get these three variables right at the design stage and expansion becomes a straightforward field task. Get them wrong and you are looking at CPU replacement, new cabinets, or both.

If you have already confirmed your MELSEC-Q component families and are ready to check availability, visit LeadTime.ca for current stock and lead times — the team ships worldwide.

Who Should Use This Guide — and What It Will Not Cover

This selection guide is written for controls engineers and panel designers who have already chosen the MELSEC System Q platform and now need to size a rack configuration that will support realistic system growth. It is also relevant to maintenance and reliability engineers adding I/O to an existing MELSEC-Q rack that is approaching capacity.

This guide is right for your project if:

  • You are designing a new MELSEC-Q panel and expect to add I/O or special function modules over the next five to ten years
  • You need to choose between the Q00CPU/Q01CPU entry group, the Q02UCPU mid-range, the standard non-redundant QCPU range, or the redundant Q12PRHCPU and Q25PRHCPU
  • Your total planned module count — current plus forecast — is approaching or exceeding 24 modules, and you need to understand the next CPU tier's limits
  • You are working in a large cabinet where extension cable routing could approach the 13.2 m maximum
  • You are specifying a high-availability system and need to understand the additional constraints that apply to redundant QCPU extension configurations

If your system is already confirmed as a small, fixed-I/O machine with no realistic growth beyond 24 modules, the entry-level Q00CPU or Q01CPU group with a single main base is likely sufficient and this level of planning overhead is not needed. For every other scenario, read on.

On this page:

MELSEC System Q Rack Architecture: What You Are Actually Sizing

The MELSEC System Q Programmable Logic Controller is a modular rack-based PLC family from Mitsubishi Electric. Understanding its physical structure is the prerequisite for every sizing decision in this guide.

Every MELSEC-Q system is built from five classes of hardware that must be selected together:

  • QCPU module — the CPU that runs the program and sets the hard limits on how many extension base units and total modules the entire system can support. Different CPU models have different documented maximums, and this is the single most important variable in rack expansion planning.
  • Main base unit — the physical rack that holds the QCPU, at least one power supply module, and the first set of I/O and special function modules. The main base connects to the first extension base through an extension cable using the OUT connector.
  • Extension base units — additional racks connected in a chain from the main base, each providing more module slots. Each extension base requires a power supply module, connects to the previous base via IN connector and to the next via OUT connector, and must be assigned a unique sequential stage number using jumper settings.
  • Power supply modules — required in every base unit, including each extension base. Power supplies must be sized for the current draw of the modules installed in that specific base, with margin for future additions.
  • I/O and special function modules — digital input/output, analog, communication (Ethernet, fieldbus), motion, safety, and other function modules that occupy the slots in the main and extension bases.

Where this system sits in a typical control architecture: the QCPU connects upstream to an engineering workstation via GX Works or Ethernet, communicates laterally to HMIs and SCADA systems through communication modules, and connects downstream to field devices through I/O modules distributed across the main base and extension bases. Adding new machines or processes to the system means adding I/O modules — and that means having extension base slots available and extension cable length remaining.

What You Need to Know Before Sizing Any MELSEC-Q Rack

Rack sizing errors almost always trace back to starting the hardware selection before gathering the right inputs. Before choosing a CPU tier or a base unit slot count, confirm each of the following:

  • Current I/O and module count — list every digital input, digital output, analog input, analog output, and special function module required at commissioning. This is your baseline slot demand.
  • Forecast module count over system life — estimate what will be added over the next five to ten years: new machine stations, measurement instruments, communication options, safety modules. This forecast, not the baseline, should drive CPU tier selection.
  • Redundancy requirement — if the process requires CPU redundancy, the Q12PRHCPU or Q25PRHCPU must be specified from the start, along with dedicated redundant bus modules. Redundancy cannot be retrofitted into a standard QCPU system without hardware replacement.
  • Panel dimensions and cable routing paths — the physical positions of the main base and extension bases inside the cabinet determine whether the 13.2 m extension cable limit is achievable. Measure before you finalize base unit count and positions.
  • Communication and networking modules — Ethernet, MELSECNET/H, fieldbus, and motion modules all consume slots and power. Plan these explicitly; do not treat them as add-ons to a slot count derived only from I/O.
  • Environmental conditions — temperature, vibration, and humidity affect both module reliability and clearance requirements between bases, which in turn affects cabinet sizing for future expansion.

The table below maps each input to how it drives rack sizing decisions:

Design Input How to Quantify It How It Affects Rack Sizing
Current I/O count Count all DI, DO, AI, AO modules at commissioning Sets minimum slot count for main base and first extension bases
Forecast module growth Estimate new stations, options, instruments over 5–10 years Determines CPU tier; forces upgrade if underestimated
Special function modules List Ethernet, motion, safety, fieldbus modules Consumes slots and power independent of I/O count
Redundancy requirement Yes or No — determined by process criticality Forces Q12PRHCPU or Q25PRHCPU and dedicated bus modules from the start
Panel space available Measure usable cabinet height, width, and depth Limits number of physical base units and cable routing options
Extension cable routing Measure planned base positions and cable path lengths Must remain within the 13.2 m total extension cable limit
Power per module Sum current draws from module datasheets per base Determines power supply model and whether additional supplies are needed at expansion

QCPU Expansion Limits by Tier: The Numbers That Drive Every Rack Decision

The QCPU you specify sets non-negotiable hard limits on two things: the number of extension base units the system can accommodate and the total number of modules across all bases. These limits are defined in the Mitsubishi Electric MELSEC System Q User's Manual (Hardware) and are the most critical numbers in any rack expansion plan.

Entry-Level Group: Q00CPU, Q01CPU, Q00UCPU, Q01UCPU

The entry-level CPUs support a maximum of four extension base units and a total of 24 modules across the entire system — main base and all extension bases combined. This group is appropriate for compact machines and control panels where future expansion is limited and the total module count including growth will comfortably stay within 24. If your forecast puts you close to 24 modules including communication and special function modules, move to the next tier at the design stage rather than at the point of expansion.

Mid-Range Compact CPU: Q02UCPU

The Q02UCPU supports a maximum of four extension base units and a total of 36 modules. This makes it the right choice for medium machines where the entry-level 24-module cap is a concern but the system does not require the larger extension capacity of the standard non-redundant QCPU range. The Q02UCPU is a practical step up when you expect to add analog instrumentation or additional communication modules over the system's life without exceeding 36 modules total.

Standard Non-Redundant QCPU Range

Other QCPU variants in the standard non-redundant range support up to seven extension base units with a maximum of 64 slots in total. This tier is appropriate for line-level and plant-level controllers where multiple new stations or process areas may be added over the system's life. The combination of seven extension bases and 64 total slots provides substantial headroom for long-term growth, and it is the natural choice when the Q02UCPU's 36-module ceiling is likely to be reached.

Redundant CPUs: Q12PRHCPU and Q25PRHCPU

The Q12PRHCPU and Q25PRHCPU are the MELSEC-Q redundant CPU models for high-availability applications. Extension capability for these CPUs depends on serial number — earlier serial numbers do not support extension bases, while later serial numbers (from ranges specified in the hardware manual) allow up to seven extension base units and up to 63 modules when used with dedicated redundant bus interface modules such as the Q65WRB and Q68RB. If you are specifying a redundant MELSEC-Q system with expansion plans, confirm the serial-number requirement in the hardware manual before ordering and verify with your distributor that the units being supplied meet the required serial range.

Slots vs. Modules: Why the Terminology Confusion Causes Real Mistakes

One of the most frequently reported sources of sizing errors in MELSEC-Q rack planning is the distinction between slots and modules. These are not always the same number, and using one when the hardware manual means the other leads to over-specifying or under-specifying the system.

A slot is a physical position in a base unit that can accept a module. A module is a physical hardware unit installed in a slot. In most MELSEC-Q configurations each module occupies one slot, making the counts equal. However, some double-width or special-function modules can occupy two slots while counting as one module toward the CPU's module limit. Conversely, the CPU module and power supply module occupy slots on the main base but are not counted as modules toward the CPU's documented module limit in the same way as I/O and function modules.

The practical rule: when checking whether your planned configuration falls within the CPU's documented limit, always refer to the specific QCPU's hardware manual for exactly what counts toward its maximum. Do not assume that filling all physical slots in your bases will stay within the CPU's module limit, and do not assume that the CPU's documented module limit equals the total number of physical slots across your bases.

Extension Base Units, Jumper Settings, and Numbering Order

Adding extension base units to a MELSEC-Q system requires correct physical connection and correct jumper configuration. Errors in either area cause system malfunctions that can be difficult to diagnose.

  • Extension bases connect in a chain starting from the main base's OUT connector to the first extension base's IN connector, then from that base's OUT to the next base's IN, continuing to the last extension base in the chain.
  • Each extension base must be assigned a unique sequential stage number using jumper settings on the base unit. The first extension base connected to the main base is stage 1, the next is stage 2, and so on up to the maximum allowed by the CPU.
  • Setting the same stage number on two extension bases, omitting a stage number, or leaving jumpers in an incorrect state causes the CPU to fail to recognize one or more bases or to malfunction — this is one of the most common commissioning errors in multi-base MELSEC-Q systems.
  • When adding a new extension base to an existing system, assign the next available sequential stage number. Do not reassign existing bases' stage numbers unless the hardware manual explicitly requires it for your configuration.
  • Every extension base requires its own power supply module. Verify that the power supply module type is compatible with the extension base being used, as confirmed in the hardware manual.

Extension Cable Length Limits and Cabinet Layout Consequences

The total length of extension cables from the main base unit to the last extension base unit must not exceed 13.2 m. This is a hard limit defined in the MELSEC System Q hardware documentation and applies to the cumulative cable length of the entire extension chain, not to individual cable segments between adjacent bases.

In practical terms, this limit has a direct impact on cabinet layout decisions:

  • In tall cabinets where bases are mounted vertically at different heights, the cable runs vertically between bases, consuming the 13.2 m budget faster than engineers often expect when they first measure only horizontal panel dimensions.
  • Where bases are distributed across multiple enclosures or large marshalling cabinets, the cable routing path — not the straight-line distance between bases — is what must stay within 13.2 m.
  • Reserve extension cable routing paths when designing the cabinet, not after hardware is installed. Routing that requires bends, detours around other equipment, or cable tray runs will add length quickly.
  • For field I/O cables (signals between modules and field devices), the hardware documentation allows general cable lengths up to 100 m, with a recommendation in documentation to keep cable lengths around 20 m to help avoid interference. Plan I/O cable routing accordingly.
  • If your system's physical scale means the 13.2 m limit cannot accommodate the required number of extension bases, consider remote I/O over a network such as MELSECNET/H rather than further local rack extension — this is the documented approach for large systems that have outgrown local rack expansion.

Power Supply Sizing for Expansion

Every base unit in a MELSEC-Q system — main and extension alike — requires its own dedicated power supply module. Power supply sizing is an expansion planning task, not just a commissioning task, because a power supply selected only for current modules will reach its rated capacity as new modules are added.

  • Calculate the total current consumption of all modules in each base unit individually. Power is not shared between bases; each base draws from its own supply.
  • Select a power supply module for each base that provides margin above the current consumption of the modules installed — the hardware manual specifies available power supply options and their rated outputs for the MELSEC-Q range.
  • Account for future modules when sizing power supplies at the initial design stage. Adding a higher-capacity power supply to an installed base later requires physical access and reconfiguration work that could have been avoided.
  • Communication modules, motion control modules, and safety modules often have higher current draws than standard I/O modules. Identify these in your module list and include them explicitly in your power budget.
  • If any base's planned future module mix pushes the power budget beyond the highest available supply rating for that base, consider distributing modules across more bases to keep each base's load within a manageable power budget.

Step-by-Step Rack Sizing Workflow

A structured sizing process prevents the most common expansion planning failures. The following workflow should be completed before issuing any RFQ or purchase order for MELSEC-Q hardware:

  • List all current and forecast modules including I/O, communication, motion, safety, and special function modules. Sum the forecast total and add a margin of 20–30% for unforeseen additions — this is your target module capacity.
  • Match the target module capacity to the documented limit of each QCPU tier. Select the lowest CPU tier whose documented maximum comfortably exceeds your target, leaving spare capacity rather than selecting a CPU at exactly its limit.
  • Select main and extension base units with sufficient total slots to accommodate the target module count, keeping at least 10–20% of slots unoccupied at commissioning for future use.
  • Plan the physical positions of all base units in the cabinet. Measure the cable routing path from the main base to each extension base and confirm the cumulative total stays below 13.2 m for the maximum number of bases you may ever install.
  • Estimate power consumption per base, select appropriate power supply modules with margin, and document the power budget alongside the module list.
  • Assign and document extension base stage numbers in sequential order. Record jumper settings in the design documentation so future engineers adding bases continue the numbering correctly.
  • Cross-check every decision against the latest Mitsubishi Electric hardware manual for your specific QCPU model before finalizing the bill of materials.

Five Real-World MELSEC-Q Rack Expansion Configurations

The following scenarios illustrate how the sizing framework above applies to typical engineering situations. These are representative configurations — always verify against the hardware manual for the specific QCPU and base units you select.

Scenario CPU Selection Base Configuration Expansion Strategy Key Constraint to Watch
Small machine, limited future growth Q00CPU or Q01CPU (entry group) Main base only, or main plus one extension base Design with 20–30% spare slots; reserve cabinet space for one additional extension base 24-module total system limit across all bases
Medium machine with known future options Q02UCPU (if forecast stays within 36 modules); otherwise standard non-redundant QCPU Main base plus two extension bases initially, with routing and space for up to four Allocate slots for communication and future analog modules; monitor running total against 36-module limit 36-module total limit for Q02UCPU; shift to higher tier if forecast exceeds this
Large line controller with long-term growth Standard non-redundant QCPU range Main base plus several extension bases; cabinet space reserved for additional bases up to seven total Centralise I/O; design extension cable routes within 13.2 m; plan for remote I/O if limits may be exceeded 64 total slots maximum; 13.2 m cable length across all extension bases
High-availability redundant system Q12PRHCPU or Q25PRHCPU (confirmed serial number for extension support) Main base plus extension bases via Q65WRB (first extension) and Q68RB (subsequent bases) Plan module counts below 63 maximum; leave spare capacity for critical future I/O Serial-number requirement; mandatory use of redundant bus modules Q65WRB and Q68RB
Existing rack nearing capacity Existing QCPU — assess remaining extension base and module headroom against hardware manual Add extension bases within remaining CPU limit; verify cable length cumulative total If headroom remains: add bases and modules within limits. If limits reached: plan migration to higher QCPU tier or add remote I/O via MELSECNET/H Existing base stage numbers must not conflict; cable length cumulative total must remain below 13.2 m

QCPU Tier Comparison: Expansion Limits at a Glance

CPU Group Max Extension Base Units Max Total Modules/Slots Typical Application Size Recommended Expansion Margin Key Notes
Q00CPU, Q01CPU, Q00UCPU, Q01UCPU (entry) Up to 4 24 modules total Small machines, compact panels Leave at least 4–5 slots unoccupied at commissioning Low initial cost; limited ceiling — do not use if forecast approaches 24 modules
Q02UCPU (mid-range compact) Up to 4 36 modules total Medium machines with moderate growth Leave at least 6–7 slots unoccupied at commissioning Good step up from entry group; still limited to 4 extension bases
Standard non-redundant QCPU range Up to 7 64 slots total (per hardware manual) Line and plant controllers with long-term growth Leave 2 or more extension bases worth of capacity unused at commissioning Maximum local rack expansion capability; appropriate for multi-station lines
Q12PRHCPU, Q25PRHCPU (redundant) Up to 7 (later serial numbers only) Up to 63 modules (with Q65WRB and Q68RB bus modules) High-availability and safety-critical systems Plan well below 63-module limit; leave room for critical I/O additions Serial-number check mandatory; requires dedicated redundant bus modules

If your module forecast puts you at or near the limit of the current CPU tier, moving one tier up in CPU is the right action at the design stage — not at the point of expansion. Check current availability of all MELSEC-Q CPU and base unit families at LeadTime.ca before finalizing your bill of materials.

Expert Selection Verdict

The MELSEC System Q Programmable Logic Controller is a well-documented, mature modular platform that rewards engineers who do their expansion planning before hardware is ordered. Its tiered CPU architecture — from the entry-level Q00CPU/Q01CPU group through the Q02UCPU to the standard non-redundant range and the redundant Q12PRHCPU and Q25PRHCPU — gives engineers genuine flexibility to match hardware capability to project scale. The platform is right for controls engineers who need a centralised rack-based PLC with the ability to add I/O and special function modules over a multi-year system life, and who are prepared to plan that growth explicitly rather than relying on retrofitting.

The MELSEC-Q is not the right choice if your process requires a distributed I/O architecture from the start and local rack expansion is not a priority — in that case a remote I/O approach over MELSECNET/H may be a better fit regardless of CPU tier. It is also not the right choice if you specify an entry-level QCPU without accounting for the 24-module ceiling and then expect to grow past it — the ceiling is real and the only remedy is CPU replacement. For redundant applications, the Q12PRHCPU and Q25PRHCPU are the correct models, but they carry meaningful additional complexity in serial-number verification and mandatory use of dedicated redundant bus modules. Engineers who are not already familiar with these requirements should verify them against the hardware manual before committing to the configuration.

From a procurement standpoint, MELSEC-Q hardware is widely used in manufacturing and infrastructure applications globally, and most CPU and base unit families are available through specialist distributors with reasonable lead times for standard configurations. However, specific QCPU variants, redundant bus modules, and higher-end base unit configurations can have variable availability depending on current market conditions — verifying stock and lead time before finalizing a design is practical, not just precautionary. The LeadTime.ca team can assist engineers in confirming which MELSEC-Q CPU and base unit families are available for their project timeline — check current availability on the product page at LeadTime.ca or contact the team directly for configuration guidance.

For volume pricing or to confirm lead time before committing to a build, contact the LeadTime.ca team directly — we ship worldwide: [https://leadtime.ca/pages/contact](https://leadtime.ca/pages/contact)

What Engineers Need to Know Before Specifying a MELSEC-Q Expansion

Community discussion specific to MELSEC-Q rack expansion planning is limited in publicly indexed forums — most detailed exchange happens in closed OEM networks and integrator communities. What is consistent across general industry engineering sentiment, however, is that the MELSEC System Q platform is well-regarded for its modular flexibility and the quality of its hardware documentation, but that the combination of CPU-specific limits, slot and module terminology, and extension cable rules creates genuine specification complexity that catches engineers who approach it without a structured planning process.

The most frequently encountered confusion points are: assuming all QCPU variants support the same number of extension bases when they do not; treating slot counts and module counts as interchangeable when the hardware manual distinguishes them; and underestimating how quickly extension cable length is consumed in large enclosures where cable must route around other equipment rather than run directly between bases. These are not exotic edge cases — they are the normal experience of engineers sizing a MELSEC-Q rack for the first time without a clear reference for the numbers.

When community reference data is sparse and the stakes of an under-specified rack are high — forced CPU replacement, panel rework, project delay — consulting a specialist distributor with direct MELSEC-Q sourcing experience is the practical approach. LeadTime.ca works with controls engineers specifying MELSEC System Q configurations globally, and can help cross-reference CPU tier requirements, confirm which base unit and bus module families are appropriate for a given expansion plan, and identify stock and lead times before an RFQ is finalized. If you are unsure whether your planned configuration stays within documented limits, that is the right moment to reach out rather than after hardware has been ordered.

Six Common MELSEC-Q Rack Expansion Mistakes — and How to Avoid Them

Before issuing a purchase order for any MELSEC-Q rack configuration, confirm that your design does not fall into any of these documented sizing and selection errors:

  1. Ignoring CPU-specific extension limits. Assuming all QCPU variants support the same number of extension bases and modules. Effect: racks that cannot be expanded as planned, forcing CPU replacement. Prevention: always check the specific QCPU's hardware manual for maximum extension bases and module slots before final design.
  2. Underestimating future module requirements. Designing only for current I/O without considering realistic growth. Effect: all slots and module capacity are consumed quickly; any expansion requires major panel rework. Prevention: include expected future stations, options and process changes in the initial sizing and add a margin for unknowns.
  3. Exceeding extension cable length limits. Mounting extension bases far apart in large cabinets without calculating total cable length. Effect: violating the 13.2 m maximum extension cable length, risking communication issues or unsupported configurations. Prevention: plan cabinet layout around documented cable length limits and verify total length for each design.
  4. Incorrect extension base jumper settings and numbering. Setting jumpers out of order, leaving jumpers off, or setting the same stage number on more than one base. Effect: system malfunction or inability to recognise extension bases correctly. Prevention: follow the hardware manual's jumper setting order from main to last base, and document the configuration.
  5. Neglecting power supply margin. Sizing power supplies only for current modules. Effect: added modules push power supplies beyond rated capacity, causing faults or brownouts. Prevention: estimate current draw with margin, and consider future modules when selecting power supplies.
  6. Forgetting panel and wiring space for expansion. Designing cabinets tightly around current hardware. Effect: no room to add extension bases, forcing new cabinet installation or relocation. Prevention: reserve physical space and wiring duct capacity for planned future bases and cables.

If your current design has any of these risks, contact the LeadTime.ca team before ordering — it is faster to adjust a specification than to rework an installed panel: [https://leadtime.ca/pages/contact](https://leadtime.ca/pages/contact)

Pre-Order Selection Checklist

Use this checklist before finalizing any MELSEC-Q rack expansion bill of materials:

  1. Confirm the selected QCPU's maximum number of extension base units and total slots/modules.
  2. Check that the planned number of modules (including future additions) does not exceed that limit.
  3. Verify that the total extension cable length from main to last extension base stays below 13.2 m.
  4. Ensure base unit slot counts match your planned module mix with at least 10–20% spare slots.
  5. For redundant systems, confirm serial-number requirements and correct use of redundant bus modules.
  6. Confirm power supply capacity with margin for future modules.
  7. Reserve physical panel space, wiring duct and cabinet depth for future extension bases and cable access.

Frequently Asked Questions

How many extension base units can I add to a MELSEC-Q system, and does it depend on which QCPU I have?

Yes, the CPU model determines the limit. The Q00CPU, Q01CPU, Q00UCPU, and Q01UCPU entry group supports up to four extension base units with a total of 24 modules across the entire system. The Q02UCPU also supports up to four extension bases but allows up to 36 total modules. The standard non-redundant QCPU range supports up to seven extension bases with a maximum of 64 total slots. For the redundant Q12PRHCPU and Q25PRHCPU, extension capability depends on serial number — later serial numbers allow up to seven extension bases and up to 63 modules when used with dedicated redundant bus modules. Always verify the specific limit in the hardware manual for the exact QCPU model you are specifying.

My existing MELSEC-Q rack is almost full — can I just add another extension base, or do I need a new CPU?

First, check how many extension bases are already connected and how many modules are already installed against your specific QCPU's documented maximums. If the CPU still has headroom in both extension base count and total module count, and the cumulative extension cable length will stay below 13.2 m with the new base added, you can add an extension base within the existing system. If any of those limits are already at or near maximum, a CPU upgrade or a move to remote I/O over a network such as MELSECNET/H is the documented path forward — adding hardware beyond the CPU's limits is not supported.

What is the 13.2 m extension cable limit, and what happens if I exceed it?

The 13.2 m limit is the maximum total length of extension cables from the main base unit to the last extension base unit in the chain, as specified in the MELSEC System Q hardware documentation. This is a cumulative limit across all cable segments in the extension chain, not a per-segment limit. Exceeding it creates an unsupported configuration that can result in communication errors between bases and unreliable system operation. Plan cabinet layout and cable routing paths explicitly to verify this total before hardware is installed.

What is the difference between slots and modules in MELSEC-Q documentation, and which limit applies to expansion planning?

A slot is a physical position in a base unit; a module is a hardware unit installed in a slot. In most configurations they correspond one-to-one, but some special-function or double-width modules can occupy two slots while counting differently toward the CPU's module limit. The CPU's documented maximum refers to modules as defined in the hardware manual, not simply to the number of physical slots across your bases. When planning expansion, count modules as the hardware manual defines them for your specific QCPU, not just the physical slot count of the base units you have selected.

Do I need to reassign existing extension base stage numbers when I add a new base to an established MELSEC-Q system?

Generally, no — add the new extension base at the end of the chain and assign it the next sequential stage number. Reassigning existing bases' stage numbers risks disrupting the operating system's recognition of all bases and should only be done if the hardware manual requires it for a specific configuration. Always document the new jumper assignment in your design records so future engineers continue the numbering scheme correctly.

When does it make more sense to use remote I/O instead of adding more local extension bases?

Remote I/O becomes the practical choice when: the system is physically distributed over distances that make 13.2 m extension cable routing impossible; the QCPU has reached its maximum extension base or module count and a CPU upgrade is not practical; or future expansion will be in a separate physical location from the main panel. The MELSEC System Q hardware documentation identifies MELSECNET/H as a network option for extending I/O beyond local rack limits. Remote I/O also reduces noise exposure on long I/O field cables by placing the module closer to the field devices.

Why Order Through LeadTime.ca

  • LeadTime.ca sources MELSEC System Q CPUs, base units, power supply modules, and accessories globally and ships worldwide — not restricted to any single region.
  • The team can help confirm which QCPU variants and base unit families are in stock for your project timeline, including less common configurations such as redundant bus modules.
  • For engineers working from hardware manuals who need a second opinion on whether a planned configuration stays within documented limits, LeadTime.ca provides application guidance alongside sourcing.
  • Volume pricing, project-based procurement, and lead-time confirmation are available by contacting the team directly before issuing a purchase order.

At-a-Glance Summary

  • Entry-level QCPU group (Q00CPU, Q01CPU, Q00UCPU, Q01UCPU): maximum 4 extension bases, 24 total modules across all bases
  • Q02UCPU: maximum 4 extension bases, 36 total modules across all bases
  • Standard non-redundant QCPU range: maximum 7 extension bases, 64 total slots per hardware manual
  • Redundant Q12PRHCPU and Q25PRHCPU (later serial numbers): up to 7 extension bases, up to 63 modules with Q65WRB and Q68RB bus modules
  • Total extension cable length from main base to last extension base must not exceed 13.2 m
  • I/O field cables can generally extend up to 100 m; hardware documentation recommends keeping cable lengths around 20 m to help avoid interference
  • Each extension base requires a unique sequential stage number set by jumpers — incorrect or duplicate settings cause system malfunction
  • Every base (main and extension) requires its own power supply module sized with margin for future modules
  • Design with at least 10–20% spare slots at commissioning; size CPU tier to the maximum forecast module count, not the current module count
  • If local rack limits are reached, remote I/O over MELSECNET/H is the documented expansion path
  • Verify all configuration decisions against the latest Mitsubishi Electric MELSEC System Q User's Manual (Hardware) before finalizing any bill of materials

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