The Q38B is Mitsubishi Electric’s 8-slot main base unit for the MELSEC-Q programmable controller series — providing one power supply slot, one CPU slot, and six I/O or intelligent function module slots in the standard MELSEC-Q main base unit for typical machine and process control system configurations, the most widely specified MELSEC-Q main base unit balancing module capacity with panel space efficiency. Atlantech Drives holds stock of the Q38B. Contact us for fast worldwide delivery and competitive pricing.
What Is the Q38B?
The Q38B is a MELSEC-Q series main base unit providing 8 total slots — one power supply slot, one CPU slot (slot 0), and six module slots (slots 1–6) for Q-series I/O modules and intelligent function modules. With 6 usable module slots, the Q38B is the most commonly specified MELSEC-Q main base unit because 6 modules typically covers the majority of standard machine and process control applications without requiring extension base units — a typical Q38B system might contain a Q02UCPU with QJ71E71-100 Ethernet, QD75MH4 positioning, QX80 inputs (×2), and QY40P outputs (×2), consuming all 6 module slots with a complete and capable control system. The Q38B supports all Q-series module types — I/O modules, intelligent function modules, motion CPU modules (in multi-CPU configuration), safety modules, and process control modules. In multi-CPU systems, the Q38B can host two CPU modules (Q02UCPU in slot 0 and Q172DSCPU motion CPU in slot 1, for example) with the remaining 5 slots available for I/O and intelligent modules. The Q38B includes the standard Q-series right-side extension bus connector for adding Q6□B or Q6DB extension base units when more than 6 module slots are required in the total system.
Key Technical Specifications
- Model: Q38B
- Total Slots: 8 (1 power supply + 1 CPU + 6 module slots)
- Module Slots: 6 (slots 1–6 — I/O and intelligent function modules)
- Compatible Power Supply: Q61P, Q62P, Q63P, Q64P
- Compatible CPUs: All Q-series CPU modules (Universal, High Performance, Process, Safety, Motion)
- Multi-CPU Support: Up to 4 CPU modules (CPUs occupy module slots 0–3)
- Extension Connector: Right-side extension bus connector
- Module Slot Width: 27.4 mm per slot
- Bus Standard: Q-series internal bus
- Operating Temperature: 0°C to 55°C
- Dimensions (W × H × D): 382 mm × 98 mm × 44.1 mm
- Weight: Approx. 0.74 kg
Typical Industrial Applications
The Q38B is the standard main base unit for the broadest range of MELSEC-Q industrial control applications — its 6-module slot capacity covers the vast majority of machine tool, packaging, material handling, and process control system configurations without requiring extension base units. In CNC machining centre auxiliary control systems where a MELSEC-Q handles coolant control, tool changer sequencing, and workpiece clamping alongside a dedicated CNC controller, the Q38B typically hosts a Q02UCPU with QJ71E71-100 Ethernet (for SCADA and HMI connection), QD75MH4 positioning (for tool changer servo axis), QX80 input (for limit switches and sensors), and QY40P output (for solenoid valves and coolant pump contactors) — 4 intelligent/I/O modules within the Q38B’s 6-slot capacity with room for future expansion. MELSEC-Q systems managing building automation and energy management functions in large commercial buildings use the Q38B with Ethernet, PROFIBUS, and multiple I/O modules to integrate HVAC, lighting, access control, and power metering systems into a unified building management system without requiring extension bases. Process control skids in pharmaceutical and chemical production — controlling multiple PID loops, valve sequencing, and alarm management — build the MELSEC-Q system around the Q38B base unit with process CPU (Q12PHCPU) and analog I/O modules in the 6 available slots. Water treatment SCADA systems with MELSEC-Q remote terminal units (RTUs) at pump stations and chemical dosing points deploy Q38B-based systems for their proven reliability in remote unstaffed installations where the complete control system must operate independently for months between maintenance visits.
Expert Tips
When populating the Q38B’s 6 module slots, place high-current-draw intelligent function modules (QD75MH4 at 0.75 A, QJ71PB92D at 0.54 A, Q172DSCPU motion CPU at 1.3 A) adjacent to the power supply module in the left-side slots — placing high-current modules near the power supply minimises the 5 VDC bus current flowing through the full backplane length, reducing voltage drop on the internal bus and improving voltage regulation at the right-side module positions. For Q38B systems with a Q62P power supply (6 A) near the rated capacity, this slot arrangement can provide 50–100 mV improvement in 5 VDC bus voltage at the right-side modules, improving module operation margin. When planning Q38B module slot assignments for a multi-CPU system (Q02UCPU + Q172DSCPU), place the motion CPU (Q172DSCPU) in slot 1 — the position immediately to the right of the sequence CPU in slot 0 — as the multi-CPU shared memory access between CPU modules is optimised for adjacent slot placement and some multi-CPU functions require the motion CPU to be in a specific relative position to the sequence CPU. In Q38B systems where module removal for maintenance is frequently required, install modules in the right-side slots (slots 4–6) for the modules most commonly removed — right-side modules are more accessible in panel installations where the Q38B is mounted with adjacent components to the left.
Frequently Asked Questions
Q: Can the Q38B support a 4-CPU multi-CPU configuration (4 Q-series CPUs installed simultaneously)?
A: Yes, the Q38B supports up to 4 CPU modules installed simultaneously — CPUs occupy slots 0, 1, 2, and 3 (the four leftmost module positions after the power supply slot). In a 4-CPU configuration, only slots 4, 5, and 6 remain available for I/O and intelligent modules — 2 fewer module slots than the standard single-CPU configuration. For large multi-CPU systems requiring both many CPU modules and many I/O/intelligent modules, the Q312B (12-slot main base, 10 module slots) provides more total capacity for mixed CPU and module installations.
Q: Is the Q38B compatible with both the Q02UCPU Universal model and the older Q02CPU Basic model?
A: Yes. The Q38B’s Q-series internal bus is backward compatible with all MELSEC-Q CPU generations — both the older Basic model CPUs (Q02CPU, Q06HCPU) and the current Universal model CPUs (Q02UCPU, Q03UDECPU, Q04UDEHCPU) physically and electrically mate with the Q38B’s CPU slot. The specific CPU model determines the system’s performance (program execution speed, memory capacity, communication features), not the base unit — all Q38B bases provide the same electrical bus interface to all Q-series CPU modules.
Q: What is the maximum total 5 VDC bus current that the Q38B can carry without bus conductor overheating?
A: The Q38B’s internal bus conductors are rated to carry the maximum current supplied by the compatible power supply modules — up to 8.5 A for the Q63P or Q64P. The base unit’s bus conductor rating is not the limiting factor — the power supply module’s rated output current (6 A for Q62P, 8.5 A for Q63P) is the effective limit. Do not attempt to install two power supply modules in a single Q38B base to combine their current outputs — the Q38B has only one power supply slot, and attempting to parallel power supplies in a single base unit is not supported and risks module damage.
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