The Q68B is Mitsubishi Electric’s 8-slot main base unit for the MELSEC-Q programmable controller series, providing eight peripheral module slots and — critically — support for connecting extension base units via QC extension cables, making it the entry point into the MELSEC-Q scalable multi-base architecture for applications requiring more than eight peripheral modules in a single system. As the smallest main base unit in the MELSEC-Q range to support extension bases, the Q68B is widely specified as the main base in systems designed for future expansion, eliminating the need to replace the main base when additional I/O or communication modules are required. Atlantech Drives holds stock of the Q68B. Contact us for fast worldwide delivery and competitive pricing.
What Is the Q68B?
The Q68B is a MELSEC-Q series main base unit with eight peripheral module slots and a built-in extension base connector, allowing up to seven extension base units (Q6B, Q12B, Q25B) to be daisy-chained from the main base via QC extension cables. This multi-base architecture supports a maximum of 64 peripheral module slots across the main and extension bases, enabling large-scale I/O configurations within a single MELSEC-Q CPU’s address space. The Q68B accepts all MELSEC-Q CPU modules in slot 0, all Q-series power supply modules in the power supply slot, and any combination of MELSEC-Q peripheral modules across its eight peripheral slots. In multi-CPU configurations, two CPUs can be installed in the Q68B (CPU slots 0 and 1), leaving six peripheral slots for I/O, communication, and special function modules. The Q68B is programmed and configured using GX Works2 or GX Works3.
Key Technical Specifications
- Model: Q68B
- Type: Main base unit with extension support
- Number of Module Slots: 8 (peripheral modules only)
- Power Supply Slot: 1 (dedicated, leftmost)
- CPU Slot: 1 (slot 0) or 2 (slots 0 and 1 in multi-CPU)
- Total Slots Including Power Supply and CPU: 10
- Extension Base Connection: Supported (up to 7 extension bases)
- Maximum Total Peripheral Slots (with extension): 64
- Internal Bus: MELSEC-Q high-speed internal bus
- Compatible Power Supply Modules: Q61P, Q61P-D, Q62P, Q63P, Q64P, Q61SP
- Compatible CPUs: All MELSEC-Q series CPU modules
- Operating Temperature: 0°C to 55°C
- Weight: Approx. 0.56 kg
Compatibility & System Integration
The Q68B’s extension base support is implemented via a dedicated extension connector on the right side of the base unit, accepting QC05B (0.45 m), QC06B (0.6 m), QC12B (1.2 m), QC30B (3.0 m), and QC50B (5.0 m) extension cables. Each QC cable connects the Q68B main base to the first extension base, and additional extension bases are chained from the first extension base’s output connector. The MELSEC-Q internal bus signal is transmitted across the extension cable with no performance degradation for cable lengths up to 1.5 m; for longer extensions (QC30B, QC50B), Mitsubishi recommends verifying signal integrity by checking for communication errors (SD8065–SD8068 special register values) after installation. The Q68B is fully compatible with the QJ61BT11N CC-Link Ver.2 master module, QJ71E71-100 Ethernet module, QJ71LP21-25 MELSECNET/H module, and QD75MH/QD77MS positioning modules, enabling complex multi-network, multi-axis system architectures within a single base.
Installation Advice
When installing the Q68B in a system that will use extension bases, always plan the extension cable routing before mounting the base units on the DIN rail — QC extension cables have a minimum bend radius of approximately 50 mm and cannot be routed through sharp corners without risk of internal conductor damage. Mount the Q68B main base and extension bases on the same DIN rail section where possible, with extension bases to the right of the main base in ascending order (extension base 1, 2, 3…). Ensure that the DIN rail is securely grounded and that the Q68B’s functional earth terminal (FG) is connected to the panel protective earth — the FG terminal is the small screw terminal located at the bottom-right of the base unit PCB and must be connected with a wire no smaller than 2 mm² to comply with Mitsubishi’s installation guidelines. After completing the hardware installation, power up the system and check SD8060–SD8068 (base unit information registers) in GX Works2’s device monitor to verify that all extension bases and their installed modules are correctly recognised by the CPU.
Frequently Asked Questions
Q: How many extension bases can be connected to the Q68B, and what is the maximum total module count?
A: The Q68B supports up to seven extension base units connected in a daisy chain via QC extension cables. Using the maximum configuration of seven Q12B extension bases (12 slots each) plus the Q68B’s eight peripheral slots, the total peripheral module count reaches 8 + (7 × 12) = 92 slots. However, the MELSEC-Q CPU’s maximum addressable I/O is 64 slots, so the practical limit is 64 peripheral slots regardless of physical slot count. Mitsubishi recommends configuring the base unit assignment in GX Works2’s PLC parameter settings to match the actual hardware configuration.
Q: Can the Q68B be used as an extension base instead of a main base?
A: No. The Q68B is a main base unit and has a CPU slot and power supply slot that extension bases do not have. Extension bases (Q6B, Q12B, Q25B) have only peripheral module slots and an extension connector — they do not accept CPU or power supply modules. The Q68B must always be used as the main base in a MELSEC-Q system.
Q: What is the recommended power supply module for a Q68B with eight fully populated peripheral slots?
A: Calculate the total 5 VDC current consumption of all eight peripheral modules from their respective hardware manuals. A typical mix of four QX41 digital input modules (0.15 A each) and four QY41P digital output modules (0.16 A each) totals approximately 1.24 A — well within the Q61P’s 6 A rating. For systems with high-current special function modules (such as QD75MH positioning modules at 0.50 A each), the Q62P (3 A) may be sufficient, but the Q61P (6 A) or Q63P (6 A with wide input range) provides the greatest headroom for future module additions.
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