Omron CJ1W-SRM21 is a CompoBus/S Master Unit

The Omron CJ1W-SRM21 is a CompoBus/S Master Unit for the CJ1 series PLC (including CJ1G, CJ1H, and CJ1M) and the newer CJ2 series. It acts as a dedicated gateway to Omron’s CompoBus/S fieldbus—a high-speed, simplified remote I/O bus designed for efficient communication over short to medium distances.

CompoBus/S is favored for high-speed applications like packaging and small-scale assembly lines where rapid I/O updates are more critical than complex data transmission.

1. Key Technical Specifications

The SRM21 defines how many remote points your PLC can “see” on a single bus.

2. Physical Setup & Addressing

Before the unit will work, you must set the physical switches on the front of the module:

• Unit Number Switch: A rotary dial (0–95) that determines the memory location in the PLC’s Special I/O Unit Area.

• DIP Switches: These control the communication settings:

  • SW1 (Speed): High-speed vs. Long-distance mode.

  • SW2 (I/O Capacity): Determines if the unit handles 128, 64, or 32 points (shorter counts result in faster refresh cycles).

  • SW3/4: Settings for “Communications Error” behavior (Clear vs. Hold outputs).

3. Wiring the Bus

CompoBus/S uses a simple 4-wire setup: two wires for data (BD H and BD L) and two wires for the 24V DC power supply that powers the bus interface of the slave units.

• Topology: Supports “Bus” (daisy-chain), “Star,” or “Branch” configurations, though a main trunk with short branches is most common.

• Termination: A terminating resistor (typically 121 ohms) is required at the furthest end of the bus in high-speed mode to prevent signal reflection.

4. Memory Mapping in CX-Programmer

The SRM21 does not require complex “Message” instructions (SEND/RECV). Instead, it uses Direct I/O Mapping:

• The PLC automatically reserves a block of memory in the CIO Area based on the Unit Number you set.

• For example, if the Unit Number is 0, the Inputs from Slaves might start at CIO 2000 and Outputs at CIO 2001.

• Data is updated every PLC scan, making the remote I/O feel as fast as local rack I/O.