During a recent plant inspection, a 1771-ASB/E module stopped communicating with the PLC-5 rack. The PLC continued running, but the serial link to the downstream devices was lost, which caused alarms for I/O data updates.

The first thing I checked was physical connections. Serial communication is extremely sensitive to loose or oxidized connectors. On this module, the DB-9 connector often suffers from bent pins or corrosion. Cleaning and reseating the connector restored partial communication in some previous cases.

Next, I verified the baud rate and node address configuration. The ASB/E module requires exact matching settings with the connected devices on the DH-485 or RS-232 network. A mismatch, even by one parameter, can result in no data transmission. Using RSLogix 5, I confirmed the settings aligned with the rest of the network.

I also looked at cable integrity and termination. Long serial cables or improperly terminated DH-485 networks often cause intermittent or complete communication failure. I measured resistance and checked the shield continuity. Adding proper termination resistors solved communication drops on several older racks I’ve serviced.

Another step was to observe the module LEDs. The OK and COMM indicators provide immediate insight. A blinking COMM LED often indicates network activity but with errors, while no LED activity can point to an internal fault. In this case, the LEDs blinked irregularly, confirming the problem was more likely network-related than a hardware defect.

Finally, I performed a module reset and power cycle. This sometimes clears transient faults in the module’s firmware, restoring normal serial communication. After verifying the wiring, settings, and termination, the module resumed proper operation, and the serial link stabilized.

Field insight: With 1771-ASB/E modules, most communication issues are caused by connector problems, configuration mismatches, or network termination, rather than intrinsic module failure. Always isolate physical and configuration causes first.

References:

• Rockwell Automation. PLC-5 1771-ASB/E Serial Communication Module User Manual, 2017.

• Rockwell Automation. DH-485 Network Guidelines, 2016.