On the SI-EM3 (Single), SI-EM3D (Dual), SI-EM3/V (Single), SI-EM3D/V (Dual) and CM090 option cards, the TCP layer requires a response, otherwise a "loss of message" or "loss of communication" is declared and represented by a BUS Fault on the keypad of the drive. On the PLC side, the fault code can not be read if communication is lost between the PLC and drive. A "Heartbeat", also known as a "Watchdog", is generally a signal used to confirm two devices can communicate. Since the “BUS” fault is internally detected on the drive and therefore not accessible via a register, a psudo heartbeat can be used to check if communication was lost.

Confirm the PLC and Drive can communicate by having the PLC logic write a "1" to a bit then read the status of the selected bit to verify the "1" was written. After a selected time has expired, write a "0" to the same bit and read the bit was set to a "0".  If the read bit did not match what was written, communication has been lost.

Below is an example using digital input terminal S8 on a A1000 drive: 

1. On the drive, set parameter H1-08 = F (through mode)
2. On the PLC, write a “1” to register 1H bit B as showin in Table C.5 below.
   
3. On the PLC, read register 2BH bit 7 (as shown in the table below) to verify that “1” was written.
   
4. Repeat steps 2 and 3 but this time write a "0" to register 1H bit B, then read register 2BH bit 7 is set to "0". If the read does not correspond with the write, communication has been lost.