Instead of a second PC and a null modem cable, you can also use virtual null modem software (vnullmod20 - downloadable from the Internet or Eltima or TAL software) and run both RTU master and slave on one machine. Just do not use a COM port that already exists on your machine (such as COM1). This can also be run in the Remote Labs. IMPORTANT: Change the slave ID to ‘4’ and set coils 12/13 (absolute addressing in decimal) so that your data (bit values) with a ‘read coil’ command (FC01) becomes 03 hex (i.e. 0000 0011) in the response message. IMPORTANT: Address ONLY these two coils.
a) Provide screenshots of BOTH the Modbus Poll and Modbus Slave screens showing the requests and responses. (DisplayCommunication). Capture a minimum of four Modbus transactions: one with both coils cleared, one with both coils set, and one each with one of the coils set.