Despatch Protocol Plus Modbus Communications - Manual (Page 53)

Chromalox Instruments and Controls
A-51643 Rev. 6 10/06/03 47
7.2 Additional Communications Troubleshooting Hints
Communications problems occur primarily due to physical wiring problems or mismatched settings. It is
important to verify that the Protocol
TM
Plus controller‘s Communication Card is properly installed, and its
jumpers are properly set to match the wiring option used (RS-232, RS-422, or RS-485). Make sure the
controller has MODBUS communications enabled, and that communications is programmed to match the
controller‘s Address, Baud Rate, and Parity settings.
For programming communications, make sure that the serial data format uses two stop bits as mentioned
above; using one stop bit will not work.
Programming should allow silent intervals between messages as described in the Modbus User‘s guide, in
addition to transmitting bytes within a message continuously.
Finally, the Protocol
TM
Plus controller requires that the ―Communications Security Code‖, Holding
Register 0, be properly set before accessing some registers via Modbus. A program may want to set this
register initially, with the Level 1 or 2 password associated with the controller, in order to avoid errors
trying to read critical registers later on.
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