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Despatch Protocol Plus Modbus Communications User Manual

Page 53

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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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