User manual

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Communication Notes:

1. The GSC400 only supports commands 0x03 (read multiple registers) and

0x06 (write single register). If an attempt is made to send another

command the GSC400 returns “incorrect command”.

2. Can only request to read 20 registers at a time. If over 20 GSC400

returns “illegal data address”.

3. Also need to watch out for discontinuities in register map. The GSC400

will return invalid data for non-existing registers (often returns a value of

0).

4. If they try to read registers that don’t exist GSC400 will return values for

those registers that don’t exist but it will be invalid data (often 0).

5. If they try to write to a register that doesn’t exist, GSC400 will ignore write

request and return OK.

Refer to Appendix E on page 123 for more detailed Modbus information and the

Modbus map.

Troubleshooting steps:

1. Are Modbus +, -, and ground connected properly. Try reversing + and -.

2. Are the device address and baud rate set properly in GSC400 Modbus

menu (under Advanced menu).

3. Is the cable type suitable for RS485 communications? How long is the

cable and how noisy is the environment.

4. Communication errors. Even if sent an invalid command, read/write

request for an invalid register, or a read request over 20 registers, the

GSC400 will return an error message to the master. Do not just look for