rs485 communication issues

Hi,

I am trying to get data from a digital indicator to Siemens PLC S7-200 series(two port) using modbus 485. The indicator supports modbus 485 communication. While purchasing the indicator, i verified communication using modbus tools for windows by connecting it to PC using 485 to 282 converter and the communication was perfect. Now when i am trying to get data from the indicator(register=40011) in my ladder using sample programs from Siemens forum, i receive 0 as value whereas no communication error is identified. I've grounded the shield as well to avoid any noise that might create disturbance. Strangely the the communication light also blinks when connected to PLC. Any help and suggestions would be highly appreciated.

Thank you.

I’m not sure your issues is related to the 485. In your addressing are you using a base 0 or a base 1 scheme? PLC’s typically use a base1 meaning that the addressing starts with 1 not 0. It could be that if your PLC is configured to poll address 40011 in reality it’s polling 40010 which, depending on what it is, could be responding with 0. Try polling address 40012.

Firejo said:

I’m not sure your issues is related to the 485. In your addressing are you using a base 0 or a base 1 scheme? PLC’s typically use a base1 meaning that the addressing starts with 1 not 0. It could be that if your PLC is configured to poll address 40011 in reality it’s polling 40010 which, depending on what it is, could be responding with 0. Try polling address 40012.

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Hi Firejo,

Many thanks for your reply, yes you were right my problem was not with 485, i just switched to another power supply and it worked, strange but that was the solution to my problem