modbus discrete input

[edmondchoi]

hello, i am confused about modbus discrete input.

i found an example online,
HMI is Client, PLC is server.
To read the "motor servo-on status" by HMI.

it use function code 02 which refer to
"The client reads the status of the discrete inputs
of the server."

isn't it servo-on status an "OUTPUT" of the PLC?

Please help .

 

[nhatsen]

To read the "motor servo-on status" by HMI.
it use function code 02 which refer to
"The client reads the status of the discrete inputs
of the server."

isn't it servo-on status an "OUTPUT" of the PLC?

I suppose the PLC sends commands to the motor and receives a status:

    PLC
-------------+
  MODBUS     |
  TABLE      |
+--------+   |PLC output       
| COILS  |-----------------> To motor 
+--------+   |
| INPUTS |<----------------- From motor status
+--------+   |PLC Input
             |
-------------+

a) The input section of the modbus table receives the motor status. The HMI uses function 2 to read the input (status). You could say that the status is an "output" from the PLC to HMI (incorrect), but from the point of view of the modbus table is an input.

b) When you want to start/stop the motor, the HMI uses function 5 to write to the coils section (coils=outputs). Then the PLC activates the correspondent channel in the output module.