Hello everyone! 

I need to poll from the PLC a set of MODBUS RTU slave stations connected in the chain topology (the communication parameters such as baudrate, etc., are all the same) to a single RTU master station module. Could I automatically change the MB_ADDR parameter of the Modbus_Master FB connected to the master RTU port without calling Modbus_Comm_Load when the current MODBUS operation has been completed (triggered by DONE output)? If so, could you share thoughts and recipes on the schemes I described?

Barebones and incomplete pseudo code to show the technique..

if (slaveDone or slaveError):
   slaveAddress += 1 			# next address
   slaveExe = False                     # transition 0 -> 1 needed
else:
   slaveExe = (FirstScan = False)
   
if slaveAddress > highNr:
   slaveAddress = lowNr

if slaveAddress < lowNr:
   slaveAddress = lowNr

MODBUS_MASTER(Exe    = slaveExe,
              Adress = slaveAddress, 
              Done   = slaveDone, 
              Error  = slaveError)

qwazee

Barebones and incomplete pseudo code to show the technique..

if (slaveDone or slaveError):
   slaveAddress += 1 			# next address
   slaveExe = False                     # transition 0 -> 1 needed
else:
   slaveExe = (FirstScan = False)
   
if slaveAddress > highNr:
   slaveAddress = lowNr

if slaveAddress < lowNr:
   slaveAddress = lowNr

MODBUS_MASTER(Exe    = slaveExe,
              Adress = slaveAddress, 
              Done   = slaveDone, 
              Error  = slaveError)

 

Many thanks for this answer, I need to investigate the possibility to do so, but did you once tested it on the real installation? 

 

Yes, I have used a similar structure before. I built my own design from an example that is listed on this support site. Search for Modbus communications from S7-1200 to V20 drive.

These days I use something else.. a 3rd party library.
Extended Modbus library for Siemens PLCs