MD888 as MODBUS Slave AND Master

[ccdubs]

HI there,

Does anyone know if it is possible to use the MD888 as a MODBUS slave and a master?

I have a number of PLC's forming part of a network branch. I plan to use SCADA to talk to Slave PLC (address 1) via MODBUS RTU. This PLC would then become a master and communicate commands with the others.

It this acheivable?

Many thanks.

[support]

Yes, this is completely possible as the M-series PLC are designed to be operated precisely in this manner. Each PLC has two comm ports, you can connect the SCADA to the PLC's RS232 port and so the PLC act as a slave on this port. At the same time, you can program the PLC to use the other port (the RS485 port) as a master port to talk to other devices via either MODBUS RTU, MODBUS ASCII or even some proprietary ASCII or binary protocols.

[ccdubs]

Thanks for the reply.

I should have added more info to the original email. We are limited to only using the RS232 port as the PLC is connected to a radio modem which is responsible for all comms.

I was intending to let the SCADA push values into memory for PLC on address 1 using MODBUS. This PLC would then recognise the new data and use the same port to send MODBUS information to PLC's on addresses 2,3.....

Is this option possible?

[support]

You probably want to avoid using the same port for both master and slave function since unless you have good arbitration scheme it is difficult to prevent conflict on the serial port.

Note that you can buy an Auto485 to convert the PLC's RS485 port to a second RS232 port and that should work out for you very well.