Multiple MODBUS masters ?

[vijay]

In one of your application notes you show a MD PLC talking to various RS485 slave devices such as high res analog modules . You also show the RS485 connection to a 232 convertor + the Xserver ethernet interface. I assume the internet interface allows other computers to request data from the MD PLC. In this situation there would be multiple MODBUS masters - the MD in talking to the slave IO and the external computer requesting data from the MD. My understanding is that the MODBUS protocol does NOT permit multiple masters as the protocol does not tag data packets with originating addresses.

How do you expect this to work ?

The problem we have is that we are tying up the 232 COM1 port with a touchpanel. We need the COM2 port (MX+) for a bar code reader and we need to use the 485 COM3 port to talk to analog mdules (we need at least 12 bit ADC) and we also need the 485 port for MODBUS data acquisition from the PLC.

- Vijay

[support]

The XServer does not use the MODBUS protocol. It uses the PLC's native protocol for communication with the TRiLOGI software or TRi-ExcelLink.

The T100MD can talk to the slave analog board using its RS485 port, leaving its RS232 port for use with the XServer.  Alternatively, the XServer can connect to the PLC's RS485 port (so multiple PLC can share one XServer), the PLC can then use an Auto485 to convert its COMM1 to RS485 and talk to multiple  slave I/O boards.

For your case, you are using up all the serial ports that the PLC posseses, which mean that something got to give. You may want to multiplex the use of the COM2 port so that you switch the COM2 to either the bar code scanner or the analog I/O module (via Auto485 adapter). When a device is present and the bar code is ready to scan, the COM port is switched to the bar code scanner, otherwise it is switched back to the analog module.

The PLC software can be written to control this multiplex easily if the bar code scanner is under the control of the PLC. i.e. a sensor will sense the presence of an object,  it then switches the COM2 port to the bar code scanner serial port and the PLC will trigger an output to scan the barcode and get the data. After that, the PLC software will switch the COM2 to the Auto485 and continue to acquire data from the analog I/O module.

In such a case, both the COMM1 and COMM3 ports are still available for connecting to the touch panel and for MODBUS data acquisition purpose.