Flow Meter to Red Lion Data to Compaq Logic

Hello.
I'm mounting a Sarco Spirax Flow Meter going to a Red Lion Data Station Plus. From the DSP we are going to a Compaq logic controller. This is a first for me. From the flow meter to the DSP we are using RS485. The flow meter has a Mod Bus A and B terminals going to an Rs485 port. The print shows twisted pair. Any ideas on what lands on A or B?

Also. Does the red lion get added to the I/O tree in PLC program? Any info would be much appreciated.

This is erroneously posted in the 'practice posting' area. Maybe the moderator will see it and move it to the Q&A.

I can help a little on the flow meter side.

The flow meter is a Modbus RTU slave that talks over an RS-485 network link.

The DSP has an RS-485 port which needs to be a Modbus master in order to talk to the flow meter slave.

If the flow meter has only A and B connections, that it is "2 wire" RS-485. The port on the DSP is an RJ45 that is '4 wire' RS-485, but the DSP manual shows how to jumper terminals to make it "2 wire".

Usually line A on one device connects to line A on the other device. But the EIA standard does not define A/B so some vendors define it one way, others the opposite. If you have comm problems, a classic test is to swap A and B on one end and see if that makes it work. Connecting the lines backwards will not damage the drivers, but comm won't work.

When you configure the DSP's RS-485 port as a master so that it can communicate to the flow meter, It can also separately write to the PLC, if the PLC supports Modbus RTU as a slave. You'll have to check into that.

The protocol (Modbus, DH+, whatever) that the DPS and the PLC use have to match, just like the hardware transport layers have to match (RS-485 on both sides, or RS-232 on both sides, or Ethernet on both sides).