Modbus Questions - Direct Logic/CMORE

Hello,

I have a DirectLogic 260 PLC that I want to read modbus data into and issue commands to field devices.

Right now, there is a CMore HMI that is the master, and it reads the information from the field devices for trending only.

My questions are:

1. "Who" should the Master be, and Why?
2. If the PLC needs to be the master, can it be in the middle of the modbus network, or does it need to be at one end and terminated with a resistor? (PLC is located near one end of the chain, but not at the end.
3. My network is RS485 3-wire (A, B, GND) but the PLC has an 8-wire connection (TXD2 +/-, RTS2 +/-, RXD2 +, CTS2 +/-, GND) available. Do I just use the TXD + and - lines, plus GND?

Thanks,

Tim

In Modbus RTU there is only 1 master. The CMore MUST be a master. Possibly add Ethernet?

Hi Bernie,

The CMORE does have Ethernet, and is connected to the PLC for everything else but the modbus stuff.

My guess is make the PLC the master, and then push the former "modbus data" to the HMI as standard tag data over Ethernet? That way the PLC can read and write on the modbus network?

So, the master polls the network and reads/writes information to/from the salves, and the slaves just read and write information on the network?

Thanks,

Tim

Modbus works, like many other protocols, by the request-reply system, the master sends the request and one slave replies.

The problem with a serial connection is that there is nothing that prevents collisions when two devices try to send requests at the same time, so there can only be one master that sends requests and also this master have to wait to receive the reply before sending another request.

This limitation does not exist with Ethernet since it has the means to avoid collisions, if both the HMI and the PLC support Modbus TCP you should use it instead of Modbus RTU, then the PLC can also act as a master and if those field devices do not support Ethernet you would have to add a Modbus TCP to Modbus RTU conversion gateway

Thanks lfe for your reply.

The data must travel nearly 1,000m and the wiring that was already there was the 3-wire 485-type, so I used what I had. I believe modbus TCP has the same 100m limit as traditional Ethernet? If not, then I will consider wiring modbus IP since the slave devices all support modbus IP

With Ethernet over fiber optics there is no problem with 1km, but converters should be installed and done by fiber specialists.

But you could make the conversion easier with a Modbus TCP to Modbus RTU converter, which could be located near the PLC and HMI so you only have to disconnect the RS-485 cable from the HMI and connect it to the converter, so you could continue using the rest of wiring to the field devices.

The HMI configuration would have to be changed to communicate with Modbus TCP instead RTU.

TimD said:

Hello, My questions are:

2. If the PLC needs to be the master, can it be in the middle of the modbus network, or does it need to be at one end and terminated with a resistor? (PLC is located near one end of the chain, but not at the end.

Click to expand...

On an RS-485 network the master can be at either end or at any node along the way. But only the end points get termination resistors, whether they're slaves or a master.

TimD said:

3. My network is RS485 3-wire (A, B, GND) but the PLC has an 8-wire connection (TXD2 +/-, RTS2 +/-, RXD2 +, CTS2 +/-, GND) available. Do I just use the TXD + and - lines, plus GND?

Click to expand...

The Tx(+) and Rx(+) jumper together and the Tx(-) and Rx(-) jumper to together to make 2 wire half duplex.