I had a problem which I am unable to understand. I was checking Quantum PLC 113 for communication by Foxboro DCS & ABB Advant 450. Both DCS don't have Modbus Plus Hardware. We want to establish communication between one DCS & quantum PLC.We had used cross RS-232 Cable for communication instead of what is specified by quantum catalog. We had no communication on Modbus Port by Foxboro & ABB DCS. By mistake we had connect RS-232 DCS port to Modbus Plus Port & it start communication. Modbus Plus is not supposed to communicate on serial link. We than make RS-232 connection as per quantum catalog Pin 2 & 3 Cross 4 & 6 Cross, 7 & 8 short on both sides 1 & 5 short. After this connection Both DCS start communicate on Modbus Port of Quantum PLC.

Now My question why Quantum PLC communicate on RS-232 cross cable connection on Modbus PLUS port? DCS don't have any Modbus PLUS Hardware. Request All expert to revert back ASAP.

Regards

 

Modicon modbus RTU ports are configured for baud rate, parity, data bits etc through the Concept programming package. By default 9600,8,E,1. Now these ports are full hardware handshaking so unless you cross (or tie together on one end) RTS/CTS and DSR,DTR and DTD comms will not work.

Your DCS cannot communicate on Modbus PLUS - implossible. Modbus PLUS is a variant of RS485 and is a token passing network.

Now I can't remember for the 113 controllers (it is the case for 434/534 controllers) there are two slide switches on the font panel which changes the communication parameters between what is configured in Concept vs dafult settings of 9600,8,E,1. Check these vs Concept HELP files.

 

Dear Rajesh,

What you observed like "communication", that was something else. Modbus Plus is an RS 485 based network with peer-to-peer communication.

Modbus Port is using RS-232 signaling and different communication stack. Master polls the slaves after each other. In your case the Modbus Port #1 acts as Master port. Your DCS devices as a slave, I think.

Ignore that "communication" signals.

 

Dear Mr. Brad,
I full agree with you that Modbus PLUS should not communicate on RS-232 cable. But we all Foxboro,ABB engineers had seen by issuing query & getting response on Modbus Plus port. We also confused on this behaviour. Request Pl. answer if you have. We don't have word IMPOSSIBLE because there are many cases where Vendors says it is impossible & we had seen same in front of them.

 

Modbus Plus protocol uses RS-485 for its multi-drop feature and is a Master-Slave protocol. It is supported by many manufactures. I have found it standard (or an option) in many medium voltage protection relays and meters. I suspect Foxboro and ABB offer a Modbus Plus interface module. These interfaces will allow engineers to configure tables to specify blocks of boolean/integers/real/word data to & from the PLC.

If your DCS has a serial port then use Modbus protocol to pass data between it and the PLC. It uses RS-232 for a point-to-point serial connection. Connect using an off the shelf serial cable c/w null modem between the DCS serial port and the Quantum CPU534 Comm 1 port or buy a Schneider Modbus communication cable. Set the PLC slider selector switch to RTU and configure communication settings to 9600, even, 8, 1.

Have you considered Modbus Plus over Ethernet (MBE)?

 

The cable connection principle are the same device application will be used cross cable connection while for the different application (ex: Pc to sw) will be used direct connection i.e.straight cable.

tq

 

Crossovers depend upon what the devices are. DTE to DCE (Data Terminal Equipment to Data Communication Equipment) connections are 1:1. EG PC to Modem, PLC to MODEM. However, when connecting DTE to DTE you will likely require a crossover EG PLC to PC or HMI. However, with the wide interpretation of serial port pinouts and lack of standards you will discover expections.

Check the pinout designations and look for connection examples.