rs 485

Is it possible to connect DCS to PLC via Rs 485 by using modbus protopcal
srik

srik said:

Is it possible to connect DCS to PLC via Rs 485 by using modbus protopcal
srik

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In theory it is, however few actually suport modbus RS485.
Most modbus is RS232. Modbus RS 485 seemed to come along about the same time as Modbus Ethernet, which is probably the better protocol.

So far, the only PLC I've seen Modbus 485 on is some Modicon Momentums. You may be able to use an RS232 to 485 converter set to 9600bps.

Note, Modbus RS485 is different than Modbus Plus. Even though both are RS485 based, and both are based on the original Modbus protocol, they are not compatible.

Generally YES! But you should check the followings:

1. Both DCS and PLC support Modbus protocol.

2. Both DCS and PLC support RS485 as physical layer of this Modbus.



Modbus is just a protocol which it's physical layer can be both RS232 and RS485.

I have seen modbus plus used for plc to pc communication and modbus used for plc to plc communication for the same application

Most PLCs these days support Modbus RTU somewhere along the line. I believe all Schneider PLCs support it. Siemens do but you do not want to know about the price. The new AB Micrologix 1100 has Modbus built in. Very easy with Omron PLCs - the latest ones use CX-Protocol and a standard serial port card that can have 2 x RS232 or 1 x RS232 and 1 x RS422/485 port. I have just set a PLC up with Modbus RTU master RS232 to a Lonworks/Modbus RTU slave converter and the other as an RS422 slave to a BMS system. The new Omron CP1 shoebox that is just about due has Modbus built in.

One can go on about many other brands that support, particularly, Modbus RTU. As far as I am aware it is still the most popular industrial network to a variety of devices in use today.
It is also very easy and convenient to use an RS232 port with an RS232/422/485 converter. I have been using the Adam 4520 for this purpose for years. It is 24VDC powered and is also isolated.

Mr bob
Actually I want to use the rs 485 for energy meters and PLCs to communicate with main DCS system.Is it required to seperate the energy meters and PLCs or can continue in same network?

If the PLC is setup to be a Modbus RTU slave, it can exist on the same RS485 network as the energy meters. A Modbus RTU slave is a Modbus RTU slave no matter what the device is.

Thanks mr bob
Now it's clear