I have an old ABB plc that has a single "Modbus over RS-232" port for communication. I want to connect two devices to this PLC. But I read that RS-232 can connect to only one device. I was wondering if I could use a RS-232 to RS-485 interface and connect multiple devices.

 

See bb-electrocics.com for lots of good info on RS232 to RS485 converters. A good choice is their model 485SD9R because it uses RS232 RTS, DTR and TXD for power and most importantly it handles the Key Up and Key Down automatically for all Baud rates. Another good read is the National Semiconductor Application Note AN1057. There are two good things to know about RS485, one is it is a 3 wire system Labeled A, B and GND. The second good thing to learn about is RS485 Fail Safe Biasing. Fail Safe Biasing is the application of bias resistors to guarantee all RS485 receivers are biased OFF when there are no devices transmitting.

 

RS-232 is normally 1 to 1
Device 1 Tx ------> Rx Device 2
Device 1 Rx <------ Tx Device 2
Device 1 Gnd ------ Gnd Device 2
When not in use the Tx pin is held so it's not possible to have 2 Txs at either end.
However it is possible to have more than one listener. I'm not sure if RS-232 supports addressing however it would be possible for all recievers to get the same message. for instance if you downloaded a table of data the individual listener would just pick out the data it wanted.
RS-232 is supposed to have a distance limitation of 50 ft however it can go several hundred as long as the baud rate isn't too high.
Yes, it's also possible to use converters, that gives you the advantage of 2 wire daisy chain configuration.
Roy

 

There is a cheap and dirty method of connecting multiple RS232 slaves to one host using RS232 if you want to go cheap on this. Run the Host RS232 TX to all Slave RXs. Then diode auctioneer each Slave Tx to the Host Rx with a diode (one diode for each slave Tx). Diode auctioneering is the simplest form of an OR gate where any of the Slave Txs can pull the Host Rx UP ONLY, not down. To ensure the Host Rx goes down when not being pulled up, bias the Host Rx to GND (Common) through a bias resistor, any R = 4K7 to 10K0 will do just fine, 1/8 Watt type of thing. Remember in Modbus it is the Slave address that makes each slave unique, nothing to do with the wiring.

 

Good idea Harvard, wish I had thought of that!

Regards
Roy

 

We at Phoenix have several devices that will allow you to create a multipoint network back to a single RS232 port. Options include Rs 485 converters, Fiber Optic converters and RS 422 Converters. All these allow more than a Point to Point network. The question is how will you parse the messages at the ABB controller.

 

Some interesting hardware solutions here, but if you are using an ABB master type controller (MP200, AC400 etc), the answer to your question is probably no.

These controllers poll the PLC using Modbus Function code 7. If the RTU doesn't respond, then the comms link will not function. This is built into the system software, as is the complete network configuration. The controller firmware will not allow you to configure multiple remote RTU addresses on a single serial port.

So while you can connect multiple RTUs on the port, there is no way to address them from the Controller.

Rob
www[.]lymac.co.nz

 

My friend,

What you need is a device that allows you MULTIDROP in RS-232. It is possible.

Checkout the website http://www.hedintex.de/ they offer a solution that can provide a multidrop with RS-232. It is also cheap.

I have used it myself and it works great.

Regards,
Saad.

 

According to the Gould Modbus Protocol Reference Guide PI:MBUS-300 Rev B, a Function Code 7 has a 4 byte poll sequence, they are: Slave address, Function Code 7 and a 2 byte CRC. While there is no data tacked onto a Function Code 7 packet, the start of the message does have a Slave Address. So, my take on this host device is, it does not support true Modbus Protocol, something like "Enron Modbus" maybe?

 

As you mentioned that with one RS232 modbus port you want to add multipal devices then use RS232/RS485 Convertor for the same.You can use DIGI convertor also if the devices you want to connect heaving ethernet port