RS-232 Communication

ceilingwalker

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ceilingwalker
Join Date
Mar 2010
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Phoenix, AZ
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Good day all. I have a Red Lion display that is connected to a SLC50/04 processor via the RS-232 (DB-9) connector. Whilst it was all on my bench it worked great. Now that I have put it in its permanent place the displays are no longer displaying the values as they did on my work bench. The displays and the processor are about 70-80 feet away from each other. I originally thought it was within the 50 foot range however I didn't include the length it needed to creep down the wall. If it is a distance issue, is there something I can do to boost the signal? Thank you.
 
Last edited:
Mickey said:
You might try slowing to baud rate a little.
Click to expand...

I read somewhere online that the specs for RS-232 were 300 baud, stop bits 2, data bits 8, and according to the source it should run over 100'. I'm reluctant to change the channel config for Channel 0 to 300 baud as it is warning me that I will lose communications. Will it be ok to try this and be able to log back onto the processor?
 
ceilingwalker said:
I read somewhere online that the specs for RS-232 were 300 baud, stop bits 2, data bits 8, and according to the source it should run over 100'. I'm reluctant to change the channel config for Channel 0 to 300 baud as it is warning me that I will lose communications. Will it be ok to try this and be able to log back onto the processor?
Click to expand...


You don't have to go that low, I would try 9600.
 
Here is what I found, and I don't pretend to be savvy with communications but this seems odd to me. Looking at the attachment and the way I have it wired; when I disconnect either the "RXD" or the "Sig Comm" traveler wires (the wire's that jump from the first units terminal point 2 or 3)the display works. I can actually have both of the traveler wires disconnected that go to the other displays and the first display will work fine. As soon as I reconnect everything the way it is displayed in my diagram, the first display no longer communicates with the PLC. I ohm'd out the traveler wires and they they are fine. No shorts from terminal to terminal and no opens. I don't understand my results but I bet I wired something stupid wrong, right?

display.jpg
 
Thank you for posting the diagram: it adds a great deal of information.

RS-232 is designed to be a point-to-point serial connection between two devices.

This doesn't keep plenty of vendors from promoting "bending" the rules and recommending multiple devices connected to an RS-232 wire. The fact that your wire run is significantly longer than the recommended two-device length is making it worse.

The diagram you show might work with RS-422 or RS-485.

What exact Red Lion devices are you using ? Are they meant to communicate read-and-write with the SLC controller, or do they just accept one-way data ?
 
RS232 is for connecting BETWEEN TWO UNITS ONLY. Disconnect all of that. Fortunately, if the units meet the specification of RS232 then nothing should have been damaged but you can't have more than two units.
 
On another type unit we've used a daisy-chain RS232 serial. Master Xmit -> Slave1 RCV, internal link to Slave1 Xmit -> Slave2 Rcv, internal to -> Slave 2 Xmit etc

Each slave retransmits the message out its Xmt. If an address field doesn't equal their station they do nothing else. If there is a response required from a specifically addressed slave it tacks on its reply just after retransmitting the originating message. The address of a reply is that of the Master and there is a final link back to the Master from the last slave.

But of course that requires that specific protocol to keep things straight. I had trouble coping with the fact that the Master (in this case a CompactLogix PLC) hearing its own transmission echoed back to it from the last slave.
 
What I don't understand is, I had this all set up on my work bench and it displayed all three units fine, and displayed exactly what it was supposed to. I don't understand what difference it made installing it.
 

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