RS232 display

I work on 2 specific machines that use RS-232 comm's in a loop to multiple items. Definitely not RS-485 as I test them when needed.


One is an electroplating system that has 8 rectifier power supplies and 4 pulse modulator controls wired in a single serial loop. Each item has a specific RS-232 address the HMI/SCADA sends commands to set with a bank of DIP switches.


The other is actually a group of high resolution printers that have 10 servo controlled motors and 4 IR cameras all controlled through a single RS-232 cable. Each servo control and camera has a unique address from 16384 to 32767 programmed from the factory that has to be changed in the program if replaced.

What we have to be completely clear on is that RS-232 is an electrical standard only. It does not specify any standard for the communications protocol, or language, used on the wires.



If RS-232 is used to communicate to multiple devices then it must be using a proprietory communications protocol that is manufacturer specific.

If you buy another display it should come with some documentation. Is it just one display you're using or multiple. It depends on what you need to display, some manufacturers do bespoke to order.

Mr. Ro Bot said:

The Siemens PLC is connected to the display with a PPI Multi Master Cable that's converting it to RS232.

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From this, I guess that your PLC is an S7-200, and the cable you mention is a Siemens serial MPI/PPI PC Adapter.
This adapter has a serial RS232 port on the side that connects to the display, and an RS485 port that connects to the PLC, hence the two wires on the PLC side.
As far as I remember the RS232 side on the PPI/MPI adapter was default 19200 baud, and then a newer version had 19200/38400 selectable by a switch.
I am guessing that indeed the display must be setup with the serial settings that matches the PPI/MPI adapters settings.

You could take a lot of the guesswork out by specifying the exact type no's of all the devices in question.

So, for the 3rd time, please state the type no's of all the devices.

Mr. Ro Bot said:

This one I bought from NEWCAL (the display manufacturer) instead (savings of $500).

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And there is the exact reason why the OEM charges $500 more than the manufacturer of the display, they do this work for you and carry the warranty...

We used to get this all the time with temperature controllers, we would sell them out at £1000 configured, somebody looks at the list of £800 online an thinks they are being ripped off.

Aabeck said:

I work on 2 specific machines that use RS-232 comm's in a loop to multiple items. Definitely not RS-485 as I test them when needed.


One is an electroplating system that has 8 rectifier power supplies and 4 pulse modulator controls wired in a single serial loop. Each item has a specific RS-232 address the HMI/SCADA sends commands to set with a bank of DIP switches.


The other is actually a group of high resolution printers that have 10 servo controlled motors and 4 IR cameras all controlled through a single RS-232 cable. Each servo control and camera has a unique address from 16384 to 32767 programmed from the factory that has to be changed in the program if replaced.

Click to expand...

If it's truly RS232 then having multiple slave devices on one communications cable won't work. RS232 uses transmit and receive lines with the transmit of the master going to the receive of the slave and vise versa. When either the master or slave is not transmitting, it's transmit line is being held low (-3 to -15V) and when it transmits it pulses the line high (3 to 15V). If you had more than one slave on the same transmit line, when one of them would transmit the transmitter line of the other would squelch it's signal. The only way to get multiple slaves on a multidrop network is to isolate them all from each other so that one can transmit while the others are not. This is typically done through modems but there are some RS232 splitters out there that will allow you do to it but you can't do it with cable alone.

If the two RX/TX leds are not blinking then there is no data being sent so I think you have a problem with the communications on the MPI adapter or the PLC, I assume this was working before with the original display then only suggestion that if the original display went down then it may have damaged the adapter.
One thought is that the MPI/232 adapter surely will require power, this is often supplied from the 232 port of the PC or PLC, also the display may well be a non Null 232 so TX/RX could be swapped (if it worked before it's unlikely).
Do you know what commands are sent to the display i.e. connect a pc & use a terminal emulator to see if there is any data being sent.

parky said:

One thought is that the MPI/232 adapter surely will require power, this is often supplied from the 232 port of the PC or PLC, also the display may well be a non Null 232 so TX/RX could be swapped (if it worked before it's unlikely).

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Parky, The op is using a PPI (S7 200) adapter, not MPI, I asked for a part number but I was ignored, the only thing they wrote is was a Multimaster cable...

Sorry my mistake, however If the TX/RX leds are not working it still begs the question is there any data being sent? So ignoring the type of adapter it seems to me that it may not be the display, I have had a quick look at the website of the supplier and quite frankly there's no real info. With a supplier like this I guess there is little support from them. There's nothing in their downloads section, no docs, when you select displays it takes you to a simple page with pics of about 4 displays with no info.

I have started to write this post, and then deleted what I have written, several times. Too many expletives.
I can only say, that I have added the op to my ignore list.

I can see your frustration, If people can think they can just sign up & expect us to do the job for them then they are mistaken, I will help anyone within limits if they are genuine as I'm sure many others here will, But they must give us as much info as possible as we can only go on what info they supply, we are not there, cannot see the problem & can only base our replies on what info we are given or try to decipher from the posts.

Firejo,

They are true RS-232. When testing I connect them to a standard serial port, not 232 - 485 adapter. It is a daisy chain of cables going from one unit to the next after the first.

There are instructions on how to have other programs, like a HMI or SCADA, control them also by serial driver.

When an RS232 port is idle, its transmit line voltage is being held to its negative state (-3 to -15V). When it starts to transmit it switches to a positive state (3 to 15V) and then starts cycling between high and low to create "space" and "mark" or "1's and 0's. If you connect two or more transmit lines together, when they are all idle they are all being held low but when one starts to transmit it needs to switch to high but can't because the other are still being held low. It equates to trying to put 12VDC onto a line that is grounded. The +12VDC will go to ground not raise the ground to 12VDC. The only way to "daisy chain" RS232 is to isolate the transmit lines from each other but in doing that you dramatically reduce the signal levels and thus range.
Having said that, with a serial port that is part of the PC's hardware (not a USB to Serial connection) it is possible to do something other than Asynchronous RS232 on the port. Rockwell used to do that with the PIC driver.

I think this post is dead, there has been no response recently from the original poster so he has sorted the problem without notifying us or maybe he finds us overpowering lol