RS 232 comms

Hi ,

I have a weigher connected to a PLC via RS 232

It is 2 way communications between the weigher and the plc

I have very little experience in comms

The weigher is a very old Chronos Pc 454 and the plc is a Gem 80
My ultimate aim is to replace the old weigher with a Chronos pc 456

The 456 uses RS 485

I can replace the weigher and use a RS 232 to RS 485 Converter unit

What id like to check first is can I wire the comms cable to the new weigher in one direction , from the plc to the weigher , so I don't risk sending data to the plc
Just incase I cause a problem by sending comms from the new weigher to the plc

I am asking if I can test the new weigher by only connecting the rx side of the incoming plc comms cable to the weigher , just so I can see if the weigher will respond to the incoming comms

As a separate question
To understand comms better , I have bought a null modem cable and want to connect 2 x laptops together using terminal software

I would like to use a terminal program on both laptops and open a terminal window on each laptop and see comms in each direction

Can you recommend software to do this


Would this software allows me to view incoming Rs 232 from the PLC if I connect the laptop to the incoming data cable at the weigher

I am wanting to " view " the comms from the plc to the weigher

Yes to all those queries. I've made temporary connections with paper clips when I didn't have a M-M gender changer. Obviously dealing with the ground may be an issue when connecting send and receive to separate devices

Lots of comms app options, I used to use kermit or hyperterminal, but I expect there are newer apps to do this. Kermit had a scripting language that could be used to execute a dialogue, e.g. use AT commands to dial a modem and connect.

You have the right idea in mind. It's best to test out using PC to simulate either side of the communication.

What's missing from your description is the protocol involved or I missed it. ASCII or Modbus are typical for application like this.

The possibility of sending comms in only 1 direction and the tools to test it with depends on the protocol. A lot of weigher stuff seems to be simple ASCII that can be typed in with a terminal emulation program. Modbus is much more complex. RTU writes cannot be typed in from a terminal as they are 8-bit characters that have to be sent all at the same time. Modbus ASCII is not the same as a simple ASCII interface.

Need more information here.

Many years ago, I used to do this, I built two cables both null & serial (as there are two standards where pins 2 & 3 are direct or crossed over, spliced in two other cables where pins 2 & 5 to one & 3 & 5 to the other, this meant I could connect a piece of equipment to another to allow it to run and use a PC with two ports to monitor both lines, I created my own communication program so I could monitor the traffic both ways, as even ascii often has control (non printable) characters I could swap between ascii & decimal or hex so as to look at the protocol. Not ideal, but in all the years doing this have never damaged a 232 port they are pretty hardy even if connected wrong.

Hi gents ,

I think its basic ascii ( certainly not Modbus )
Its a 30 year old weigher electronic control unit
being replaced by a 15 year old weigher unit

I think I can identify the incoming comms to the weigher
So I'm going to try the weigher powered up , alongside the
Old one and connect just the incoming RS 232 to the weigher

Thanks for the replies

With ASCII, any terminal program can work. Hyperterminal, Putty, etc..