PLC Serial Communication

Hi,

I have an application which requires a PLC to communicate with four balances via RS232 and at the moment I am planning on using a CompactLogix L32E and two 1760-ASCII modules to achieve this. The balance weights needs to be read constantly with tare and calibrate commands being sent as required. Having not communicated with more than one serial device at a time I was wondering if I would encounter any problems with this setup such as being able to receive data from all four balances at once or if anyone else has successfully used an alternative setup to communicate with multiple serial devices. Any advice is greatly appreciated.

I do not believe a RS232 serial connection will do what you want.

Look at RS485 or other "Mult-drop" connection schemes.

RS 232 to 485 converting adaptors are not expensive.

Sorry, I don't have experience with the details. Check in with the maker of your scales, They might be a good reference.

Cheers

As another possibility, Real Time Automation makes an ASCII-to-Ethernet/IP Gateway that I have found to be very easy to setup and use. All the incoming and outgoing data are tied directly to tags that are monitored by the unit. Pretty slick.

435NBX

Do you mean 1769-ASCII

It looks like this might work with each I/O module connected to 2 balances. I have not used this but the critical questions will be how fast the data changes vs how long the program scan and I/O card scans are and if the processor supports using two of thee cards at a time.

Other than that you will need to brush up on ASCII codes and string parsing in RSLogix.

We use the 1769-ASCII to communicate with several RS232 devices per line at our plant. I believe the cards have 2 channels each, so i would just buy 2 cards have 4 identical programs running to get the information you need. Once you get one working like you want, you should be able to have the other 3 running in about 5 minutes.

Thanks for all the suggestions, I did mean 1769-ASCII. My main concern is being able to communicate with the four balances at once, I have successfully been able to communicate with one balance in the past using the PLC’s built-in RS232 port.

Given Helliana’s use of a similar setup and that the 1769-ASCII module has a simultaneous mode to communicate using both ports on the module at once I think I will go ahead with the setup. I will also look deeper into gateways, the 435NBX certainly looks simple to setup and use. I’d be keen to hear from anyone else that has had experience with using the 1769-ASCII modules.