PLC with Raspi

[advmin]

Hello all,

I am here to get your opinions and suggestions on the project I am working on now.
PLC : MICROLOGIX 1500 LRP

I use a barcode scanner (ZEBRA DS 457) to read the barcodes on trays of different sizes.
A Raspberry Pi with 8-relay board is used.
The barcode scanner is connected to the Raspi via USB and it is programmed in such a way that the decimal digits on the barcode are converted to 8-bit binaries and powered on the relays accordingly (1 for ON and 0 for OFF).
So by using the relays, programs corresponding to different trays can be selected in Rslogix 500.

I think there would be better ways than this for this process.
Please share your views and opinions on this. (Please also share the drawbacks of this setup,so that i can rectify it in future).

cheers

 

[drbitboy]

1500 LRP has Ethernet and RS-232, use one of those instead?

 

[Old Man]

If this is already working, what is your concern and why do you think it should be changed?

 

[JesperMP]

If this is already working, what is your concern and why do you think it should be changed?

You don't think that the existing solution is a cludge ?
I think that advmin asks a very relevant question, if there could be a better way.

(I dare not mention that the Micrologix 1500 is totally obsolete and should not be used for new developments).

 

[alan_505]

1500 LRP has Ethernet and RS-232, use one of those instead?

The ML1500 does not have ethernet only DF1 or DH-485.
You would need to use a net-eni to get it to talk to ethernet.

 

[drbitboy]

The ML1500 does not have ethernet only DF1 or RS-485.
You would need to use a net-eni to get it to talk to ethernet.

Whoops, nice catch, thanks.

 

[JesperMP]

The ML1500 does not have ethernet only DF1 or RS-485.
You would need to use a net-eni to get it to talk to ethernet.

With "RS-485", you mean DH485 ?
I think the obvious better way is to connect directly with the scanner via serial ASCII. This may mean you have to think about changing the PLC and maybe also the scanner.
Changing the PLC should be under consideration anyway, if I may say so.

The Micrologix 1500 is totally obsolete and should not be used for new developments

 

[Ken Roach]

What sort of software are you using on the Raspberry Pi ?

The MicroLogix 1500 does support Modbus RTU Slave, so if your software has a serial library that includes that protocol, you might consider replacing the hardwired interface with a serial interface. That could allow you to send more data, might be easier to troubleshoot, and will free up discrete inputs on that small controller.

The "LRP" version of the MicroLogix 1500 should support Modbus on the second serial port as well as the round DIN8 Channel 0 port.

 

[Old Man]

You don't think that the existing solution is a cludge ?
I think that advmin asks a very relevant question, if there could be a better way.

(I dare not mention that the Micrologix 1500 is totally obsolete and should not be used for new developments).

I think you are Correct! I just wanted a better understanding of what he/she thinks the problem is and why he/she might be looking for a better solution.

WOW, can't even ask questions anymore, what a shame, such a good site (use to be) going down the tube.

This is exactly why I very seldom post/reply to anything on this site anymore.

 

[JordanCClark]

Would one of RTA's modules help?

https://www.rtautomation.com/product/460mrsa/

 

[JesperMP]

WOW, can't even ask questions anymore, what a shame, such a good site (use to be) going down the tube.

This is exactly why I very seldom post/reply to anything on this site anymore.

?
I haven't seen any posts in this thread that weren't relevant including yours and mine. And I haven't seen any post that had a negative tone.
I think you are reading something into my comment that isn't there.

My comment was a real question.
Don't you think that reading in the scanner values via a raspberry Pi, outputting to relays, reading in via digital inputs, interpret the inputs as a binary value is something of a cludge ?

 

[JeremyM]

I second Ken Roach's software question and defer to his expertise.

If the Pi is driving field IO, I would recommend nothing less than a battle-tested RTOS for the Pi.

 

[Peter Nachtwey]

I agree with JesperMP except I think all the solutions proposed so far are kludges.
This thread has already used more engineer time than the project is worth. Just buy something that will do the whole project with a minimal of effort. I find it hard to believe there aren't more modern and small PLCs that can't read bar codes and handle a few I/O.

 

[Ken Roach]

I figure if you've got a MicroLogix running the process, and you don't need *more* functionality, and you don't need to build more machines, then a question about improving the interface is sensible.

Once upon a time in the mid 1990s I built an interface between MicroLogix 1000 and a PLC-5/40 with 120V AC I/O points. I proudly showed my boss how a double-length clock pulse started the data frame and how I was able to communicate all the preset values to the MicroLogix, which ran a tiny motorized subsystem, in just a couple of seconds.

He went and got his copy of The TTL Cookbook.

"You've invented the world's slowest shift register."


I think in this case the challenge is getting a device to be a USB host for an HID device: the barcode scanner almost certainly emulates a keyboard. Among modern PLCs that lack a Linux or Windows OS, that's still a rare feature. I would probably go with a Wago PFC or CC100 if I was going to recontrol this sort of system.

But if I was merely asked to improve the reliability or bandwidth of the connection between the existing Raspberry Pi and the MicroLogix 1500, then RS-232 Modbus RTU protocol would be my method.

 

[advmin]

Thank you guys for sharing your views and really sorry for the late response.

What sort of software are you using on the Raspberry Pi ?

In Raspi, I use PYTHON programming through "THONNY" software.

The "LRP" version of the MicroLogix 1500 should support Modbus on the second serial port as well as the round DIN8 Channel 0 port

Thanks, Ken for your suggestion. I am using channel 0 for external access(laptop) and channel 1 for the HMI.
Could you please guide me on using Modbus in this setup?

cheers

cheers