Getting proprietary serial into a CompactLogix PLC

darkspark

Member
Join Date
May 2017
Location
Brisbane
Posts
5
Hello everyone,

I am trying up upgrade some proprietary hardware to PLC controlled so we can have some better control over it, and support for the future.

Currently there is a serial communications that happens between 2 pieces of hardware, the data doesn't look to be ascii based, there is a start pulse then a 32 bits of data and an end pulse.

Is there a way to read this into a CompactLogix? I don't think I will have the speed to read an input directly and collect the data.

Thanks in advance.
 
Welcome to the forum !

A serial protocol like that is... unusual. It's certainly not a normal RS-232 UART.

Do you know what physical media it's on ? Are the start and end pulses on the same pins as the data or are they on handshaking pins ?

If you can identify the serial protocol and method there may be a commercially available converter to give you RS-232. Or (shudder) this might be one of those cases where a microcontroller like an Arduino is the right solution.
 
@AutoMax: They are some pieces of equipment from glass container inspection machines.

@Ken Roach: Thanks for the welcome. Its funny you mentioned Arduino, its my hobby and I was talking to my colleagues about how easy this would be to implement with one of them, but unfortunately I won't be able to sell the company on a custom solution as there aren't enough people in my trade (Electrician) who are competent on PLC's let alone microcontrollers.

I have my logic analyzer at work, but I haven't had time to get some data with it on this system. From what I can infer from the conversations I have had with my colleagues, its either rs232 or rs422. I haven't had a chance to look at the drawings to see the wiring.

I will get back to you when I have some more information.
 
Ok, so I had a chance to put the logic analyzer on this comms. Getting a closer look I can see that it is RS422, but it seems to be at 1200 baud, which is a lot slower that I assumed, it also doesn't use a stop bit. It seems to transmit 4 bytes at a time, with each bit representing the states of the system. So I am fairly confident that I will be able to read this with an ASCII card, I found an old BASIC card in a SLC5/04 PLC, but I cant take that PLC offline to rescue that card for a while, so Ill post another update when I can try and read it with the PLC.
 
Most UARTS are configurable for between 5 and 9 data bits (7 and 8 are most common), but your initial description makes it sound like there would are 32 data bits.

I've never seen a serial link with no stop bits. I suppose it's theoretically possible, but I don't think you could set up an SLC or Logix family serial module to work that way because they all use common UART chips.

Is it possible you're seeing three bytes, framed as 11-bit serial ? That would explain what looks like a Start bit with 32 bits of data.

I don't think there are any SLC or Logix family serial interfaces with the hardware that would let you do a bit-banging serial interface.
 

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