I have been asked to investigate using a 1769-L23-EQBFC1B as part of a control system where it needs to communicate with an off brand PLC that we have used for years. I would really rather not install a DF1 driver in the other PLC (it eats a lot of memory I would rather not use), but setting up ASCII strings isn't as efficient as I would like either.

I would like to set up a scheme where the Compact Logix sends a simple frame to the other controller like <Address><Value><AddChk>. The other controller could then echo or respond with an <ACK> or <NAK>. The other controller could send a similar frame back when appropriate (alarm detected, etc.). For example, <180><25><205> to set address 180 to a value of 25. The address field would relate to the off brand PLC address.

The RS232 Port on the off brand PLC is limited to 9600 baud, so I would like to make the framing as succinct as possible while still handshaking.

I have looked at the Compact Logix manuals and they seem to dictate ASCII or MSG instructions, which I believe use the DF1 framing. I just want to send 3 bytes out of the serial port that all have the possibility of being from 0-255. Likewise, I would like to be able to read a 3 byte frame received at the RS232 port. If I use the ASCII read and the value lies outside 0-127 will that cause an error?

Has anybody tried to do something similar? Are there any obstacles I need to avoid?

Thanks for any suggestions.

Ian