dustin (Customer) asked a question.
I have several Productivity PLCs with Modbus RTU devices connected to the RS485 port. Until now I have been able to configure all of the modbus devices to communicate at 19200 8,N,1. I need to add in a new modbus device that needs to be 8,N,2.
I don't think I can mix 8,N,1 and 8,N,2 on the same RS485 network. What is my best option to get these connected to the PLCs on a separate modbus network? Some of the CPUs are P1-540 and P1-550, I don't see a separate communications card for those. Some of the CPUs are P2-550 and I suppose I could add a P2-SCM module. One PLC is a newer P2-622 and it looks like the the RJ12 RS232 port is configurable for RS485 but I can't find any info in the manual about how to wire that or what adapter to use.
Any advice would be appreciated.
You could try setting the Master devices (PLC) to 8N2 because the second stop bit is just another character count of idle time on the line and most 8N1 devices will just interpret this as the line being inactive before the next query.
The problem of mixing 8N1 and 8N2 could occur in timing between the responses from Slave devices set to 8N1 and Master device (PLC) set to 8N2 since the PLC would expect the 2 stop bits and if another response came in too quickly then the stop bit would look like a 1 instead of 2 and the PLC would discard the response.
However, if the response time between device is long enough for the bus to remain idle, then packet loss may not be a problem as the PLC would interpret that as 2 stop bits. I ran a quick test using Modbus Slave on a PC set to 8N1 and a P1-540 set to 8N2 and everything seemed to function.
Regarding the P2-622, take a look at the product insert on page 4 (P2-622 Insert.) which shows the wiring pinouts for the two configurable serial ports. No adapter needed but you'll have to configure the port in the project and then transfer the project onto the PLC.
As you mentioned on the P2-550 the P2-SCM is one option you could pursue, but since the P1-550/540 do not offer those comm modules you could also look into an FA-ISOCON which allows an RS-232 comm port to get onto an RS-485 network easily by setting a few dip switches if those ports are free on your P1000 PLC's.