The PLC talks over ModBus to Mach, it means you just have one cable from the PLC to the computers serial port and all communication goes via that. I use the DL06 PLC from Automation Direct. I use the D0-06DR model which has relay outputs.
Hood
I just sold off the DL06 that was in my machine. Replaced it with an Automation Direct DoMore BRX plc. Programming software is free. The BRX models go up to 36 digital I/O, and 4 analog inputs, 2 analog outputs and 8 of the inputs are highspeed to 250Khz, 8 of the outputs are highspeed to 250Khz, Built in Ethernet and serial, plus a POM module that can add another ethernet port, or serial, or Profibus. Add on expansion modules up the wazoo, no rack limitations. Has an SD card slot for backup or data storage, and can send emails and text messages. Way better programming techniques and software, and the Autotune PID loops will blow your socks off!
We are so impressed that we are switching our whole product line over from another AD PLC and it is saving us $1200 in hardware costs alone!