We will now communicate Modbus Serial RS485 from the Horner Master controller to the Solo process temperature controller. The Horner APG XL4 all in one controller has several different ports for communications. We have both serial and Ethernet-type ports that can be used for many different protocols. These protocols can be downloaded from the Horner APG website and transferred into the controller. We will be looking at the Modbus RTU protocol. Using the serial RS485 port on the XL4 we will communicate to a Solo process temperature controller. We will read the process value (PV) into the Horner APG XL4 controller and write the set value (SV) back into the process temperature controller. This is all accomplished without any ladder logic in the PLC. The Horner APG XL4 is one of the easiest networks to set up. Continue Reading!