Here’s another application note from Microchip – Modbus TCP for the Microchip TCP/IP Stack (PDF!)
Modbus® is an industrial Transmission Control Protocol
(TCP) used for remote monitoring and control
applications. This application note describes a basic
Modbus TCP server implementation and how it can be
extended in a Modbus application.
The application software that accompanies this
document was developed using Microchip’s TCP/IP
stack (v5.42) as a base framework. Although the
software was developed using the PIC32MX360F512L
microcontroller, it is easily ported to other PIC
microcontrollers that support TCP/IP functionality