I am trying to implement a Modbus Slave on a RCM6760. Is there a board specific Modbus library for the RCM6760 already written or do I need to just modify one that already exists?
Modbus master and slave libraries and samples exist for both Ethernet (Modbus TCP) and serial (Modbus RTU, Modbus ASCII) protocols, but as you’ve noted not specifically for any Rabbit 4000+ board. Porting the Rabbit 2000, 3000 Modbus example code to Rabbit 4000+ is expected to be quite straight forward, given some familiarity with Rabbit and with Modbus protocols.
You can get the Dynamic C 9.62 (for Rabbit 2000 + 3000 products) at the following link under development software and sample code: