Thanks for the information the project looks great
I have tried to compile your code and the original SImpleModbusSlave example and both give the same error
SimpleModbusSlaveExample.ino: In function 'void setup()':
SimpleModbusSlaveExample:85: error: 'modbus_configure' was not declared in this scope
SimpleModbusSlaveExample.ino: In function 'void loop()':
SimpleModbusSlaveExample:95: error: 'modbus_update' was not declared in this scope
There is some information that this is related to the version of Arduino and I ahve compiled in 1.01 and the error is the same
Any ideas?