02/08/2012 Uploaded version 1.00.  In this demo the master has been set-up to request the following I/O from a single slave:

Discrete outputs (Coils) x 8 starting at 1
Discrete inputs (Status Inputs) x 8 starting at 1
Holding Registers (16 bit analogue registers) x 2 starting at 1
Input Registers (16 bit analogue input registers) x 1 starting at 1

Supported functions:
01 Read coils
02 Read status inputs
03 Read holding registers
04 Read input registers
05 Force single coil
06 Preset single register
15 Force multiple coils
16 Preset multiple registers

To simulate the write operations, PORT D has been configured as input with buttons 1 to 4 performing the following:

Button 1, writes to a single coil to address 1 (Function 05)
Button 2, writes to 7 coils, starting at address 1 (Function 15), the value written is b111101
Button 3, writes to a single holding register to address 1 (Function 06), value 0xcaca
Button 4, writes to 2 holding registers, starting at address 1 (Function 16), values 0x04d2 and 0x162e.

Summary of usage:
RAM used 1480 bytes 97.7%
ROM used 7454 bytes 22.7%

For Modbus Slave embedded implementation:

http://www.libstock.com/projects/view/355/modbus-slave