Good Resources
http://www.rtaautomation.com/modbusrtu/
http://www.rtaautomation.com/modbustcp/
Serial Modbus
Serial MODBUS connections can be ASCII and RTU (binary coded).
Modbus RTU:
Messages are a smaller size and therefore faster,
Uses timing of bus silence
Modbus ASCII
messages are easily human readable.
All nodes within one MODBUS network must use the same transmission mode.
UART Setup
No specific baud rate is specified by the MODBUS: typical baud rates are 9600 or 19200.
Modbus RTU via serial port
Standard MODBUS RTU node addresses are 1-254. 0 is reserved for broadcast messages.
Only the master node may initiate a command
Modbus TCP via Ethernet
Any device can send out a Modbus command, although usually only one master device does so.