// SimpleModbusSlaveV9a for Arduino based on https://code.google.com/p/simple-modbus/ // -- Adding function 06 ( Preset Single Register) by Iyus

/* SimpleModbusSlave allows you to communicate to any slave using the Modbus RTU protocol.

This implementation DOES NOT fully comply with the Modbus specifications.

Specifically the frame time out have not been implemented according to Modbus standards. The code does however combine the check for inter character time out and frame time out by incorporating a maximum time out allowable when reading from the message stream.

SimpleModbusSlave implements an unsigned int return value on a call to modbus_update(). This value is the total error count since the slave started. It's useful for fault finding.

This code is for a Modbus slave implementing functions 3,6 and 16 function 3: Reads the binary contents of holding registers (4X references) function 6 : Preset single register function 16: Presets values into a sequence of holding registers (4X references)

All the functions share the same register array.

Note:
The Arduino serial ring buffer changed in V1.05 it is now 64 bytes or 32 unsigned int registers. Most of the time you will connect the arduino to a master via serial using a MAX485 or similar.

In a function 3 request the master will attempt to read from your slave and since 5 bytes is already used for ID, FUNCTION, NO OF BYTES and two BYTES CRC the master can only request 58 bytes or 29 registers.

In a function 16 request the master will attempt to write to your slave and since a 9 bytes is already used for ID, FUNCTION, ADDRESS, NO OF REGISTERS, NO OF BYTES and two BYTES CRC the master can only write 54 bytes or 27 registers.

Using a USB to Serial converter the maximum bytes you can send is limited to its internal buffer which differs between manufactures.

The functions included here have been derived from the Modbus Specifications and Implementation Guides

http://www.modbus.org/docs/Modbus_over_serial_line_V1_02.pdf http://www.modbus.org/docs/Modbus_Application_Protocol_V1_1b.pdf http://www.modbus.org/docs/PI_MBUS_300.pdf

*/