Fox Thermal FT3 - Communications; Modbus Protocol

Scope - Modbus Communications
This portion of the manual describes the Modbus implementation using RS485 serial communication
physical layer for the Fox Thermal FT3 mass ow meter based on the Modicon Modbus Protocol (PI-
MBUS-300 Rev. J). A Modbus-enabled FT3 includes "MB" in the model number.
Modbus Protocol
Modbus Protocol is an application layer messaging protocol that provides client/sever
communications between devices. Modbus is a request/reply protocol and oers services specied by
function codes.
The size of the Modbus Protocol Data Unit is limited by the size constraint inherited from the rst
Modbus implementation on Serial Line network (max. RS485 Application Data Unit = 256 bytes).
Therefore, Modbus PDU for serial line communication = 256 – Server address (1 byte) – CRC (2 bytes)
= 253 bytes.
RS485 ADU = 253 + Server address (1 byte) + CRC (2 bytes) = 256 bytes.
For more information on Modbus go to the web site http://www.modbus.org/.
Command Request:
<Meter Address> <Function code> <Register start address high> <Register start address low>
<Register count high> <Register count low> <CRC high> <CRC low>
Command Response:
<Meter Address> <Function code> <Data byte count> <Data register high> <Data register
low> ... <Data register high> <Data register low> <CRC high> <CRC low>
NOTE! The data shown in brackets < > represents one byte of data.
Modbus Indicators
Orange LED indicator LP2 blinks when Modbus signals are received and yellow LP1 blinks when
Modbus signals are transmitted. The LEDs are located on the Modbus communication board next to
the wiring connections.
Modbus Function Codes Supported by the FT3
The FT3 supports the following commands:
1) Command 03: Read multiple registers
2) Command 04: Read single register.
3) Command 06: Write single register
Communications | 90
Model FT3
Communications