Fox Thermal FT3 - Modbus Introduction; Modbus Protocol

Scope
Thank you for purchasing the model FT3 thermal gas mass ow meter from Fox Thermal.
The model FT3 is one of the most technically advanced ow meters in the world. Extensive
engineering effort has been invested to deliver advanced features, accuracy measurement
performance and outstanding reliability.
This document 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).
Modbus Protocol
MODBUS Protocol is an application layer messaging protocol that provides client/sever
communications between devices. MODBUS is a request/reply protocol and offers 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>
i
Note: The data in shown in brackets < > represents one byte of data.
Model FT2A
Model FT2A
FT3 Modbus
4
Modbus Introduction
MODBUS PROTOCOL