User Manual

Commissioning
SmartRadar FlexLine Part No.: 4417.762_Rev07
6 - 8 Service Manual
6.3.3 Commissioning the HCI-GPU - Modbus Protocol
6.3.3.1 Introduction
The Modbus protocol has been developed by Modicon and is used to
establish master-slave/client-server communication between intelligent
devices. Modbus is a de facto open standard and the most widely used
network protocol in the industrial manufacturing environment.
There are different flavors of Modbus. The most common is Modbus
RTU (Remote Terminal Unit) which is based on serial (twisted pair)
communication like RS-485 and RS-232. Honeywell Enraf has
implemented this Modbus RTU protocol in its gauges as an option.
As with RS-485 communication a bus structure can be built, it is
possible to connect a maximum of 32 gauges (as slaves) to one host
(master). Each slave has its unique address.
The host (master) initiates the communication by addressing one of the
slaves in its query. Only the addressed slave (gauge) will respond.
With RS-232 communication there can be only one slave (the gauge)
and one master (the host).
6.3.3.2 Modbus Protocol Description
A Modbus message is placed by the transmitting device into a frame
that has a known beginning and ending point.This allows receiving
devices to begin at the start of the message, read the address portion
and determine which device is addressed, and to know when the
message is completed.
RTU mode is a binary mode of data representation. Messages start with
a silent interval of at least 3.5 character times. This is most easily
implemented as a multiple of character times at the baud rate that is
being used on the network (shown as T1T2T3T4 in next figure).
The first field then transmitted is the device address. The gauges
monitor the bus continuously, including during the silent intervals. When
the first byte (the address byte) is received, each gauge decodes it to
find out if it is the addressed gauge.
Following the last transmitted byte, a similar interval of at least 3.5
character times marks the end of the message. A new message can
begin after this interval.