Fairbanks X SERIES PR 5410 Process Indicator User Manual

Page 112

Section 4: Commissioning

10/08 112

51207 Revision 1

4.12 MODBUS

J-BUS

ROTOCOL

4.12.1 Communication

The MODBUS/J-BUS protocol implemented in the instrument permits fast, simple and reliable
communication between a PC or a PLC and up to 127 instruments. PR 5410 fully supports Modbus-
RTU including functions 1, 2, 3, 4, 5, 6, 8 (sub-function 0), 15 and 16.
J-bus is a French 'clone' of the Modbus. There is a small difference: J-bus addresses count from 0
(instead of 1) to hex FFFF (instead of dec. 9999). Some Modbus masters automatically subtract 1
before sending a message, and some Modbus slaves subtract 1 to get the requested address. Thus
it may happen that access to an address shifted by 1 is made; this is the only point which must be
taken into account. In everyday practice, no other problems when connecting J-bus and Modbus
instruments should appear.
Binary data from and to the SPM of PR 5410 are transmitted using this protocol. Any data exchange
includes two telegrams: a command from the PC to PR 5410 and a reply from PR 5410 to the PC.

Command

PR
5410

Each telegram consists of four blocks:

1. Device address

2. Functional code

3. Data (addresses and values)

4. Control word (CRC)

Note: All 2-byte values (16-bit values/word) have the Motorola notation. Consequence: MSB - LSB

A reply to every correct command is sent. At 9600 bauds, the reply time is typical 4 ms and maximal
8 ms.
A faulty command received by PR 5410 (e.g., parity error in the data, or CRC error) is ignored and
no reply is sent.
The pauses between the individual characters and a command must not exceed the 3.5-fold value
of a character length; otherwise PR 5410 detects a premature end of the command.
If the received command is correct but cannot be executed nevertheless (e.g. due to a faulty
address or faulty data), reply is with an error telegram.

A telegram sent to Slave 0 is executed by all PR 5410 units, but not replied by anyone.