ModBus Protocol
PR 5500 Instrument Manual
EN-420 Sartorius
7 ModBus Protocol
7.1 General Description
The J-Bus/ModBus protocol implemented in the device permits fast, simple and reliable communication between
a PC or a PLC.
Device fully supports
-
ModBus-RTU (via a serial interface), see Chapter 7.2
-
ModBus-TCP (via a network interface), see Chapter 7.3
-
ModBus-UDP (via a network interface), see Chapter 7.3
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. Therefore the address access may shift by
1; this is the only point which must be taken into account. In everyday practice, no other problems when
connecting J-Bus and ModBus devices should appear.
Binary data from and to the SPM of device are transmitted using this protocol. Any data exchange includes two
telegrams: a command from the PC to device and a reply from device to the PC.
Command
Command
PC
Reply
Reply
Device
Note
All 2-byte values (16-bit values/word) have the Motorola notation. Consequence: MSB - LSB
A reply is sent to each error-free command.
If the received command is correct but cannot be executed nevertheless (e.g. due to a faulty address or faulty
data), an error telegram is the reply.
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