ABB M101 - Table 3 Modbus RTU Mode of Transmission

MODBUS STANDARD INTRODUCTION
10 1TGC908004M0201
Characteristic
RTU (
8
-
bit)
Coding Syste
m
8
-
bit
Number of bits per character:
Start bits
Data bits (least significant first)
Parity
Stop bits
1
8
1 (1
bit set for even or odd parity, no bits for no
parity)
1
or
2
Error Checking
CRC (Cyclical Redundancy Check)
Table 3 Modbus RTU mode of Transmission
2.2.1.5. Error Detection
There are two types of errors, which may occur in a communication system:
• Transmission error and
• Programming or Communication error
The MNS Digital Gateway deals with either type of error as specified in MODBUS specification.
The most frequent cause of communication error is noise, unwanted electrical signals in a
communication channel. These signals occur because of electrical interference from
machinery, damage to the communication channel, impulse noise (spikes), etc. Character
framing, a parity check, and a redundancy check detect these errors. When the error occurs,
the message is unreliable and the processing of the last received erroneous message stops.
Programming or operational errors are those involving illegal data in a message or difficulty in
communicating with a slave. These errors result in an exception response either from Master
or Slave station.