B&B Electronics MESR321_SL_ML - Manual - Manual (Page 54)

Modbus Help Vlinx MESR321 Modbus Gateway
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Modbus ASCII/RTU Basics
The Modbus protocol emerged in the mid-1970s as an early protocol for linking
terminals with Modicon PLCs using a master/slave (sometimes called a
master/client) relationship. A simple, open, message-based protocol, it caught on
quickly and became a de facto standard in the industry. It supports asynchronous
point-to-point and multidrop communications and can be used with a variety of
serial interfaces (RS-232, RS-422, RS-485, modems, etc).
The original Modbus specification included two possible transmission modes:
ASCII and RTU. Modbus RTU mode is the most common implementation, using
binary coding and CRC error-checking. Modbus ASCII messages, though
somewhat more readable because they use ASCII characters, is less efficient
and uses less effective LRC error checking. ASCII mode uses ASCII characters
to begin and end messages whereas RTU uses time gaps (3.5 character times)
of silence for framing. The two modes are incompatible so a device configured
for ASCII mode cannot communicate with one using RTU.
All Modbus communications are initiated by Modbus masters using a polling
query/response format. The master can send broadcast messages (using a slave
address of 0), which all slaves accept, but do not reply to. More commonly the
master polls individual slaves sequentially. In each poll it sends a message
containing a device address, followed by a function code, any data that maybe
required, and an error check field. The addressed slave responds with a similar
message structure. Typically it repeats back its address and the function code,
and then sends a field indicating the number of bytes of data it is sending,
followed by the data and the error check field.
Slave addresses can range from 1 to 247. Function codes include several
common ones typically used in all applications, and additional ones that may be
implemented in specific cases. Common function codes include: Read Coil
Status (01), Read Input Status (02), Read Holding Registers (03) and Read Input
Registers (04).
When a master sends a message to a slave it expects to receive a valid
response within certain length of time. If the slave does not receive the message,
or if the slave receives the message but an error is detected, it does not respond.