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BB-MESR90x Modbus Gateways
7.
MODBUS HELP
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 "master/client"-) relationship. As 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 multi-drop 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 start 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 that 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. If the slave cannot respond appropriately for some
other reason (e.g. it does not recognize the function code), it will return a message containing
an exception response.
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