Table of Contents
Advertisement
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. This simple, open, message-based protocol 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, are less efficient and
use 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 a device 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 may be 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 common ones typically used in all applications
and others that may be implemented in specific cases. Common function codes: 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.
HINTS AND TIPS
A few simple suggestions that may assist you if your system is experiencing problems include:
•
Slowing down the polling rate may be helpful if power cycling does not cure the problem.
•
A common misperception is that every serial network must terminate with a resistor. While
this was true of early serial network configurations, it is typically the wrong answer.
Modbus Gateway – BB-MESP211T
52
Advertisement
Table of Contents
