Glossary of MODBUS terms
MODBUS, like many other networking systems, has a set of unique terminology. Table below contains a
few of the technical terms used in this guide to describe the MODBUS interface. They are listed in
alphabetical order.
Address field
It contains the Slave address.
Application Process
The Application Process is the task on the Application Layer.
Application protocol
MODBUS is an application protocol or messaging structure
that defines rules for organizing and interpreting data
independent of the data transmission medium.
ASCII transmission mode
When devices are setup to communicate on a MODBUS serial
line using ASCII (American Standard Code for Information
Interchange) mode, each 8–bit byte in a message is sent as
two ASCII characters. This mode is used when the physical
communication link or the capabilities of the device does not
allow the conformance with RTU mode requirements
regarding timers management.
Bus
A bus is a communication medium connecting several nodes.
Data can be transferred via serial or parallel circuits, that is, via
electrical conductors or fibre optic.
Client
A Client is any network device that sends data requests to
servers.
MODBUS follows the Client/Server model. MODBUS Masters
are referred to as Clients, while MODBUS Slaves are Servers.
Cyclic Redundancy Check
(CRC)
Error-checking technique in which the frame recipient
calculates a remainder by dividing frame contents by a prime
binary divisor and compares the calculated remainder to a
value stored in the frame by the sending node.
Data encoding
MODBUS uses a ‘big-Endian’ representation for addresses and
data items. This means that when a numerical quantity larger
than a single byte is transmitted, the most significant byte is
sent first.
Exception code
Code to be returned by Slaves in the event of problems. All
exceptions are signalled by adding 0x80 to the function code
of the request.
Exception response
MODBUS operates according to the common client/server
(Master/Slave) model: the Client (Master) sends a request
telegram (service request) to the Server (Slave), and the Server
replies with a response telegram. If the Server cannot process
a request, it will instead return a error function code
(exception response) that is the original function code plus
80H (i.e. with its most significant bit set to 1).
Function code
MODBUS is a request/reply protocol and offers services
