Amtech Axpert-Eazy Series Instruction Manual page 192

requested data to the master, or by taking the action requested in the query. Typical master devices
include host processors and programming panels. Typical slaves include programmable controllers.
The master can address individual slaves, or can initiate a broadcast message to all slaves. Slaves
return a message (called a 'response') to queries that are addressed to them individually. Responses
are not returned to broadcast queries from the master.
The Query–Response Cycle
The Query: The function code in the query tells the addressed slave device what kind of action to
perform. The data bytes contain any additional information that the slave will need to perform the
function. For example, function code 03 will query the slave to read holding registers and respond with
their contents. The data field must contain the information telling the slave which register to start at
and how many registers to read. The error check field provides a method for the slave to validate the
integrity of the message contents.
The Response: If the slave makes a normal response, the function code in the response is an echo
of the function code in the query. The data bytes contain the data collected by the slave, such as
register values or status. If an error occurs, the function code is modified to indicate that the response
is an error response, and the data bytes contain a code that describes the error. The error check field
allows the master to confirm that the message contents are valid.
The Two Serial Transmission Modes
Standard Modbus networks using either of two transmission modes:
10
1. ASCII
2. RTU (Remote Terminal Unit)
This Device supports only the RTU mode. Only the RTU mode is described in this document.
11
Both the master and slave must be configured in RTU Mode.
2 RTU Mode
When controllers are setup to communicate on a Modbus network using RTU (Remote Terminal Unit)
mode, each 8–bit byte in a message contains two 4–bit hexadecimal characters. The main advantage
of this mode is that its greater character density allows better data throughput than ASCII for the same
baud rate. Each message must be transmitted in a continuous stream.
The format for each byte in RTU mode is:
Coding System:
Amtech
11.4
Device Address
Function Code
Query
from
Eight Bit
Master
Data Bytes
Error check
8–bit binary, hexadecimal 0–9, A–F
Two hexadecimal characters contained in each
8–bit field of the message
Device Address
Function Code
Response
from
Eight Bit
Slave
Data Bytes
Error check