Aeg Modicon Modbus Protocol - GE Multilin 565 Instruction Manual

FEATURE INFORMATION

AEG MODICON MODBUS PROTOCOL

Overview
The 565 Feeder Management Relay implements a subset of
the AEG Modicon Modbus serial communication standard.
Modbus protocol is hardware-independent. That is, the physi-
cal layer can be any of a variety of standard hardware
configurations. This includes RS232, RS422, RS485, fibre
optics, etc. Modbus is a single master/multiple slave type of
protocol suitable for a multi-drop configuration as provided
by RS485 hardware. The 565 Feeder Management Relay
Modbus implementation employs two-wire RS485 hardware.
Using RS485, up to 32 slaves can be daisy-chained together
on a single communication channel.
565 Feeder Management Relays are always Modbus slaves.
They can not be programmed as Modbus masters. Comput-
ers or PLCs are commonly programmed as masters.
Modbus protocol exists in two versions: Remote Terminal
Unit (RTU, binary) and ASCII. Only the RTU version is
supported by the 565 Feeder Management Relay.
Both monitoring and control are possible using read and
write register commands. Additional commands are sup-
ported to provide additional functions.
This information is also available on diskette for customers
who wish to write their own communications programs.
Electrical Interface
The hardware or electrical interface in the 565 Feeder Man-
agement Relay is two-wire RS485. In a two-wire link data
flow is bidirectional. That is, data is transmitted and received
over the same two wires. This means that the data flow is half
duplex. That is, data is never transmitted and received at the
same time.
RS485 lines should be connected in a daisy chain configura-
tion with terminating resistors installed at each end of the link
(ie. at the master end and at the slave farthest from the
master). The value of the terminating resistors should be
approximately equal to the characteristic impedance of the
line. This will be approximately 120 ohms for standard #22
AWG twisted pair wire. Shielded wire should always be used
to minimize noise.
Note: polarity is important in RS485 communications. The '+'
terminals of every device must be connected together.
Data Frame Format and Rate
One data frame of an asynchronous transmission to or from
a 565 Feeder Management Relay consists of 1 star t bit, 8
data bits, and 1 stop bit. This produces a 10 bit data frame.
This is important for transmission through modems at high
bit rates (11 bit data frames are not supported by Hayes
6-6
modems at bit rates of greater than 300 bps).
Modbus protocol can be implemented at any standard com-
munication speed. The 565 Feeder Management Relay sup-
ports operation at 1200, 2400, 4800 and 9600 baud.
Data Packet Format
A complete request/response sequence consists of the fol-
lowing bytes (transmitted as separate data frames):
Master Request Transmission:
SLAVE ADDRESS
FUNCTION CODE
DATA - variable number of bytes depending
on FUNCTION CODE
CRC
Slave Response Transmission:
SLAVE ADDRESS
FUNCTION CODE
DATA - variable number of bytes depending
on FUNCTION CODE
CRC
SLAVE ADDRESS - This is the first byte of every transmis-
sion. This byte represents the user-assigned address of the
slave device that is to receive the message sent by the
master. Each slave device must be assigned a unique ad-
dress and only the addressed slave will respond to a trans-
mission that starts with its address.
In a master request transmission the SLAVE ADDRESS
represents the address of the slave to which the request is
being sent.
In a slave response transmission the SLAVE ADDRESS
represents the address of the slave that is sending the
response.
NOTE: A master transmission with a SLAVE ADDRESS of 0
indicates a broadcast command. All slaves on the
communication link will take action based on the
transmission but no response will be made.
FUNCTION CODE - This is the second byte of every trans-
mission. Modbus defines function codes of 1 to 127. The 565
Feeder Management Relay implements some of these func-
tions.
In a master request transmission the FUNCTION CODE tells
the slave what action to perform.
In a slave response transmission if the FUNCTION CODE
sent from the slave is the same as the FUNCTION CODE
sent from the master then the slave performed the function
as requested. If the high order bit of the FUNCTION CODE
sent from the slave is a 1 (ie. if the FUNCTION CODE is >
127) then the slave did not perform the function as requested
and is sending an error or exception response.
DATA - This will be a variable number of bytes depending on
the FUNCTION CODE. This may be addresses, actual val-
ues or setpoints, sent by the master to the slave or by the
slave to the master.
CRC - This is a two byte error checking code.
- 1 byte
- 1 byte
- 2 bytes
- 1 byte
- 1 byte
- 2 bytes