2
489 GENERATOR MANAGEMENT RELAY – COMMUNICATIONS GUIDE
The 489 is always a slave; it cannot be programmed as a master. Computers or PLCs are
commonly programmed as masters. The Modbus protocol exists in two versions: Remote
Terminal Unit (RTU, binary) and ASCII. Only the RTU version is supported by the 489.
Monitoring, programming, and control functions are performed with read/write register
commands.
CG.1.3 Data Frame Format and Data Rate
One data frame of an asynchronous transmission to or from a 489 is default to 1 start 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 modems at bit rates of greater than 300 bps). The parity bit is optional as odd or
even. If it is programmed as odd or even, the data frame consists of 1 start bit, 8 data bits,
1 parity bit, and 1 stop bit.
Modbus protocol can be implemented at any standard communication speed. The 489
RS485 ports support operation at 1200, 2400, 4800, 9600, and 19200 baud. The front panel
RS232 baud rate is fixed at 9600 baud.
CG.1.4 Data Packet Format
A complete request/response sequence consists of the following bytes (transmitted as
separate data frames):
1. A
Master Query Message
consisting of: a 1-byte
Slave Address
, a 1-byte
Func-
tion Code
, a variable number of
Data Bytes
depending on the Function Code,
and a 2-byte
CRC
code.
2. A Slave Response Message consisting of: a 1-byte
Slave Address
, a 1-byte
Function Code
, a variable number of
Data Bytes
depending on the Function
Code, and a 2-byte
CRC
code.
The terms Slave Address, Function Code, Data Bytes, and CRC are explained below:
•
SLAVE ADDRESS
: This is the first byte of every transmission. 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 address and only the addressed
slave will respond to a transmission 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. The RS232 port ignores the slave
address, so it will respond regardless of the value in the message. Note: A master
transmission with a Slave Address of 0 indicates a broadcast command. Broadcast
commands can be used for specific functions.
•
FUNCTION CODE
: This is the second byte of every transmission. Modbus defines
function codes of 1 to 127. The 489 implements some of these functions. 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 indicating the slave performed the function
as requested. If the high order bit of the Function Code sent from the slave is a 1 (i.e. if
the Function Code is greater than 127) then the slave did not perform the function as
requested and is sending an error or exception response.
