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MODBUS PROTOCOL
Data frame format
and data rate
Data packet format
2–2
120 Ω for standard 24 AWG twisted pair wire. The value of the capacitors should be 1 nF.
Shielded wire should always be used to minimize noise. Polarity is important in RS485
communications. The '+' terminal of every device must be connected together for the
system to operate.
One data frame of an asynchronous transmission to or from a GE Multilin 745 consists of 1
start bit, 8 data bits, and 1 stop bit. This produces a 10 bit data frame. The 745 can be
configured to include an additional even or odd parity bit if required, producing an 11 bit
data frame.
All ports of the GE Multilin 745 Transformer Protection System support operation at 300,
1200, 2400, 9600, and 19200 baud.
A complete request/response sequence consists of the following bytes transmitted as
separate data frames:
Master query message:
Slave address
(1 byte)
Function code
(1 byte)
Data
(variable number of bytes depending on the function code)
CRC
(2 bytes)
Slave response message:
Slave address
(1 byte)
Function code
(1 byte)
Data
(variable number of bytes depending on the function code)
CRC
(2 bytes)
A message is terminated when no data is received for a period of 3½ character
transmission times. Consequently, the transmitting device must not allow gaps between
bytes larger than this interval (about 3 ms at 9600 baud).
•
Slave address: This is the first byte of every message. 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 message that starts with its address. In a master query
message the slave address represents the address of the slave to which the request is
being sent. In a slave response message the slave address is a confirmation
representing the address of the slave that is sending the response. A master query
message with a slave address of 0 indicates a broadcast command. All slaves on the
communication link will take action based on the message, but none will respond to
the master. Broadcast mode is only recognized when associated with function codes
05h, 06h, and 10h. For any other function code, a message with broadcast mode slave
address 0 will be ignored.
•
Function code: This is the second byte of every message. Modbus defines function
codes of 1 to 127. The 745 implements some of these functions. In a master query
message, the function code tells the slave what action to perform. In a slave response
message, 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 (i.e. if the function code is >
7Fh) 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 include actual values, setpoints, or addresses sent by the master to the slave or
by the slave to the master.
745 TRANSFORMER PROTECTION SYSTEM – COMMUNICATIONS GUIDE
CHAPTER 2: MODBUS PROTOCOL
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