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6.1 MM3 MODBUS PROTOCOL
A complete request/response sequence consists of the following bytes (transmitted as separate data frames):
Master Request Transmission:
SLAVE ADDRESS: 1 byte
FUNCTION CODE: 1 byte
DATA: variable number of bytes depending on FUNCTION CODE
CRC: 2 bytes
Slave Response Transmission:
SLAVE ADDRESS: 1 byte
FUNCTION CODE: 1 byte
DATA: variable number of bytes depending on FUNCTION CODE
CRC: 2 bytes
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. Note: A master transmission with a SLAVE ADDRESS of 0 indicates a broadcast com-
mand. Broadcast commands can be used only in certain situations; see Section 6.4: APPLICATIONS on page
6–13 for details.
FUNCTION CODE: This is the second byte of every transmission. Modbus defines function codes of 1 to 127.
The MM3 implements some of these functions. See Section 6.2: MM3 SUPPORTED FUNCTIONS on page 6–
6
4 details of the supported function codes. In a master request transmission the FUNCTION CODE tells the
slave what action to perform. In a slave response transmission the FUNCTION CODE tells the master what
function was performed 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 > 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 Actual Val-
ues, Setpoints, or addresses sent by the master to the slave or by the slave to the master. See Section 6.2:
MM3 SUPPORTED FUNCTIONS for a description of the supported functions and the data required for each.
CRC: This is a two byte error checking code.
The RTU version of Modbus includes a two byte CRC-16 (16 bit cyclic redundancy check) with every transmis-
sion. The CRC-16 algorithm essentially treats the entire data stream (data bits only; start, stop and parity
ignored) as one continuous binary number. This number is first shifted left 16 bits and then divided by a charac-
teristic polynomial (11000000000000101B). The 16 bit remainder of the division is appended to the end of the
transmission, MSByte first. The resulting message including CRC, when divided by the same polynomial at the
receiver will give a zero remainder if no transmission errors have occurred.
If a MM3 Modbus slave device receives a transmission in which an error is indicated by the CRC-16 calcula-
tion, the slave device will not respond to the transmission. A CRC-16 error indicates than one or more bytes of
the transmission were received incorrectly and thus the entire transmission should be ignored in order to avoid
the MM3 performing any incorrect operation.
6-2
MM3 Motor Manager 3
6 COMMUNICATIONS
6.1.4 DATA PACKET FORMAT
6.1.5 ERROR CHECKING
GE Power Management
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