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MM3 MOTOR MANAGER 3 MODBUS PROTOCOL
NOTE
8.1.3
Data Frame
Format and
Data Rate
8.1.4
Data Packet
Format
NOTE
8–2
be used to minimize noise. The shield should be connected to all of the MM3 Motor
Manager 3s as well as the master, then grounded at one location only. This keeps the
ground potential at the same level for all of the devices on the serial link.
Polarity is important in RS485 communications. The '+' (positive) terminals of every
device must be connected together.
Refer to Serial Communication Port on page 2–4 for additional information.
One data frame of an asynchronous transmission to or from a MM3 Motor Manager 3
typically consists of 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 MM3
Motor Manager 3 has the capability of adding an odd or even parity bit if necessary.
Modbus protocol can be implemented at any standard communication speed. The MM3
Motor Manager 3 supports operation at 1200, 2400, 4800, 9600, 19200, and 57600 baud.
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
The function of each of these bytes is 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.
A master transmission with a slave address of 0 indicates a broadcast command.
Broadcast commands can be used only in certain situations; see Storing Addresses using
the Broadcast Command on page 8–12 for additional details.
•
Function code: This is the second byte of every transmission. Modbus defines function
codes of 1 to 127. The MM3 Motor Manager 3 implements some of these functions.
See Supported Modbus Functions on page 8–5 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
CHAPTER 8: COMMUNICATIONS
MM3 MOTOR MANAGER 3 – INSTRUCTION MANUAL
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