Rockwell Automation 1756-DMF30 ControlLogix Drive Module - Manual (Page 24)

Publication 1756-UM522B-EN-P - February 2003
2-2 Drive Comm (Drive Communication)
Point/Feedback Data Exchange” to transfer data. The Drive Module
sends set point data to the PMI processor, which returns feedback
data to the Drive Module.
Initialization At boot time, the Drive Module and PMI processor use an
asynchronous master-slave protocol to transfer the PMI operating
system and establish synchronized communication. The PMI
processor is the master, and commences by requesting its operating
system. The Drive Module responds by downloading the operating
system in a special message, containing up to 256 byte packets.
When the operating system transfer is complete, the Drive Module
sends a configuration message and a gain message. The configuration
message contains information that fixes the communication frequency
at 4 mS. Once the Drive Module and PMI processor have
synchronized communications, normal Set Point/Feedback data
transfer can begin.
Synchronized
Communication The synchronized protocol is called “Set Point/Feedback Data
Exchange”. The drive module sends set point data to the PMI
processor. The set point data contains reference data, for drive
control, and a time stamp. This time stamp enables the PMI processor
to calculate what the proper time is for the next feedback message.
The PMI processor sends feedback messages to the Drive Module.
The feedback messages contains data about drive operation.
Timing
The velocity loop updates (reads from, and writes to Drive
Communication) every 2 mS. After every other velocity loop update
(at a 4 mS interval), the position loop performs its update. After every
position update, the Drive Module sends set point data to the PMI
processor. After the next velocity update (but before the following
one), the PMI processor sends feedback data to the Drive Module.
Refer to the following timing chart.