ProSoft Technology MVI56-LTQ - Manual (Page 82)

Reference MVI56-LTQ ♦ ControlLogix Platform
User Manual Limitorque Valve Actuator Master Communication Module
Page 82 of 98 ProSoft Technology, Inc.
August 30, 2010
5.8 Polling Schemes
In Looped Mode, the LTQ provides communication redundancy to each
configured slave on the network. The LTQ monitors the health of each
communication path between port 1 and each configured slave and between port
2 and each configured slave. LTQ port 1 communication status between port 1
and the addressed slave is recorded in the slave Channel A status bit. LTQ port
2 communication status between port 2 and the addressed slave is recorded in
the slave Channel B status bit. Both Channel A and Channel B status bits are
located in the slave Status register, bits 10 and 11(Word 1, bits 10 and 11).
On a healthy network where all configured slaves are communicating; the LTQ
will first poll all slaves via port 1, then poll all slaves via port 2, back to port 1, and
so on. As each slave is successfully polled, the respective Channel bit is set to 0
in the slave Status register. Remember the LTQ port 1 equals Channel A and the
LTQ port 2 equals Channel B.
Should a slave not be reached on a poll, the LTQ will set the corresponding
Channel Fail bit to 1, switch to the other port and attempt to communicate with
the same slave. Should the slave not communicate from the second port, the
corresponding Channel Fail bit will be set to 1, and the LTQ will resume polling
on the original port. After the LTQ has completed polling all configured slaves on
the first port, the polling routine will switch to the other port and repeat the above
process.
Example:
There are 5 slaves on the network and slave number 3 has been turned off. The
LTQ is currently polling the slaves through the LTQ port 1 (Channel A). Slave
number 1 and 2 respond to the LTQ port 1 poll. Slave number 3 does not
respond to the port 1 poll causing the LTQ to set slave 3 Channel A bit to 1. The
LTQ now changes to port 2 (Channel B) and polls slave number 3. Slave number
3 does not respond to the port 2 poll causing the LTQ to set the slave 3 Channel
B bit to 1. Next the LTQ changes back to port 1 and attempts to poll slave
number 4. This communication attempt is successful and the LTQ now polls
slave number 5 through the LTQ port 1. Slave number 5 responds completing the
port 1 poll.
Next the LTQ repeats the process through port 2 (Channel B). Slave 1 and 2
respond, slave 3 does not respond and the LTQ sets the slave 3 Channel B bit to
1. The LTQ changes to port 1 (Channel A) and attempts to communicate with
slave 3. Slave 3 does not respond, the LTQ sets the slave 3 Channel A bit to 1,
switches back to port 2 and resumes polling the remainder of the configured
slaves. Once slaves 4 and 5 have been successfully polled via port 2, the LTQ
then switches to port 1 and repeats the polling process. The port alternation
process described above continues until slave 3 is powered on and the
communication fault clears.
Commands for slave control interrupt the polling process and are issued through
the current poll port. After the slave has acknowledged the command, the LTQ
resumes the polling process. In the event of a communication fault between the
current poll port and a commanded slave, the LTQ will issue the command
through the other communication port.