Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 108)

Publication AG-UM008C-EN-P - February 2005
3-40 Configuring MicroLogix 1100/1200/1500 Controllers
Standard Polling Mode With Multiple Message Transfer
For standard polling mode with multiple message transfer per poll
scan, the maximum poll scan would be achieved when every slave
had multiple maximum-sized message packets to transmit when
polled. If the worst case scenario is 2 maximum-sized message packet
to transmit, then the maximum poll scan time is approximately 2 times
the maximum single message transfer poll scan time plus the
minimum poll scan time.
The minimum poll scan time occurs when the master sends out a 4
byte poll packet to each slave and in return receives a 2 byte response
from each slave. This time is approximately the master modem
turnaround time or RTS Send Delay (whichever is greater) plus the
slave modem turnaround time or RTS Send Delay (whichever is
greater), multiplied by the number of slave stations in the poll range.
For example, if the master modem turnaround time and RTS Send
Delay are both 0 ms, and the slave modem turnaround time is 50 ms
and RTS Send Delay is 60 ms, the minimum poll scan time would be:
To finish the example:
For this example, the MSG Timeout value should be at least
7 seconds.
Minimum Slave MSG Block Message Timeout
The minimum slave MSG Block Message Timeout should allow for the
Master to go through Message Retries plus one number of maximum
poll scans before timing out. Therefore, the minimum MSG Block
Number of slave
stations Master modem
turnaround time
= *
Use only the greater of
these two values
or
RTS Send Delay The minimum poll
scan time
Slave modem
turnaround time or
RTS Send Delay
+
Use only the greater
of these two values
10 (slave stations) 0 + 60 ms =
*
600 ms
Maximum single
message transfer
poll scan time
Minimum poll scan time =
The maximum multiple
message transfer poll
scan time
# of MSG instructions
per slave +
*
2 = (3.20 seconds) + .600 seconds = 7 seconds
=
The minimum MSG
Timeout