Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM User Manual
Page 179

Publication AG-UM008C-EN-P - February 2005
Configuring SLC 5/03, 5/04, and 5/05 Processors 4-41
Therefore, if there are 10 slave stations and the ACK Timeout is
320 ms, then the maximum single message transfer poll scan time
would be:
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:
10 (slave stations)
.320 seconds =
*
3.20 seconds
Round up the MSG Timeout value to the nearest second (4).
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