Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 238)

Publication AG-UM008C-EN-P - February 2005
7-12 Configuring Logix Controllers
Determining Minimum Master Serial Port ACK Timeout
To determine the minimum ACK Timeout, you must first calculate the
transmission time by multiplying the maximum sized data packet for
your controller by the modem rate in ms/byte. For an example we will
assume communications with SLC 5/03 slaves (103 data words or
224 bytes total packet size including overhead) and a 9600 bps
modem, which transmits at approximately 1 ms/byte. Therefore, the
message transmission time is 224 ms. For approximate modem
transmission rates, see the following table.
Next, you need to determine the average slave program scan time. For
this example, lets assume an average slave program scan time of
20 ms. Remember, program scan time will vary by application.
Finally, you must determine the larger of two values, either the
configured slave RTS Send Delay or the turnaround time of the slave
modem. The RTS Send Delay time can be found by double-clicking
on the slave’s Channel Configuration icon and looking at the Chan. 0
System tab of the Channel Configuration screen. Note that the RTS
Send Delay time is in intervals of 20 ms, so with a value of 3 in the
box, the RTS Send Delay time would be 20 ms multiplied by 3. Using
this value (60 ms) for our example, and assuming that the turnaround
time of the modem is 50 ms (which will vary by modem) you would
choose to use the RTS Send Delay time of 60 ms for your calculation.
Having determined the maximum message transmission time
(224 ms), the average slave program scan time (20 ms) and the largest
of either RTS Send Delay (60 ms) or the modem turnaround time, the
minimum ACK timeout is simply the sum of these values.
Table 7.3 Approximate Modem Transmission Rates
modem bps approx. ms/byte
4800 2
9600 1
19200 0.5
Parameter Example Values (in ms)
Max message transmission time 224
Average program scan time 20
RTS Send Delay 60
modem turnaround time 50
calculated ACK Timeout 304
round up to nearest 20 ms 320
Use only the largest of these two values.