Installation manual

Publication 1747-RM001G-EN-P - November 2008
13-62 SLC Communication Channels
Determining Minimum Master ACK Timeout
To determine the minimum ACK Timeout, you must first calculate the
transmission time by multiplying the maximum sized data packet for your
processor by the modem rate in ms/byte. For an example we will assume an
SLC 5/03 processor (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. In RSLogix
500, double click on the Processor Status icon and then locate Average on the
Scan Times tab. 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 13.18 Approximate modem transmission rates
modem bps approx. ms/byte
4800 2 ms/byte
9600 1 ms/byte
19200 .5 ms/byte
Table 13.19 Sum of the Transmission Rates
Parameter Example Values (in ms)
Max message transmission
time
224
Average program scan time 20
RTS Send Delay 60
Use only the largest of these
two values