Appendix D
Calculating the PCP Transmission Time
The formula below can be used to calculate the transmission time for a PCP message.
Formula:
TD = TL + Gm( OD, N ) * Z + TL7
Gm = ( OD, N ) = (( N + OD - 1 ) / m ) + 1
TD
Transmission time of a PCP service in milliseconds
TL
Latency 2 * Z (milliseconds)
OD
Service dependent overhead
N
User data
Z
Interbus cycle time (milliseconds)
TL7
Layer 7 runtime typical 4.0 milliseconds
m
Width of parameter channel (in bytes) - 1 (control information)
Gm( OD, N )
Number of cycles that are required to transmit the overhead and user data
The division through m should be an integer division.
Example:
The following example uses the above formula to calculate the PCP transmission time for a Write
service.
Write Request:
Read Response:
OD = 7 bytes for one write request
OD = 4 bytes for a read response
N = 128 bytes (e.g. one parameter set)
N = 0 bytes (no user data)
Z = 1.5 milliseconds
Z = 1.5 milliseconds
Description
m
Gm
Td
Write Request: Parameter channel with a width of 1 word
2 - 1
135
209.5 ms
Write Request: Parameter channel with a width of 2 words 4 - 1
45
74.5 ms
Write Request: Parameter channel with a width of 4 words 8 - 1
20
37 ms
Read Response: Parameter channel with a width of 1 word 2 - 1
4
13ms
Read Response: Parameter channel with a width of 2 words 4 - 1
2
10 ms
Read Response: Parameter channel with a width of 4 words 8 - 1
1
8.5 ms
