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8 What you need to know about SmartWire-DT
8.3 SWD cycle time
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8.3 SWD cycle time
The SWD cycle time is the time required for the cyclic exchange of data
between the coordinator and all modules using a common data frame on the
SWD line.
The SWD gateway is the coordinator.
The SWD cycle time does not depend on the number of modules on the
SWD line as much as it does on the amount of payload data that the summa-
tion frame needs to transmit. Within this context, "payload data bytes" refers
to the number of input and output bytes of a module.
There are commonplace modules, such as switches, that transmit little pay-
load data. However, there are also modules with multiple settings and read-
out values, such as motor-protective circuit-breakers.
To calculate the SWD cycle time, each byte is counted as 10 bits, since a
start bit and a stop bit are required for each byte for synchronization pur-
poses. The SWD cycle time can be calculated using the following formula:
n = Number of user data bytes.
C = Baud rate [kBit/s]
n
TN
= number of modules
Example:
The following values are given:
Input bytes = 13 bytes
Output bytes = 8 bytes
Modules: 9
Baud rate = 125kBit/s
n = Number of user data bytes = 21
SWD cycle time t
p
[ms] = 1 n · 10 bit + 2 bit · n
TN
+ 30 bit · 10
C
SWD cycle time t
p
[ms] = 1 21 · 10 bit + 2 bit · 9 + 30 bit · 10 = 4.224 ms
125 kBit/s