Chapter 5: DURAPULSE MODBUS Communications
5–76
Additional Message Delay Times
So we have the total transmission time for sending a message and receiving a
reply but this does not include all of the delays for a given message. The
receiving device must have time to process the receipt of a message and formulate
a reply. The amount of time that the receiving device needs will vary greatly
depending upon the hardware platform and other processes that the device is
running.
For the previous example message, the GS Drive responds in 4ms when the drive
is stopped and will respond in 5ms when the drive is running. This may vary
somewhat depending upon the specific parameter values and the size of the
request.
Modbus-specified Delays Between Messages
There is one additional time delay required in the Modbus protocol. The protocol
specifies at least a 3.5 character delay between messages. For the settings above,
a 3.5 character time in our example would be about 2ms.
So the total time required for the message sent above would be:
7.4 ms (Transmission time for sending message)
5.0 ms (response delay from GS Drive when drive running)
4.6 ms (Transmission time for reply message)
+ 2.0 ms
(Modbus message wait delay)
19.0 ms (approximately)
Remember from our description, this is purely the time from when the message
leaves the serial port to when the reply is received back in to the serial port.
DURAPULSE AC Drive User Manual
1st Ed. Rev. D 05/2013
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