Other Delays; Communication Delay Timing Diagram - AutomationDirect DURAPULSE Series User Manual

Other Delays

Depending upon the master device, there may be additional delays. For example:
In the DirectLogic PLC, the serial communications are serviced in the
housekeeping portion of the PLC scan. So if the communications instruction is in
rung #1 of a ladder program, the serial communications message does not get
sent until the end of the total PLC scan. Likewise, if the reply message was
received into the serial port at the beginning of the PLC scan, it would not be
serviced until the end of the PLC scan.
So you would need to add an additional possible two PLC scan times to the
number above to truly calculate the time necessary to read or write an event to
the GS drive.
These delays are shown in the following Communication Delay Timing Diagram.

Communication Delay Timing Diagram

Modbus Master
(typically PLC)
Communications
Instruction Enabled
t
1
t
2
t
4
t
5
Data Processed and
t
Available to Logic
6
1st Ed. Rev. D 05/2013
Chapter 5: DURA
PULSE
GS AC
Drive
t = Scan delay from the point of turning on a
1
communications instruction to when it actually goes
out of the serial port.
t = Transmission time to send Message request
2
(read or write).
t = Response delay from GS drive to receive the reply
3
t
and formulate the response.
3
t = Transmission time to send Reply message.
4
t = Scan delay from the point of receiving reply,
5
processing it and placing in PLC memory for Logic
usage.
t = Wait time required by Modbus spec (3.5 byte times).
6
This may or may not be present depending upon the
Scan delay, but safer to factor in.
DURA
PULSE
MODBUS Communications
AC Drive User Manual
5–77