Communications Response Time - Omron SYSDRIVE 3G3MV User Manual

Communications

7-9-6 Communications Response Time

The communications response times for communications with an Inverter via the
RS-422/485 port of an OMRON-made Communications Board are detailed below. Use
this information as a reference when deciding the number of Slaves to be connected to
one network, and when considering the timing of input and output signals.
H Communications Time for One Message
·A wide variety of programs for RS-422/485 communications can be created using the
protocol macro function. The communications times will vary according to the contents
of the program.
·In general, the communications time for one message can be calculated using the fol-
lowing formula.
Communications time = [No. of bytes in DSR message ´ 10 (see note 1) ´ (1/baud
rate) ´ 1,000 (ms)] + [No. of bytes in response ´ 10 ´ (1/baud rate) ´ 1,000 (ms)] + [24
´ (1/baud rate) ´ 1,000 (ms)] + send wait time setting n156 (ms) + protocol macro
waiting time (see note 2) (ms)
Note 1. The reason that the number of bytes in the DSR message and response is mul-
tiplied by 10 is because both the start bit and the stop bit require one bit each.
(1 byte = 8 bits) + (start bit: 1 bit) + (stop bit: 1 bit) = 10 bits
Note 2. With RS-422/485 communications, set at least 20 ms as the protocol macro
waiting time.
D Calculation Example
The communications time required for one Slave in the protocol macro created in 7-9-7
Creating a Project File, can be calculated according to the following formula. (Baud rate
= 19,200 bps.)
Communications time = [(DSR message to write data (13 bytes) + DSR message to
read (8 bytes)) ´ 10 ´ (1/19,200) ´ 1,000 (ms)] + [(write response (8 bytes) + read
response (7 bytes)) ´ 10 ´ (1/19,200) ´ 1,000 (ms)] + [24 ´ (1/19,200) ´ 1,000 (ms) ´
2] + [10 (ms) ´ 2] + [20 (ms) ´ 2] = 81.2 (ms)
Note If there are N Slaves, the total communications time will be N ´ 81.2 ms. Conse-
quently, the more Slaves that are used, the longer the communications time will
be. If the number of Slaves is too high, it is possible that the detection time of 2 s
for communications time-over will be exceeded. In this case, either disable the
time-over detection function and use a different sequence to detect communica-
tions errors, or increase the number of Masters thereby decreasing the number of
Slaves per Master.
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Chapter 7