Omron SYSDRIVE RV Series User Manual page 347

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.
Communications Time for One Message
A wide variety of programs for RS-422/485 communications can be created using the protocol macro func-
tion. 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 following formula.
Communications time = [Number of bytes in DSR message x 10 (See note 1.) x (1/baud rate) x 1,000 (ms)]
+ [Number of bytes in response x 10 x (1/baud rate) x 1,000 (ms)] + [24 x (1/baud rate) x 1,000 (ms)] + send
wait time setting (ms) + protocol macro waiting time (See note 2.) (ms)
The reason that the number of bytes in the DSR message and response is multiplied 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
With RS-422A/485 communications, set at least 20 ms as the protocol macro waiting time.
Calculation Example
The communications time required for one Slave in the protocol macro created
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)) x 10 x (1/
19,200) x 1,000 (ms)] + [write response (8 bytes) + read response (7 bytes)) x 10 x (1/19,200) x 1,000 (ms)]
+ [24 x (1/19,200) x 1,000 (ms) x 2] + [10 (ms) x 2] + [20 (ms) x 2] = 81.2 (ms)
If there are N Slaves, the total communications time will be N x 81.2 ms. Consequently, 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 communications errors, or increase the number of
Masters thereby decreasing the number of Slaves per Master.
I/O Response Time
The communications processing times for the Inverter are as follows.
Inverter communications input scan: 8 ms
•
Inverter communications output scan: 8 ms
•
Internal processing time for the Inverter: Approx. 20 ms
•
The I/O response times for the Inverter are illustrated in the following diagram.
Ladder program cycle time
Communications time x 2
Inverter I/O scan
Internal processing for
the Inverter
Max. input
response time
Fig 6.76 I/O Response Time
Communications Functions
under Creating a Project File
Max. output
response time
, can
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