7-58
7-9-6 Communications Response Time
The communications response times for communications with an Inverter via the
RS-422/485 port of an OMRON-madeCommunications Board aredetailed below. Use
this information asareference when decidingthe number ofSlaves tobe connected to
one network, and when considering the timing of input and output signals.
H Communications Time for One Message
·A wide variety of programsfor RS-422/485 communications can becreated using the
protocolmacrofunction.Thecommunicationstimeswillvaryaccordingtothecontents
of the program.
·In general, the communications time for one message canbe 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.ofbytesinresponse´ 10´ (1/baudrate)´ 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. ThereasonthatthenumberofbytesintheDSRmessageandresponseismul-
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 theprotocol macro created in 7-9-7
Creating a Project File,canbecalculatedaccordingtothe followingformula.(Baudrate
= 19,200 bps.)
Communications time = [(DSR messagetowritedata (13 bytes) +DSR messageto
read (8 bytes)) ´ 10 ´ (1/19,200) ´ 1,000 (ms)] + [(write response (8 bytes) + read
response(7bytes))´ 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 usea different sequence to detect communica-
tionserrors,orincreasethenumberofMasterstherebydecreasingthenumberof
Slaves per Master.
Communications Chapter 7
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