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7 Serial Communications
NX-series Communications Interface Units User’s Manual (W540)
7-8 Communications Performance
7
7-8-2 Adjusting Divided Data Sizes to Improve Communications Performance
The communications performance is determined by the transfer time between the CPU Unit or commu-
nications master and CIF Unit and the transfer time
between the CIF Unit and serial line.
Therefore, you can achieve efficient communications if you optimize the divided data sizes for the I/O
refr
esh cycle between the CPU Unit or communications master and CIF Unit and the baud rate
between the CIF Unit and serial line.
You can use the following formula to calculate the optimum divided data size.
However, the divided data sizes must b
e
set in increments of 4 bytes, so use the next higher multiple of
4 bytes from the value found with the above formula.
Also, the smallest divided data size that you can set is 4 bytes, so if the result of the above formula is
less tha
n 4 bytes, set the divided data size to 4 bytes. Also, the largest divided data size that you can
set is 80 bytes, so if the result of the above formula is greater than 80 bytes, set the divided data size to
80 bytes.
For example, the data send times for various baud rate an
d I/O re
fresh cycles are given in the following
table for these parameter values.
Optimum divided data size [bytes]
7-8-2 Adjusting Divided Data Sizes to Improve Communications Perfor-
mance
Parameter Value
Number of I/O refreshes 3
Bits per character 10
Baud rate
[bps]
I/O refresh cycle [ms]
0.5 1 2 4
1,200 4 4 4 4
2,400 4 4 4 4
4,800 4 4 4 8
9,600 4 4 8 12
19,200 4 8 12 24
38,400 8 12 24 48
57,600 12 20 36 72
115,200 20 36 72 80
230,400 36 72 80 80
Baud rate [bps]
Optimum divided data size [bytes] =
Bits per character × 1,000
× × Number of I/O refreshes
I/O refresh cycle [ms]
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