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920MHz Band Wireless Device (Korea) USERS MANUAL EM-9121 Rev. 1
7.1.15 SIMPLE CALCULATION OF SAMPLING CYCLE
This section describes how to calculate sampling cycle in using wireless devices.
The values here are approximate, and may greatly vary depending on the radio wave environment.
With the model configuration in Figure 7.2 as an example, the conditions and Equation to calculate the sampling cycle for a data
acquisition device to collect data from sensors are shown below.
The command response time is calculated with considering only the sensors connected to child devices.
Retransmissions of collect data commands due to timeouts, etc. are not considered in this calculation.
■
CALCULATION CONDITIONS
[Table 7.18] Calculation conditions
ITEM
VALUE (UNIT)
REMARKS
Reference command response
time based on transmission data
size
161 (msec.)
Refer to the response time for 1 point Input Register (3X)/Holding Register (4X)
in Table 7.20 “Ready reckoner for command response time.”
Delay time per hop
80 (msec.)
■
EQUATION
Rough indication of command response time of child device
= (reference command response time + delay time per hop × (number of hops
*
– 1)) × number of times of access.........Equation 7.1
*
Number of hops = number of repeaters (child devices excluding low-speed moving devices) to pass through.
Parent Device
No. of times of access = 1
No. of times of access = 2
No. of times of access = 5
Number of hops = 1
Number of hops = 2
Number of hops = 3
Child Device 1
Child Device 5
Child Device 3
Child Device 4
Child Device 2
No. of times of access= 2
No. of times of access = 3
Figure 7.2 Model configuration for calculation of command response time
The table below shows command response time per child device in turning this model configuration into the Equation 7.1.
[Table 7.19] Command response time per child device in the model configuration
DEVICE
RESPONSE TIME
(ms)
NO. OF HOPS
NO. OF TIMES
OF ACCESS
COMMAND RESPONSE
TIME PER DEVICE
COMMAND RESPONSE TIME
(ms)
Child device 1
161
1
1
(161 + 80 × (1 – 1)) × 1
161
Child device 2
1
2
(161 + 80 × (1 – 1)) × 2
322
Child device 3
1
2
(161 + 80 × (1 – 1)) × 2
322
Child device 4
2
3
(161 + 80 × (2 – 1)) × 3
723
Child device 5
3
5
(161 + 80 × (3 – 1)) × 5
1605
The sampling cycle of data from the sensors connected to Child devices 1 to 5 is calculated as follows:
Sampling cycle = 161 + 322 + 322 + 723 + 1605 = 3133 (msec.)
Therefore, the sampling cycle must to be set to 3133 (msec.) or longer.
The table below is a ready reckoner for command response time used in the above calculation.
[Table 7.20] Ready reckoner for command response time
Coil (0X)/Input (1X) (bit unit)
Number of points
1
200
400
600
800
1000
Response time (ms)
148
182
195
224
240
257
Input Register (3X)/Holding Register (4X) (word unit)
Number of points
1
20
40
60
80
100
120
Response time (ms)
161
177
211
240
267
303
332
