Measurement-computing CB-7000 Utilities Uživatelský manuál stáhnout pdf (Strana 38)

4 Frequently Asked Questions

4.1

7000 FAQ

Q: How is the checksum computed?

A: The steps to compute checksum are as following:

Step 1: Checksum=0;

Step 2: For overall command byte with index, do this loop:

Checksum = Checksum + Command byte [index]

Step 3: Checksum = Checksum & 0xff

Step 4: Convert checksum to ASCII high byte and ASCII low byte

For example,

Command = $012[Enter]

Checksum = '$'+'0'+'1'+'2' = 0x24+0x30+0x31+0x32 = 0xB7

Checksum & 0xff = 0xB7

Checksum high byte = 'B'

Checksum low byte = '7'

Command with checksum = $012B7[Enter]

When to do I use a repeater in the RS-485 network?

A: The us er should use repeater when the network has m ore than 256 m odules, or it is over

1.2 km long.

Q: What is the maximum scan rate in the RS-485 network?

A: The s can rate depends on t he baud rate of RS-232 COM port, checksum status, module

numbers, channels, command to send, result string that received and system performance.

•

For the environment to test:

•

7012 module: x 1

•

Channel: x 1

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Baud Rate: 115200 bps

•

Command: #AA (cr)(null) ==> 5 Chars

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Result: >SDDDDDD (cr) ==> 9 Chars

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1 Char: 1 Start bit + 8 Data bits + 1 Stop bit = 10 bits

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Every communications: Command string + Result string

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= 14 Chars = 140 bits

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140 (bits/communication) / 115200 (bits/sec)

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= 0.00122 (sec/communication)

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Hardware delays (every communications): 1 ms

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Scan rate: 1/ (0.00122 + 0.001) = 450 (communication/sec)

•

Estimated scan rate:

•

450 times/sec maximum.

•

Tested value of scan rate:

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440 times/sec maximum without system message processing.

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360 times/sec maximum with system message processing.

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