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Chapter 8 Protocol Decoding RIGOL
MSO2000A/DS2000A User’s Guide 8-5
RS232 Decoding (Option)
RS232 serial bus consists of the transmitting data line (TX) and the receiving data
line (RX).
The industry standard of RS232 uses “Negative Logic”, namely high level is logic “0”
and low level is logic “1”.
1
0 0 0 0
1 1 1 1 1
Figure 8-3 RS232 I ndustrial Standard Schematic Diagram
By default, RS232 uses LSB (Least Significant Bit) transmission sequence, namely
the lowest bit of the data is transmitted first. While for MSB (Most Significant Bit),
the highest bit of the data is transmitted f irst.
Bit0
Bit5
Bit2
Bit3
Bit6
Bit1
Bit7
Bit4
t
Bit7
Bit2
Bit5
Bit4
Bit1
Bit6
Bit0
Bit3
t
Endian (LSB) Endian (MSB)
In RS232, you need to set the start bit, data bits, check bit (optional) and stop bit of
each frame of data.
Start Bit: represent when the data starts outputting.
Data Bits: represent the number of data bits actually contained in each frame of
data.
Check Bit: used to check the correctness of the data transmission.
Odd Checkout: the total number of “1” in the data bit and check bit is an odd.
For example, when 0x55 (01010101) is sent, a 1 needs to be filled in the check
bit to make the number of 1 be an odd.
Even Checkout: the total number of “1” in the data bit and check bit is an even.
For example, when 0x55 (01010101) is sent, a 0 should be filled in the check bit.
None: there would not be check bit during the transmission.
Stop Bit: represent when the data stops outputting.
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