Rigol SD-RS232-DS4 - RS232 BUS Serial Decode Option

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”.

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 first.

In  RS232, baud rate is used to represent the transmitting rate (namely bits per second) of the data. The commonly used baud rates include 4800bps, 9600bps, 19200bps, 38400bps, 57600bps and 115200bps.

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. Setting the Polarity is equivalent to specifying the “Start Bit”.

Data Bits: represent the number of data bits actually contained in each frame of data.

Even-OddCheck: be used to check the correctness of the data transmission.

• Odd-Check:  the  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 to be an odd.
• Even Check: the number of “1” in 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.

Press Decode1 → Decode to select “RS232” to open the RS232 decoding function menu.

1. TX and RX Channel Setting

Press TX to select any channel (CH1 to CH4) as the transmitting channel and when “OFF” is selected, no transmitting channel is set. Use the same method the set the RX channel. What’s more, you need to set the thresholds of the input channels of TX and RX. Switch the menu page and press TXThreshold and RXThreshold respectively to input the desired threshold values.

2. Polarity Setting

Press Polarity to select “-” (negative polarity) or “+” (positive polarity) and the default is negative polarity. The oscilloscope will select the rising or falling edge as the start position during decoding.

3. Endian Setting

Press Endian to select “LSB” or “MSB” and the default is “LSB”.

4. Baud Rate Setting

Press Baud to select the desired baud rate and the default is 9600bps.

5. Data Packet Setting

As mentioned before, in RS232, you need to set the start bit, data bits, check bit (optional)  and  stop  bit of  each frame of data. “Start  Bit” is specified by the “Polarity Setting”. The setting methods of other parameters are as follows:

• Press Data Bits to set the data width of each frame. It can be set to 5, 6, 7, 8 or 9 and the default is 8.
• Press Stop Bit to set the stop bit after each frame of data. It can be set to 1 bit, 1.5 bits or 2 bits.
• Press Even-OddCheck to set the even-odd check mode of the data transmission. It can be set to None, Odd Check or Even Check.
• Press Packet to enable or disable the packet end. When packet end is enabled, several data blocks are combined according to the packet end.
• Press PacketEnd to set the packet end during data transmission and it can be set to 00 (NULL), 0A (LF), 0D (CR), 20 (SP) or FF.

6. Display-related Setting

Press Format to set the display format of the bus to Hex, Decimal, Binary or ASCII.
Press Offset and use     to adjust the vertical display position of the bus.
Press BusStatus to turn the bus display on or off.

7. Decoding Table

The decoding table displays the decoded data, the corresponding time and error information on TX and RX data lines in table format. It can be used to observe relatively longer decoded data to solve the problem that some data could not be seen clearly on the screen.

Press Event Table → Event Table to select “ON” (note that this operation is only available when  BusStatus is set to “ON”)  to enter the decoding table interface as shown in the figure below. The decoding table lists the decoded data in time order. If error occurs during the decoding, the corresponding error information is displayed. If an USB storage device is currently connected to the instrument, press Export to export the data table to the external USB storage device in CSV format.

8.  The Error Expression during Decoding

DS4000 makes full use of the resources such as color and view to express the results of the protocol decoding effectively so as to let users find the desired information quickly.

End Frame Error:
Errors generated when the end frame condition is not met. When the stop bit is set to 1.5, red error mark (note that the red mark is displayed in different modes according to the horizontal time base setting; when the horizontal time base is
relatively smaller,      is displayed; otherwise,     is displayed) will be displayed if the stop bit is less than 1.5.

Check Error:

When check bit error is detected during the decoding, red error mark will be displayed. For example, when the transmitting terminal is set to none check and the decoder is set to odd check, the following check error occurs:

(00100000,LSB)

Wherein, there are odd number (1) of 1 in the 8 bits data 00100000 and the check bit should be 0; but the check bit detected on the TX is 1, thus check error occurs.

After the decoder is set to none check, the decoding becomes normal.