serial data communication protocol Monitoring – ADC200/20
This experiment demonstrates how a serial data communication protocol can be successfully monitored. Serial data communications represents the fundamental method by which information is transported in our modern world.
Convergence between communications and computers has caused all forms of information to be digitized for easy transport in serial format. Unlike analog waveforms such as sine waves and square waves that are periodic in nature, serial data comm waveforms are generally aperiodic. Using a modern digital instrument such as the PICO ADC200/20 makes capture and viewing of serial data easy.
This experiment demonstrates:
- What serial data looks like.
- Monitoring this data using the PICO ADC200/20 oscilloscope.
This experiment is suitable for:
- First introductory data communications course for college/university.
- Electronics experimenter such as a amateur radio operator.
- ADC200/20 scope, scope probe.
- PC with serial port.
- Terminal program such as Hyperterminal for Win95/98, Terminal Win3.1 or Telix Linux.
Always keep in mind that the ground of the ADC200/20 is common to the ground of the PC and thus common to AC ground.
The experimental setup is shown in Figure 1. Serial data is accessed from the serial port on a standard PC. The serial port is generally either a DB9 connector or DB25 connector. Figure 2 shows a picture of a standard PC DB9 type serial port. An appropriate serial cable consisting of female/male DB9 connectors is connected to this port.
The ADC200/20 CH A input is connected to the Transmit Data and Ground connections on the serial port via the serial cable. Note that you can use the same PC that the ADC200/20 is connected to for the serial transmission or you can use a standalone PC. Figure 1 shows separate PCs for convenience.
Figure 1 Experimental Setup
Figure 2 Photo of PC Serial Port DB Male Connector
1. Terminal Software Setup
One of the simplest serial data comm protocols is sending ASCII characters via a PC terminal program. Several popular terminal programs exist, depending on the PC operating system (Win95/98, Win3.1, Linux, DOS, etc). Let�s examine Hyperterminal that commonly comes as part of the Win95/98 operating system.
To access Hyperterminal, follow the Start Button:
Once you have located the Hyperterminal directory, find the Hypertrm program and double click it. The program screen is shown below in Figure 3.
Figure 3 Hyperterminal Screen
Now you must setup the terminal parameters. Under File, locate New Connection. Give the connection the name Picotech and click OK. The Connect To box appears. Select the Connect using parameter to Direct to Com1 (assuming that serial port used is Com1. Check with your PC manual for settings for serial port). Click OK. Next the Port Settings box appears.
Accept the parameters as listed below:
- Bits per sec = 2400
- Data bits = 7
- Parity = none
- Stop bits = 1
- Flow control = hardware
Finally under File, go to the Properties parameter and select the Settings tab. Go to ASCII Setup
and check off the box Echo typed characters locally. Save all settings under Picotech.htm. The terminal program is now setup. The proper settings are shown below in Figure 4.
Figure 4 Hyperterminal Terminal Properties Setup
2. ADC200/20 Scope Setup
Once the terminal program is configured, we are ready to send serial data. The terminal program will send data via the serial port in RS-232 format. Figure 9 shows the pinouts of the DB9 and DB25 type RS-232 connectors commonly found on serial ports.
Connect the ADC200/20 scope probe with the center conductor connected to the Transmit Data Terminal #3 and the ground conductor to Ground Terminal #5. Note this setting is for a DB9 connector. Adjust accordingly for a DB25.
In order to capture the one time asynchronous signal, the scope must be set for one time trigger.
Set the instrument as follows:
- TxData to Input A, +/- 20V, DC, Probe x 1, Input B off
- Time base = 500usec/div
- Trigger = single, Input A, rising, 1000mV, -10% delay
Note that the scope is set to trigger with a rising voltage > 1 volt on input A.
3. Sending Serial Asynchronous Data and Capturing Results
With the Terminal PC we are now ready to send and capture ASCII data. Try sending an ASCII G first. To do this, first set Cap Lock on the keyboard to on. Then simply press G. Ensure that the correct character is sent out, as it will echo on the Hyperterminal display. If you are using the same PC for both Terminal and the ADC200/20 display, you will have to size both windows so that they are visible.
Now in order to capture and display the result on the scope, hit the run button. The scope will now trigger with any voltage rising on input A >1 volt. Press the G again. You should see a display similar to Figure 5 below. Note the cursors have been set to measure the duration of the start bit.
Try another character, say the numeral 8. The display for 8 is shown in Figure 6. Note to clear the display press stop, then run again.
Figure 5 ASCII G As Displayed on ADC200/20
Figure 6 ASCII 8 As Displayed on ADC200/20
- Does the waveform shown in Figure 6 match what should be seen for ASCII 8 ?.
- How wide would a bit be if the data speed was changed to 4800 bit/sec?.
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