The program for the logic analyzer of signals at the inputs of the COM port. Scheme, description

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The program for the logic analyzer of signals at the inputs of the COM port. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Computers

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In amateur radio practice, sometimes there is a need to investigate low-frequency digital signals. Acquiring an expensive hardware logic analyzer or a storage oscilloscope for this purpose is in most cases impractical. It is enough to apply the signals to be analyzed to the inputs of the computer's COM port and run a program that will display all their changes on the monitor screen and save the information on the computer's hard disk.

The proposed program is designed to record and analyze digital signals received at the inputs of the computer's COM port. It can run under all operating systems of the Windows family, starting with Windows 98. The computer must have a processor of at least Pentium I with a clock speed of 166 MHz and at least 128 MB of RAM.

The program is written in the Delphi 7 integrated development environment. For direct (bypassing the Win API) access to I/O ports, it uses the smPort driver that does not require installation. This made it possible to achieve a fairly high accuracy in measuring time intervals. Sweep mode - waiting for a single adjustable duration, triggered by a change in one or more signals.

At the same time, the signals on the four inputs of the port (CTS, DSR, RI and DCD) are examined. At the same time, it is possible to maintain high logic voltage levels on the output lines of the RTS and DTR ports, which allows them to power devices with a current consumption of up to 15 mA (per line). To investigate signals whose levels differ from those received in the RS232 interface, it is necessary to apply them through the appropriate matching devices.

Main Specifications

Number of inputs ........................4
log level. 1, B............+3...+25
log level. 0, V............+3...-25
Number of power outlets
devices under study and
sensors.......................2
Output voltage, V.........+12
Output load current, mA, not more than .........................15
Time interval measurement error, %, no more,
with their duration, µs less than 20 ....... not standardized
20 .......................... 50
100 ......................... 10
more than 1000......................1
The maximum frequency of the signal under study, kHz .............................. 180
Minimum sweep duration, ms .............................1
Maximum sweep duration, s, when displayed on a scale
1:1 ........................0,5
1:20........................10
1:100......................50

Suppose we need to investigate a discrete information signal transmitted by a serial code on one line and accompanied by clock pulses on another. The impulses are continuous, and the transmission of information occurs in advance unknown intervals of time.

Connect the information line to the DCD input (pin 1 of the port connector), and the clock line to the RI input (pin 9) and run the Digital_Oscilloscope.exe program. In the "Settings" field of the window that opens, on the "Port" tab (Fig. 1), select by name (COM1 or COM2) the port to which inputs the signals under test are applied. The "Address" column will display the standard base address of the selected port. If necessary, you can enter another address here (in hexadecimal format) by pressing the screen button next to the address value.

The program for the logic analyzer of signals at the inputs of the COM port
Fig. 1

In the appropriate column, we indicate the duration of the recording of signals ("Capture time"), necessary to solve the problem. Please note that only the moments of changes in the levels of the studied signals are recorded, and not their regular samples. Checking the box "Use TSC" allows you to count the time using the hardware timer available in all processors, starting with the Pentium I, operating at a higher clock frequency, which significantly improves the accuracy of time measurement. This has to be abandoned, unchecked, only on obsolete computers whose processors do not have such a timer.

On the "Trigger on signal change" panel, check the DCD item. This means that the recording should start from the first change in the logic level of the signal at this input, which in this case corresponds to the beginning of information transfer. The RTS item checked on the "Hold signals" panel means that the voltage of the high logic level will be set at the port output of the same name.

After pressing the on-screen button "Capture", located above the field "Settings", the program will proceed to waiting for the specified changes in input signals. In this mode, the background under "Capture" will turn red. When the trigger condition is met (in this case, the level changes at the DCD input), the recording of information in the computer memory will begin and continue for the time specified during the setup. If for any reason you need to stop waiting for the start of recording or end it ahead of schedule, you can do this using the on-screen "Stop" button.

The program for the logic analyzer of signals at the inputs of the COM port
Fig. 2

At the end of the recording, the time diagrams of the signals will be displayed in the program window (Fig. 2). By going to the "Oscillograms" tab of the "Settings" field, you can use the "Show signals" panel to set the inversion of some signals, if necessary, select the color of each curve, turn off the display of unnecessary signals (this will speed up the display of diagrams on the screen). On the same tab, you can select the display scale along the time axis, enable the scale grid with or without digitization.

For accurate measurement of time intervals, two markers are provided in the chart field (vertical lines of red and green colors). They can be moved either by clicking the right or left mouse button on the diagram, or using the sliders below the diagrams. The position of each marker on the time axis and the interval between them are displayed numerically to the right of the sliders.

It should be taken into account that when recording a very large amount of information and choosing a large scale, the computer's RAM resources may be exhausted, about which a message will be displayed on the computer screen.

The program is used at the Department of Radio Electronics and Computer Engineering of the Yelets State University for research and adjustment of infrared receivers and transmitters, and also (together with the Hall sensor) as a digital tachometer.

Literature

Pashix. Hardware programming: RS-232.
Teixeira S, Pacheco K. Delphi 5. Developer's Guide. - M.: Publishing house "William", 2000.
Microsoft Windows SDK. - Microsoft Corporation, 1985-2007.

The logic analyzer program, including its Delphi project and the smPort driver, can be downloaded hence.

Author: V. Timofeev, Yelets, Lipetsk Region; Publication: radioradar.net

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