Computer Controlled Frequency Counter/Logic Probe

This circuit is a stable frequency counter accurate to 5 significant digits. The range is 0 - 30MHz with an input sensitivity of greater then 100mV. The probe connects to the PC serial port. So by using the crystal oscillator already present on your PC serial card and software calibration, the Probes' external circuitry is kept to a minimum. Probe 9 can also be used as a logic probe/analyzer using included software (LPROBE92.EXE).

Schematic

Parts

Part Total Qty. Description Substitutions R1,R2,R3,R4 4 100K 1/4W Resistor R5 1 10M 1/4W Resistor R6, R7 2 3.3K 1/4W Resistor R8 1 390 Ohm 1/4W Resistor R9 1 1M 1/4W Resistor C1, C4 2 0.1uF Ceramic Disc Capacitor C2, C3 2 100uF 16V Electrolytic Capacitor D1 1 1N4148 Signal Diode Any 200mA silicon signal diode D2, D3 2 3.3V Zener Diode D4 1 6.2V Zener Diode U1 1 74HC00 Quad Highspeed NAND Gate U2, U3, U4 3 4021 8 Stage Shift Register U5 1 74HC393 Dual Highspeed 4 Bit Counter U6 1 4040 12 Stage Binary Counter MISC 1 PC Board, Wire, Suitable Probe, DB9/DB25 Connector

Notes

1. Email Richard van Wyk with questions, comments, etc.
2. The software to use this probe can be downloaded using the following link. Note that this software is compiled for Intel x86 platforms and runs under DOS, Win95, Win98 and WinMe. It does not run under any Windows version based on NT including Windows NT 3.51, WinNT4, Win2K, Win2K3, WinXP and Windows Vista. This is because NT based operating systems do not allow direct hardware access.Download Probe Software, Zipped, 19K
   
3. SETPROBE.EXE is the frequency counter calibration program. To give accurate readings the Probe must be calibrated to your PC. SETPROBE.EXE calculates the constant error correction factor for the particular PC serial card the probe is to be used on. The frequency counter corrects for this slight constant error in crystal frequency by using the correction factor contained in PROBE.DAT. To calculate this correction factor, a reliable oscillator of known frequency (eg 2MHz Crystal Oscillator) is required. When CALIBRAT.EXE is run, the Probe will sample the frequency and then ask for the true frequency value in HZ. The frequency entered must be to 1 Hz accuracy (no decimal points) or an error will occur (for example "200123" not "200123.34" or "2003.421 kHz"). The program then calculates constant error correction factor and stores it to PROBE.DAT. Calibration is only necessary once.
4. LPROBE92.EXE is the logic analysis program . Logic states are displayed in real time. This program runs best under DOS (not a DOS window). The sampling speed is adjusted by using the left and right arrow keys.The three triggering modes are:
   
   • TRIG: Starts each scan (left-right of screen) on a negative going edge of logic signal.
   • KEY TRIG: Waits for a key to be pressed before beginning each scan.
   • FREE RUNNING: Not triggered.
   To Toggle between these use the UP / DOWN arrow keys. To quit from LPROBE press escape.
   
5. FPROBE92.EXE is the frequency counter program. The measured frequency is displayed in Hz with commas indicating KHz and MHz. To quit from FPROBE press any key.
6. Serial port pinouts are as follows.
   

   Line	9 Pin	25 Pin
   TD	3	2
   RTS	7	4
   DTR	4	20
   DSR	6	6
   CTS	8	5
   SG	5	7