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I have a Z80 which is misbehaving and want to check the signals. However I do not own an oscilloscope, so I'm looking for another way of checking the square waves on the pins.
I basically just need to determine if it is outputting any sign of life, I'm not interested in the bits per se. Is there a way?
If you got some electronic parts, then you can make a circuit that makes an LED that gets brighter with frequency.
Link to schematic.
In the simulation I am using a frequency sweeper as the input to see how the circuit behaves to different frequencies. As you can see, the higher the frequency, the brighter the LED gets.
This one won't really care if it's square waves, triangle waves or other forms of waves. As long as their amplitude is above 1.4 V and above 1 kHz, then you should see the LED light up.
If you increase the size of the 1 nF to something larger, then the LED will light up with lower frequencies.
The transistor isn't magical, it won't make the LED burn up. The 1 kΩ resistor in series with the LED will limit the current.
If you have very few parts, then you can remove the 1 µF, 10 kΩ resistor and the diode pointing to the right. But if you do that, then the LED might be too dark.
You could also remove the LED, the 1 kΩ resistor, the NPN transistor and connect the 10 kΩ resistor to the ground so it is in parallel with the 1 µF capacitor. You can then measure the voltage across the 10 kΩ resistor which might be easier to read rather than the brightness of an LED.
That circuit I just described is almost an envelope detector.
This is the circuit I'm talking about.
Here is the circuit I'm proposing, black on white. Not hidden behind words.
Possibly use a logic probe
There are usually three differently-colored LEDs on the probe's body:
Red and green LEDs indicate high and low states respectively
An amber LED indicates a pulse
There is a table with some typical specifications on this website.
I initially copied the table in, but then noticed a copyright notice. The table gives typical max frequency of 20Mhz, however the first one I found in a search on an electronics supplier website stated that its logic probe went up to 50Mhz.
As HandyHowie mentions a logic probe is a good, cheap, tool to have in your arsenal.
Another quick trick is to make yourself a little high pass filter with a capacitor and a resistor.
simulate this circuit – Schematic created using CircuitLab
If the signal is DC you should not see any AC voltage on the meter.
Also check for Logic Analyzers.
These devices can typically analyze 8 channels at the same time, and use the PC for viewing/setting up, connection is via USB.
I have a Chinese one, for about 5 euro and works remarkably well. I'm using it more often than my very old oscilloscope. But the logic analyzer only can be used for digital (TTL) signals.
Check to see if your DMM has a frequency counter mode. If it does, you might be able to use that function to check the signals. If the counter says 0, you probably don't have any output. If the counter is much greater than 0, you're probably okay.
For square waves up to a couple KHz (eg what you would get on the higher-significance address lines of a bus):
Feed the signal to a LED, take a small mirror, and shake it like a fan.
Feed it to an amplifier and loudspeaker.
Take a monostable multivibrator, add a led+resistor to its output. Let the pulse length to be long enough to be noticed, say 500 milliseconds.
You can also use a CMOS D-flipflop which is connected to reset itself through a RC lowpass filter (R=470kOhm, C=1uF) but that's misusing the IC => do not use that method in design.
The input of the monostable multivibrator or D-ff is a proper logic input. In addition, sparse sub-microsecond pulses can be detected. Many pulse detectors, which are based on a rectifier+transistor amp to drive a led, charge a capacitor, which can cause overloading the signal and sparse short pulses are left unnoticed.
Overload in a bus signal throws the program out of the rail, connecting the probe is equivalent with computer instruction GOTO HELL.
I haven't seen that suggestion: Arduino Uno can be used as cheap (USD$20-25) signal recorder and generator. Just hook up your source to the analog input, accumulate readings, and print result through serial port. I think it should work up to few Hz
All you need is a pair of wires, and a USB-B cable.
