per
Veteran Member
I finally got probes for my scope, and I then decided to test it on something computerized. I soon realized that my XT was the best target.
I didn't want to constantly hold the probe while tuning the scope, so I decided I wanted to meassure the output to the PC speaker. I attached the probe, as you see in image 166, and started the machine. When DOS came up, I entered Debug, and turned on the speaker output by typing "O 61 4B". The scope obivously started to display something else than the usual flat line. When I ajusted the x-sweep frequency of the scope, it looked about similar to image 163.
Then I started changing the values of the counter, as image 165 shows, and the frequency changed as expected. I tried to load values like 0001h, but I didn't got any clear ressult then, even my scope is able to read such high frequences. The value of 0002h gave ressults, as you see in picture 158, and note how round edges the waveform has! I also tried frequencies of round values, like 20Hz. See image 164 for this. Here you also note that the wave is not really square.
So why isn't the wave squared? There may be several reasons for this. First of all, it can be a fault in the scope, but it can also be something with how the IBM PC/XT handles the speaker output. After looking at the schematics, you can see that the signal is first run though a TTL logic AND gate, then through a 75477 IC, this signal is filtered by capacitor C53 (.01), and finally run through resistor R6 (33Ohm) before reaching the speaker connector. Maybe the capacitor is the cause of the strange waveforms, or maybe the 75477 doesn't have a clear squarewave (especially on high frequencies). I don't know.
I didn't want to constantly hold the probe while tuning the scope, so I decided I wanted to meassure the output to the PC speaker. I attached the probe, as you see in image 166, and started the machine. When DOS came up, I entered Debug, and turned on the speaker output by typing "O 61 4B". The scope obivously started to display something else than the usual flat line. When I ajusted the x-sweep frequency of the scope, it looked about similar to image 163.
Then I started changing the values of the counter, as image 165 shows, and the frequency changed as expected. I tried to load values like 0001h, but I didn't got any clear ressult then, even my scope is able to read such high frequences. The value of 0002h gave ressults, as you see in picture 158, and note how round edges the waveform has! I also tried frequencies of round values, like 20Hz. See image 164 for this. Here you also note that the wave is not really square.
So why isn't the wave squared? There may be several reasons for this. First of all, it can be a fault in the scope, but it can also be something with how the IBM PC/XT handles the speaker output. After looking at the schematics, you can see that the signal is first run though a TTL logic AND gate, then through a 75477 IC, this signal is filtered by capacitor C53 (.01), and finally run through resistor R6 (33Ohm) before reaching the speaker connector. Maybe the capacitor is the cause of the strange waveforms, or maybe the 75477 doesn't have a clear squarewave (especially on high frequencies). I don't know.