Finding the service manual for the SM124 at https://www.eserviceinfo.com/downloadsm ... SM124.html
Most Pi VGA HATS will be driving the sync signals from the 3.3V GPIOs, so are a bit low for TTL compatibility. 5-1.5 = 3.5V, so 3.3V is technically too low. VGA uses 0.7Vp-p signals, but that is unlikely to be critical.
The "Negative" reference would more normally be interpreted as inverted than actually taking negative voltages.
There looks to be part of the schematic missing between sm124_1.jpg and sm124_2.jpg.
HS is fed into a SN7406 TTL inverter, so that signal is definitely TTL.
VS appears to be fed into more analogue circuitry in the missing bit, so it's hard to say exactly what is going on.
What video mode are you feeding to the SM124? Spec says it is horizontal refresh of 35.7kHz and vertical of 71.2Hz. That's not a standard PC mode, and you will often get whines if the rate is outside of tolerance.
TTL is 0V and 5V signalling (with tolerances).Input signal: TTL signal
Video: 1V p-p positive
Audio: 1V p-p
Vertical drive: 5 +/- 1.5Vp-p Negative
Horizontal drive: 5 +/- 1.5Vp-p Negative
Most Pi VGA HATS will be driving the sync signals from the 3.3V GPIOs, so are a bit low for TTL compatibility. 5-1.5 = 3.5V, so 3.3V is technically too low. VGA uses 0.7Vp-p signals, but that is unlikely to be critical.
The "Negative" reference would more normally be interpreted as inverted than actually taking negative voltages.
There looks to be part of the schematic missing between sm124_1.jpg and sm124_2.jpg.
HS is fed into a SN7406 TTL inverter, so that signal is definitely TTL.
VS appears to be fed into more analogue circuitry in the missing bit, so it's hard to say exactly what is going on.
What video mode are you feeding to the SM124? Spec says it is horizontal refresh of 35.7kHz and vertical of 71.2Hz. That's not a standard PC mode, and you will often get whines if the rate is outside of tolerance.
Statistics: Posted by 6by9 — Wed Jun 25, 2025 10:39 am