How does the software controlled brightness on the Classic work?

[obsolete]

I was daydreaming a bit this morning and wondered how Apple implemented the software controlled screen brightness on the Mac Classic, so I had a look at the Bomarc schematic. It shows the "PBL" (Programmable Brightness Level) signal originating from BBU pin 49 through a 47ohm resistor, then passing through pin 9 on the motherboard connector to the analog board. The schematic notes the PBL signal as a 22.25 kHz square wave (same as Hsync).

I'm guessing that the duty cycle of this signal is varied to control the brightness? Has anyone ever taken a look at it to verify whether that's the case? I think it's interesting that the signal is generated by the BBU, which originated in the Mac SE, which did not have software controlled brightness. Maybe the capability was always there, or maybe the BBU was flexible enough the Apple was able to hack the feature in later?

I took a look at Bomarc's SE schematic; it shows pin 49 on the BBU as "MOT." and shows it passing through one of the Bourns filters and connecting to pin 10 of J6, the external floppy port, where it's labeled "PWM Motor Control." Aha, that's the answer. The Classic doesn't have the capability to control 3 floppy drives, only two, so that freed up a PWM output on the BBU that Apple repurposed for brightness control. In the process of writing this thread, I answered my own question. Hopefully someone enjoys reading it or the information is useful at some point.

 

[Snial]

I was daydreaming <snip> "PBL" (Programmable Brightness Level) signal originating from BBU pin 49 ["MOT"] through a 47ohm resistor, then passing through pin 9 on the motherboard connector to the analog board. The schematic notes the PBL signal as a 22.25 kHz square wave (same as Hsync). <snip> passing through one of the Bourns filters and connecting to pin 10 of J6, the external floppy port, where it's labeled "PWM Motor Control." <snip> The Classic doesn't have the capability to control 3 floppy drives, only two, so that freed up a PWM output on the BBU that Apple repurposed for brightness control.<snip>

I never thought about it before. I guess I just assumed an output port had been created, but Classic Mac hardware was very minimalist, so this is very insightful. There's no possibility of a second FD slot on the Classic, so a dual internal floppy Classic is impossible, and only an external floppy is possible: hence 2 FDs. So, there's a direct relationship between the single possible FD slot on the case and the software brightness control. Cool!

 

[Arbee]

All of the later all-in-one Macs (Classic II, Color Classic, LC520/550, etc) have dedicated brightness and contrast PWMs at $50F18000 and $50F18001 respectively. That's on my list of things to emulate.