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I wrote some firmware this morning for the 1704, it uses a fixed-length loop that gets the clock in the ballpark (359.2 kHz, says the scope) if not as accurate as the 1501 with its NCO. Like the 1501 firmware, it also does 2x oversampling and has a LUT in case some manner of compensation is...

Status update. So, a few iterations later, I have some firmware for the PIC12F1501 that's about as densely packed as I can get it. Besides generating the clock and both RxD+ and RxD- (so no RS422 IC is required), it does 2x oversampling and has a 512-position lookup table that can be selected...

Just a tiny bit more... the spec sheet for the PIC12F1501 says typical supply current is 1.09 mA, max is 1.7 mA.

Put up some two-packs of these for sale for $7, BYO pigtail:

A little more story... I went hunting for trouble and asked in ##electronics on Libera about how I could improve the shape of the waveform being output by the 2N3906, and we determined that the 2N3906 I was using either was defective, counterfeit, or not a 2N3906. Weird. At @demik 's...

Aha! Switched out the 3.3 KΩ resistor with a 470 Ω resistor and the waveform actually dips below zero for a reasonable time now. The waveform is still kind of pointy at the bottom, but it works well enough to send a test sine wave to SoundEdit. Thanks @ymk ! I'd be happy to hear any...

Tried building and running this circuit with a test PIC (not the 12F1501 yet) and it doesn't seem to be having quite the desired effect: The scope is reading the value of HSKi from the collector leg of the transistor, the logic analyzer's HSKi line is reading the value of RA2. The fast rise...

Hmm... so would this work? Here, RA2 on the PIC is generating the clock signal, and RA5/RA4 are outputting opposite signals (so a true negative voltage isn't necessary there). I feel like some protection circuitry is necessary in case TxD+ goes negative... a diode of some kind between TxD+...

The important bit of the patent appears to be: The transformer I'm using has a max interwinding capacitance of 20 pF, and a max DC resistance of 0.8 ohms, no word on the voltage isolation or the number of turns or the wire gauge. As @cheesestraws pointed out before, though, the fact that...

If we're taking power off TxD+ and TxD- like the real MacRecorder does, we have a positive and negative some-number-of-volts rail (on my Classic they appear to be +3V and -2.5V). Not sure how to interpret your schematic, does it have three power supplies?

Boards are here and working! I've got some IDC connectors coming in for the DE-9 version. The pigtail pictured is one I just had lying around, though, I need to figure out a source for them...

Two serial ports - SoundEdit has been designed with this capability in mind in case you bought two MacRecorders. From a technical perspective, it's certainly possible to interleave samples from two channels, albeit at half the rate, but as you say, the MacRecorder software and drivers weren't...

Got my MacRecorder! Its protocol is really pretty simple, I was just being thrown for a loop by the fact that my SE wasn't playing nice with either the real thing or my test PIC... my writeup is here. Notably, the clock isn't at 3.58 MHz like I expected, it's at 358 kHz, which puts it in the...

There's this one: I'm not sure how well I understand where V+ and V- are coming from... but I'm finding this approach potentially interesting, especially since it seems like a complicated/expensive undertaking to generate -5V from +5V.

I feel like we're straying from the topic a bit here, and I'm afraid I'm confusing the issue by talking about RS-232/422/423/485. To clarify, what I'm looking for is a circuit that I can connect to a 4 MHz (or so) 0V/5V square wave output that will turn it into a -5V/+5V square wave (exact +/-...

Because of the way the 26LS32 (receiver) in the Mac is wired for HSKi, with V- connected to ground, unless I'm very much mistaken, V+ has to go below ground in order for the Mac to register it as a logic low. Unfortunately, none of the driver chips I've seen actually do that. Makes sense, as...

Probably, except all the RS-422/423/485 drivers I've been able to find don't actually generate negative voltages, they just generate ~0V/~5V for high and ~5V/~0V for low. Technically "differential" but not useful for the single-ended HSKi signal. It does look like this chip generates negative...

(And the fastest RS232 transceiver ICs I can find on Digikey top out at 1 MHz.)

That'd be a great solution except for the speed involved. Assuming my assumptions about how the MacRecorder works are correct, there needs to be a clock of about 4 MHz fed to HSKi, and the MAX232 series seems to top out in the kHz range.

Snagged a MacRecorder off of ebay for surprisingly cheap! Hopefully this'll put me on my way to sniffing the protocol. The problem with the HSKi line remains, though. A couple people suggested it could be done with a couple of FETs, though I'm not sure that knowledge is enough for me on its...