I prototyped this on a solderless breadboard ( ptooie) as shown above with 5% resistors, so the actual voltages aren’t quite nominal. The scope shot shows the gate driven with a 500-Hz square wave, which is about the right width for the programming pulse. Q3 selects the output voltage: gate high = 5 V, gate low = 12.5 V. That simplifies a lot of other things about the project, too.
#2716 eprom programmer verification#
The shortest VPP pulse is a millisecond long, so that’s not a real restriction, and the verification can happen at nose-pickin’ speed. The CE/VPP supply looks like this: Schematic of VPP-VCE pin supplyĪn Arduino will drive the gates of Q2 & Q3, with all the programming logic and timing handled by software.
This being an entirely one-off project, I used good old LM317T regulators with a handful of transistor switches to vary the voltage and clamp the output to ground. Both supply voltages must switch between three levels: unnaturally high during programming, 5 V for normal operation, and 0 V for output-enable and during chip removal / installation in the programming socket. The VCC supply must cough up a staggering 90 mA during normal operation at 5 V and 30 mA at 6 V during programming. Normal EPROMs in 28-pin packages have separate pins for all those functions to make life easier.Īnyhow, the CE/VPP supply must provide 30 mA at 12.5 V as well as the usual minuscule FET logic currents at 5 V and 0 V. To make a long story somewhat shorter, the 27HC641 is a 8Kx8 EPROM in a 24-pin package with the usual 12 address lines, 8 data lines, power, ground, and a single chip-select / output-enable / programming-voltage pin. There are other ways of getting the job done, but this has the advantage of getting me some face time with my role model for being a Renaissance Man. That means we need a device programmer, as none of the burners we have know anything about 27HC641s. Being a stickler for authenticity, Eks picked up some 27HC641 chips. The original board used Mostek MK36400 8Kx8 masked ROMs, but they can be replaced by either 27HC641 or (in a pinch) a quartet of 2716 EPROMs. He verified that by swapping in a memory board from another 492 and found it worked perfectly.
My buddy Eks recently acquired a “guaranteed broke” Tektronix 492 spectrum analyzer that turned out to have a defunct memory board: the ROM holding the initial boot firmware has a bad checksum.
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