The TI-99/4A PEB flex-card “tune-up”.

The fellow at mainbytes.com has had an article up for the past decade that recommends that few TTL chips in the flex-card that connects the TI-99/4A console to the PEB should be replaced with CMOS equivalents, and swap a few old electrolytic capacitors while you’re at it.

The theory is sound. CMOS logic 0 and 1 are rail-to-rail (as opposed to TTL’s “well, anything over three volts or so is logic 1”). The signals handled by the replacement chips should attenuate less as they move the meter or so from the console to the PEB interface card. The devil, though, is in the details, and this particular devil is the obsolete logic family that he recommends.

He states that the 74LS244s be replaced with 74HCT244s. That was good advice in 1982, when the HCT family was released. However, the fanout isn’t improved and the propagation delay is doubled.

I recommend instead that they be replaced with 74ACT244 units. Fanout is quadrupled and the propagation delay stays the same as with the LS units, but draws about a quarter of the the power. ACT was introduced in the mid-nineties, if my memory is correct.

I’d also go one step further and replace the LS245s on the flex cable, and all of the LS244 and LS245s on each card in the system, with their ACT counterparts. The whole point of this exercise is to buffer the address and data lines; the original instructions buffered only the address lines, and only to the PEB physical bus. TI recommended that both address and data be buffered on each card as well, so these revised instructions follow TI protocol.

Definitely replace the capacitors, though, as they’re going to leak if they haven’t already.

If you decide to replace the chips on the cards as well, you should also replace the positive voltage linear regulators (7805 and 7812, typically) with new switching equivalents. I don’t have part numbers off-hand (there are a lot of fabs making switching drop-ins for the 78xx series), but replacing a 7805 with 0.5A switching equivalent makes a significant difference in the heat generated by the card.

(I sent portions of the above to the maintainer of the mainbyte site, with no response. I suppose that’s all that can be expected from a decade-old website …)

About Chris Kobayashi

I'm a security systems engineer, specializing in UNIX, network, and physical security. I'm in Tokyo, and I'm mostly retired now. I'm well-versed in both electrical and software engineering, with a particular interest in old computers and game consoles. You can contact me here.
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