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Converting TI disk images to 80-track double-sided single-density (DSSD)

TI-99/4A The stock TI disk controller ROMs support 40-track double-sided single-density, which comes to a little over 180k per disk. The controller, as shipped, expects track 0-39 on side 0 and 40-79 on side 1.

If the ROMs are replaced with modified EPROMs, the capacity can be doubled. We can't do much about the single-density limitation, as the WD1771 controller didn't handle double density, but we can increase the track count per side to 80. There's a note on AtariAge that describes how it's done, but I can't seem to find the post at the moment. If memory serves, we change the code to increment the disk head used (side 0 to side 1) from > 39 to > 79. A simple but elegant hack. (Update: here is the post in question)

Tony Knerr (may he rest in peace) originated the modification. I've placed a zipfile containing the EPROM images and instructions on this site here. Pre-burned EPROMs can be purchased from ArcadeShopper here.

That, however, only solves half of the problem. This modification renders the drive unable to use 40-track DSSD images, and there are a lot of those floating around out there.

One solution is to use Fred Kaal's excellent TI99Dir program to create an 80TDSSD image and copy the files to it, but it's Windows-only (runs under Wine).

I wrote up the following code to convert files in batches on FreeBSD/MacOS/Linux. To use it, you'll need the very latest version (greater than 1.5.0) of xdm99.py from the excellent xdt99 tools. That is because we need to specify the 80TDSSD disk image geometry explicitly, and that support landed in the code only a few days ago.

I've uploaded the conversion wrapper (a simple shell script), along with a working version of xdm99.py, here.

I hope this helps others in the same situation. Note: this doesn't properly extract files with "*" prefixing the name. Everything else should be fine.

The TI-99/4A PEB flex-card "tune-up".

TI-99/4A 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 ...)