Beta testing of the XT-CFv2 has uncovered some compatibility and reliability issues that will take time to fix, so in the mean time – and conscious of the long development time already of these boards – I’ve turned my attention to making a simpler, more home-assembler friendly XT-CF derivative.
Surface-mount (SMT), especially the 0.5mm pitch components, seem to have put off home-assemblers, and then there’s the need to program the CPLD. Though SMT can’t be avoided because of the CompactFlash header, using SOIC chips but still in the same PCB footprint (and ISA slot bracket), things can be made a lot easier.
So the XT-CF-Lite – here it is!
What Is It?
Just like the XT-CFv2, it’s a self-contained ISA disk device, functionally like a hard-card, providing solid-state, bootable storage to any PC with an ISA slot – right back to the original IBM PC 5150.
This board provides the same basic functionality of the XT-CFv2 – a 32K (accessible) flash-based ROM (with 24KB available for any user purposes), XTIDE Universal BIOS support and the port-based IO transfer mode used by the XT-CFv2 by default. But there are some limitations, because the CPLD on the XT-CFv2 provides a wealth of logic space not present here, so the lite version misses out on BIOS port auto-detection, memory-mapped IO, DMA, slot-8 compatibility, and reduced-wait-state operation for PC/AT and above.
Usually a buffer chip like the 74LS245 would be used between the ISA bus and the media, but I took a chance that one isn’t needed, because the CF card is directly connected (i.e. not cabled) and has 8mA output drive strength. And luckily, it seems to work!
That’s SMT – wasn’t is supposed to be easier to make?
We can’t get away from SMT and still use CompactFlash – through-hole sockets do exist on datasheets, but are near impossible to find. Compared to the XT-CFv2:
- 0.5mm lead-pitch flash chip has been replaced with an easy through-hole DIP package, and the 100-pin CPLD has been eliminated altogether
- 1206 package resistor networks have been replaced with a through-hole network
- SMT 7400-series chips have 1.27mm lead-pitch and are pretty easy to solder
And of course there’s no CPLD programming to do once it’s assembled. The activity LED and driver IC (IC6) can be left-off if need be. The tiny 0603 package capacitors and resistors remain but are also quite easy. The hardest part is the CF header, but locating points on the header hold it in position whilst it’s soldered, and the trick is to use plenty of flux and flow solder over the pins generously, then use solder wick and more flux to clean up the job.
With the official XTIDE Universal BIOS, it will do about 150-180KB/s on a PC/XT. It’s possible to increase this about 20% by switching the BIOS to 16-bit IO cycles, but at the expense of system compatibility. 16-bit port IO is faster because it reduces the instruction count and off-loads some work to the bus interface unit (BIU), but in development of the XT-CFv2 it was found that some clones have errors in BIU logic resulting in byte-swapped data delivery.