The Lo-tech ISA CompactFlash kit makes it possible to boot a vintage PC from a CompactFlash card in a straight-forward through-hole soldering project by means of a cheap CompactFlash-to-IDE adapter (like this). Whilst there’s long been interest in a similar SD-Card storage option, CompactFlash has always been just so much easier since it was designed for the ISA bus and supports an 8-bit transfer mode natively.
The development process for the (forthcoming) Lo-tech 8-bit IDE Adapter meanwhile involves testing as many different drives as possible, and it was perplexing that an SD-Card-to-IDE adapter just wasn’t detected, given that all the real hard-drives so far tested worked OK. This got me thinking.
In hacking the XT-IDE Universal BIOS (XUB) to work with the new 8-bit IDE adapter, I’ve simply re-used the existing XT-CF ‘BIU’ controller type (designed for CompactFlash and so making use of the 8-bit transfer mode) and bypassed the error checking on the 8-bit mode set command. This allows the disk to stay in 16-bit transfer mode (because generally 8-bit transfer mode isn’t supported by real hard-disks) and leaves the magic to the hardware on the 8-bit IDE Adapter. This works OK, but… what if the SD-to-IDE adapter actually supported 8-bit transfer mode?
In that case, the BIOS would set 8-bit transfers, then only ever retrieve half the sector data interspersed with random garbage, since the logic on the new adapter would be storing the high 8-bits which we’d just configured the device not to present. So could the SD-to-IDE adapter work on the ISA CompactFlash adapter?
The SD-Card adapter I was testing was based on the FC1306T chip, and the answer is right there in the datasheet: it’s actually an SD-to-CompactFlash adapter, “Fully compatible with CFA (Compact Flash Association) Standard”, which of course includes 8-bit transfer mode.
FC1306T based SD-Card to IDE adapters support 8-bit transfer mode
Performance wise these run at about the same speed as a basic CompactFlash card – around 200KB/s in an otherwise stock PC/XT. The slot-mount type need some care in mounting as they have PCI profile brackets, with the card mounted on the other side. I found it could be bent a little for use in an IBM Portable PC 5155 slot 8, providing externally accessible SD card storage and using a slot of otherwise limited use.
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