31 Dec
2008
31 Dec
'08
3:40 p.m.
Jules Richardson wrote:
* A drop-in replacement for Acorn's TUBE ULA would
be nice - I used my
last remaining spare a couple of years ago, and still have a couple of
devices with dead ones - swapping in from a working unit is a real pain.
Hmm, shouldn't be too hard with modern programmable-logic devices.
Or four FIFOs, for that matter. 24 bytes, 8 ways (4 host-to-parasite and 4
parasite-to-host FIFOs), plus a couple of status registers, with an operating
frequency of 3MHz or so. Not exactly groundbreaking (though the Tube ULA DMA
stuff doesn't seem to be documented anywhere obvious).
If you went discrete-logic (i.e. 74LS chips and FIFOs) you're going to need
rather a lot of ICs, and a fairly big PCB. Probably about ?100 worth of parts
(the cheapest FIFOs I can find are ?5 each, times 8 is ?40, plus the PCB, and
some 74LS chips).
Truth be told, the documentation is ambiguous enough that I'm not even sure
all the FIFOs *are* 24-level deep...
A CPLD or FPGA version should be doable, probably at around the ?30 mark.
Small FPGAs run about ?5 (or less), but nearly all of them use +3.3V I/Os, and
very few of them are 5V-tolerant.
--
Phil.
classiccmp at philpem.me.uk
http://www.philpem.me.uk/