Mainly, of course, it's cost, but, I know of no available FPGA's that still have 5-volt outputs, so that's a limitation in one sense. Another is the high pin count.

The CoolRunners (do you mean the XPLA3 parts, or the CR-II's) won't work with/as 5-volt hardware so one's probably stuck with the older 9500 series from XILINX (Now, the 9536 might just have enough registers ... ) or the equally aged 7000-series from ALTERA. It happens I like to use the latter, as they're readily avaiable for $2-3 for the 128 macrocell verisons on eBay, where I've repeatedly gotten them. The fastest parts I've gotten there, for cheap, have been the 7 ns versions in 128 macrocell 84-pin version. If you want enough pins, sadly, you have to use the 84-pin variety, since the PLCC-44's only have about 32 I/O's. You need a bus and a few controls, (nCS, nWR, nRD, and you need, in addition, about 24 I/O's. I think most of 'em come up a couple of pins short. Now, with the larger devices (I don't know for certain that this applies to the smaller ALTERA parts) you can program the JTAG pins to become I/O's, though this renders them JTAG-incompatible until erased in a parallel programmer. I think the PLCC-44 still comes up a mite shy of the 8255-compatible pin count. (I'll look into that ... <sigh> ...)

The old MACH-2 series from LATTICE is 5-volt compatible and comes in a 68-pin version that might be a mite more suitable. They're no longer readily available as surplus, though.

A 64-macrocell part ought to be suitable enough, though, if one can squeeze the right arrangement out of the PLCC44. With multiple power and ground pins, and with dedicated clocks, enables, etc, it's easy to trip.

One might be able to get by without the nRD input, however, since nCS in the absence of nWR would imply nRd. I guess it depends on how you do things.

RE