PALs (was Re: MITS 2SIO serial chip?)

--- ajp166 <ajp166(a)bellatlantic.net> wrote: It depends on what your limited resource is and how costly _that_ is. ISTR PALs were $2-$10 each c. 1981-1982. If someone remembers

please supply more accurage data. Yes, a single PAL cost more than

TTL popcorn chips, but given how much you could fold into a single PAL in the way of I/O select logic (a common use on the board I first

PALs on), it wasn't so expensive then. Or if board space was a fixed resource, then it's doubly costly. The oldest example I can cite from personal experience is the COMBOARD-I to COMBOARD-II design. Both were early examples of 68000 designs (an intellegent Unibus serial periperal)... one with SRAM (2114s) and TTL logic, the other with DRAMs and PALs. One similarity - about 1.5 sq ft. of board space to cram in a CPU, RAM, support logic, a sync serial port (based on the COM5025) a parallel port and Unibus DMA logic. In 1981, our designers went with older tech, but the DMA engine was horribly complicated from the standpoint of the 68000, but it was in part because there wasn't enough room on the board for a better design with TTL. The second revision (c. 1983-1984) had more onboard I/O, and implemented the DMA engine as a bank of shared memory to the 68000 (i.e., read/write to a certain range of memory from your code and it

generates a Unibus DMA cycle - most cool). I don't know for a fact, but from what I remember about the designs, I don't think a shared-memory DMA engine would have fit on a Unibus board if it had been made out of

logic. I didn't see PALs in use until the early 1980s. Yes, I know they came out in 1978, but products designed with them didn't hit right away.