There is a site called
www.jameco.com that is selling a 100 piece assortment of microprocessors , PALs , EPROM, RAMs, PROMs. Do you think its worth it for 12.95?
I don't know what devices they're selling but YES
UncleBinary said:
I need a single board system with a mini keyboard,screen,at least 8k of EPROM and 8k or SRAM,and maybe a spare parrelel port.Any suggestions for parts from jameco?
The easiest way to get a screen is to use a UART (I found the Intel 8251 to be easy, with some example code on z80.info) and attach it to a regular PC running a terminal program such as Kermit or Hyper Terminal (which I hate).
For RAM and ROM, you might as well implement 32K ROM and RAM, A because it's easier and B you'll have more space.
It's easier because you can attach an inverter (74LS04, pin 1 for input and pin 2 for output) to Address 15 of the microprocessor, the inverted signal can be the Chip Select for the upper 32K of the 64K address space, the non inverted signal is the lower. There might also be some other logic necessary if you're going to use a Z80, because they have separate address spaces for I/O ports and memory which also need to be decoded.
In this case, this is what I think could work. Get yourself a 74LS32 Quad OR gate. Attach the output of one of the gates (say pin 3) to the RAM and another (say pin 6) to the ROM. Attach the Memory Select pin of the Z80 to one of the inputs on both of the OR gates (pin 1 and 4). Attach the previously mentioned inverter's output to the remaining input on the RAM's OR gate (pin 2) and Address 15 of the Z80 to the final remaining pin on the ROM's OR gate (pin 5).
What you have now is a circuit that will enable either the RAM or ROM when the Z80 wants to address memory space, not I/O space.
Why an OR gate? Because all of the signals in question are negative logic, meaning that when the CPU or memory or whatever makes/sees the signal as a binary 1 it is at ground potential, not +5v. This isn't insane because it can be useful in certain conditions, which I won't go into here. De Morgan's law is something like an an OR gate can be made a AND gate if the inputs and the output are inverted. Because we want to AND together the Memory Select signal and the Chip Select signal (Memory AND the chip must be selected before it's enabled) we need to use an OR gate. If we used an AND gate we would have to invert both of the inputs and the output, because of De Morgan's law.
If you want I can post you a schematic that will select I/O devices too.
Whew. Hope this gives you something to work with.
Thrashbarg.
EDIT: I didn't actually answer your question. RAM is 62C256 (or a similar prefix, make sure you get static parallel RAM) and the ROM's I use are 28C256.