Address Decoding for Dummies! (was: SPDT Dip Switches...

On Sep 22, 13:11, Roger Merchberger wrote: Well, not unless you use "more interesting" voltages :-) In of like and sure I Let's see if I'm interpreting this correctly. I think you want to be able to decode addresses, but sometimes you want to be able to provide (let's say) two "enable" outputs, each active for a separate group of addresses (maybe two different ranges of four addresses each), and sometimes you want to be able to provide more finely selected enables (maybe responding to only one address in each range). I can see why you'd want to that if one day you are using a UART that needs four addresses and another time you use a a PIO or VIA that needs 8 or 16 addresses. A couple of ideas that come to mind are using a PROM or PAL to do the decoding. A PAL or GAL would probably operate faster than a PROM, but it might be easier to program a PROM (depending on how used you are to building tables for PROMs, logic rules for PALs, and what programming tolls you have access to). For example, suppose you have a PROM with 8 address and 4 data outputs. You could fill it with data such that certain data outputs were at the appropriate "active" level if (and only if) 6 of the address lines were in a certain state, ie were driven with a certain address(es). By tailoring the data appropriately, you could arrange it so that the outputs took account of all 6 address bits, or just (say) four). The other two address bits I'd use as a function selector: if both are low, the output is active for a range of addresses (only 4 bits significant); if one is high, respond to the first two addresses in the range; if the other is high, respond to the upper part of the range; if both high, respond only to the highest single address in the range. Or something like that, to suit the job in hand. Depending on the exact requirement, you might still be able to do this with a MUX or two. A multiplexer is a very useful general-purpose logic element, widely used as a circuit building block, not just for the obvious "if the clock phase is high, select the refresh/video addresses; if low, select the CPU addresses" sort of operation. you and tried I use an assortment. Often I use the drawing package from RISC OS on one of my Acorn machines. From time to time I've used 'pcb' on a Unix system. board That's encouraging. Completely home-made PCBs seem to be a relativley rare occurrence in the US, at least compared to here. Over here, almost every secondary school, college, and university makes PCBs and there are still magazines publishing designs which enthusiasts photocopy onto film (OHP acetates, aka viewgraph foils for you colonials) and then apply to photoresist copperclad board. In the USA, as far as I can see, most people tend to pass the artwork or plotfiles to professional PCB houses to be made up. Lots of people do that here too (I'd probably not try to etch my own board for a really densely packed device using PGAs, for example) but there's still a strong tradition of using your own ferric chloride (not in the wife's best stainless steel sink, though). While there's nothing at all wrong with either method, I'd caution anyone trying it, that layout software (or dry transfers) tends to be suited to one or the other but not both. For the professional house, you are more likely to want nice round pads and not too many different sizes. You can use thin tracks, and pads with fairly small margins of copper round the holes. However, for truly homemade, handetched boards, there's usually more variation in the quality of the image transferred to the board, so very fine tracks are not such a good idea. Moreover, they won't normally have plated-through holes, so it's wise to use larger pads which won't come unstuck too easily -- often oval pads for ICs, which have a relatively large copper area yet are still narrow enough to leave a gap to get tracks between the pins. Lots of PCB layput software isn't capable of doing anything other than simple round (or square or symmetric octagonal) pads and is therefore much less useful for home-etched PCBs. No offence intended Roger, but I think your IC pads on the CoCo board are smaller than I'd personally be happy with for a home-etched and -drilled board without plated holes. (See http://www.dunnington.u-net.com/tmp/coco_pcb.jpg for a modified fragment) BTW, did you notice the tracks going through the pins on pins 13 and 14 of the leftmost LS244? -- Pete Peter Turnbull Network Manager University of York