"RUN and LOAD switches S1 and S2 in Fig. 5 control the operation of the computer. With both switches set to OFF, ~LOAD is +5V and RUN is at ground potential. This resets the 1802." Note the tilde, suggesting ~LOAD is active low. m ----- Original Message ----- From: "dwight" <dkelvey at hotmail.com> To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at classiccmp.org> Sent: Saturday, April 23, 2016 1:28 PM Subject: Re: COSMAC Elf switch panel using PCBs I looked at the schematic pdf and it looks right. The ground lead is the NC. Can you tell us what page reference you think is wrong or confusing? Dwight ________________________________________ From: cctalk <cctalk-bounces at classiccmp.org> on behalf of Eric Smith <spacewar at gmail.com> Sent: Friday, April 22, 2016 5:06 PM To: General Discussion: On-Topic and Off-Topic Posts Subject: COSMAC Elf switch panel using PCBs I built a new Elf switch panel, but this time I used two printed circuit boards for the switches and the bezel. The bezel PCB has white soldermask with black silkscreen. The next revision will have black soldermask with white silkscreen, and the legend font, weight, and positioning changed to more closely match the original Elf photo in Popular Electronics. https://www.flickr.com/photos/22368471 at N04/sets/72157667455777465 The 20-pin header has the same pinout as Bob Armstrong used for the Spare Time Gizmos Elf 2000, but I don't presently have an Elf 2000 to test it with. For now the main intent is to use the panel for a new version of my FPGA Elf. I'm not sure whether I got the wiring of the LOAD switch correct; the Elf 2000 documentation refers to normally closed and normally open contacts of that switch, but for a toggle switch that doesn't make any sense to me. If anyone can tell me which pins of the Elf 2000 connector are grounded when the load switch is active vs inactive, that would be appreciated. The 20-pin header should have been right angle; since I only had a vertical header on-hand, the ribbon cable had to be plugged in before the switches were soldered in place, and the switches are not flush with the switch PCB. The toggle switches and push-button switch are C&K 7101SDV3BE and 8125SDV3BE, respectively, which have 0.42 inch actuator, 0.28 inch threaded bushing with keyway, vertical PCB mount with V-bracket, gold contacts, chrome actuator finish, and nickel bushing finish. These particular C&K switch variants are not very common, so I'll probably use different ones in the future, without the V-bracket. I don't yet have enough of the red and white toggle caps, which are C&K 896803000 and 896801000, respectively. The red button for the push-button switch is C&K 801803000.

Lf it were mine to make, it would be a spring return momentary SPDT. NC would be ground and NO would be +5V, as per the schematic. I'm not sure what the manual says about it. It is debounced with a jam latch, as per schematic. Tinker Dwight ________________________________________ From: cctalk <cctalk-bounces at classiccmp.org> on behalf of Eric Smith <spacewar at gmail.com> On Sat, Apr 23, 2016 at 11:28 AM, dwight <dkelvey at hotmail.com> wrote: It's an SPDT toggle. Neither the NC or NO pin of the switch should be tied to ground; it's the common pin that's grounded. So for a toggle switch, which side is NC and which is NO? Anyhow, apparently if I'd read it more carefully, it is explained in the manual.

On Sat, Apr 23, 2016 at 5:09 PM, dwight <dkelvey at hotmail.com> wrote: That would be fine for the Elf2K, but I want to also use the same switch panel for a different design that uses a single-ended input only, so I need it to be a true toggle rather than momentary. If it was momentary, it would have to be momentary-off-momentary, so it still wouldn't really make sense (IMNSHO) to refer to the contacts as NC and NO.

On Sat, Apr 23, 2016 at 8:18 PM, dwight <dkelvey at hotmail.com> wrote: Which still doesn't explain how a toggle (not momentary) can be said to have NC and NO pins. But at this point I'm flogging a dead horse. Yes. Elf2K uses both contacts with an S-R FF. My other system is VHDL in an FPGA and has a counter-based debouncer.