At 18:26 04/06/2005 -0400, you wrote: Rich, Check the switches - I had an IMSAI panel which exhibited the same symptom on one address bit, and it turned out that the original builder had done a very poor job of soldering the switches, and a cold solder joint was causing the switch not to pull the bit down. An easy test is to set the suspect switch and all switches BELOW it to ON, and all switches ABOVE it to OFF - perform an EXAMINE, and the lights should match the switches (all LEDs above OFF, the suspect and all below ON). The do EXAMINE NEXT which should wrap to the next bit. The suspect and all LEDs below should go OFF, and the LED next up should go on ... If the syspect LED goes OFF, then most of the panel->bus interface is working, and the problem may be a switch or early-on buffer/latch. If it stays ON then you will have to look further. Also check/clean the S-100 connectors, and the "umbilical cord" between the panel and the CPU - poor connections here can cause individual malfunctioning front panel bits. Many S-100 CPU cards have a connector for the IMSAI front panel, so if you have other systems, it may be easy for you to try another CPU - keep in mind that there is usually a jumper on the card to control the WRITE signal on the bus being generated by the CPU card or the Front panel. If the above gets you nowhere, then it's just a matter of sitting down with the schematic and a scope and following the signal from that switch all the way through to the bus at some point you should see that the zero from the switch is not making it through a component or connector. I don't recall the details of the IMSAI panel, but you might have to trigger on the read strobe to catch the signal being clocked through the panel. Regards, Dave -- dave04a (at) Dave Dunfield dunfield (dot) Firmware development services & tools: www.dunfield.com com Collector of vintage computing equipment: http://www.parse.com/~ddunfield/museum/index.html