With advice from Tom Lafleur and others I've been making some progress on my Altair restoration. This machine was killed by a lightning induced power surge many years ago. I've installed new, modern power supplies, and I've removed all the chips from the front panel and the CPU. These were replaced with sockets. I learned that apparently, you can't always replace 74xx with 74LSxx. I learned this when the system clock would not run when I tried two different 74LS04 chips in the clock circuit but works fine with a 7404. Anyway, after repopulating the chip sockets with mostly LS parts, the Altair exhibits very strange and unstable behavior. I almost don't know where to begin to describe it. Let's just say that the results of a reset are fairly random. I thought I'd ask a few specific questions: If I "stop" the Altair and check the voltage of an address line on the bus that is high, I get about 3.4 volts. The only boards in the system are the front panel and the CPU. In this configuration, all that's going on is the 8080 address lines are buffered through 74LS367Ã¯Â¿Â½s onto the bus. The front panel is involved only insofar as the address lines each go through an LED and a 220 ohm resistor to ground to display the address line's state. Hard to imagine a simpler situation. The output from the 8080 is a healthy 4.9V where it goes into the 74LS367, but on the output side it is only 3.4V. This low voltage value for a high logic state seems like a potential problem to me. Am I right? Lest you think it might be a bad 74LS367, be aware that I previously had the functionally equivalent 8T97 chips in there and had essentially the same result. Also, the CPU and front panel regulators were replaced and I get a healthy 4.96V on the +5 side of the regulators. So the immediate questions are: 1. Am I right in saying that the 3.4V level is an issue? 2. If so, any ideas what could be causing this? 3. I'm attaching the oscilloscope trace of the system clock as seen on bus line 49. Does that look OK, or is there too much ringing? CPU Schematic here: http://www.s100computers.com/Hardware%20Manuals/MITS/8080%20CPU%20Board%20Sc hematic.pdf Thanks for your ideas. - Eric Osman -----Original Message----- From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com]On Behalf Of Eric Osman Sent: Saturday, February 02, 2013 12:48 PM To: n8vem...@googlegroups.com Subject: RE: [N8VEM-S100:1357] New/Old Project Douglas - Thanks for the introduction and summary. I'm on Andrew's list for an extender, and I'll probably be looking to obtain a prototyping card as well. These are driven by my initial goal, which is just to get my Altair working again. Longer term I'll be looking to enhance it a bit. I was intrigued enough by the Raspberry Pi to get one, but of course, that is not the focus of this board. I'll be looking over the Wiki's and mail group archives that you mentioned. Thanks again. - Eric -----Original Message----- From: n8vem...@googlegroups.com [mailto:n8vem...@googlegroups.com]On Behalf Of Douglas Goodall Sent: Saturday, February 02, 2013 10:52 AM To: n8vem...@googlegroups.com Subject: Re: [N8VEM-S100:1355] New/Old Project Eric, Our group consists of lots of old-timers and youth as well. Apparently all of us like to fiddle with hardware, and there is quite a lot of fun going on. The major interests within the group are focused on a range of hardware, as simple as a single board computer (See the Zeta), and more sophisticated buss oriented systems, both S-100 and ECB. Andrew supplies bare circuit boards for us, mini boards, SBC's, and a highly integrated machine we started calling the N8 (originally named "Home Computer"). John sells S-100 bare boards, CPU cards, memory boards, ... There are two main Google mail groups, one for Andrew's focus (n8...@googlegroups.com) and one for John's (n8vem...@googlegroups.com). There is another one recently formed for the scsi to ide project, aka S2I. Information about the boards, schematics, board layouts, etc are found on the wiki (n8vem-sbc.pbworks.com). Building these boards is a learning experience, and we gain knowledge about sourcing parts, building up boards and then debug them. The community members are very happy to help each other get things working the google groups are a constant stream of questions and answers about aspects of the hardware and software. There are a number of different BIOSs written by community members, some of which are more specific and some of which are more productized and full featured. If you want to find out more about the boards, look under board information on the wiki. There is a software information section as well. Welcome to our community, and don't be shy to communicate with us via the lists or privately. Regards, Douglas Goodall On Feb 2, 2013, at 1:23 AM, Eric O <ewok...@gmail.com> wrote: > Andrew Lynch suggested I join this group and seek assistance with my "project". > > Background: > Back in 1975 I was a 20 year-old electrical engineering college student and electronics hobbyist and saw the famous Popular Electronics article on the Altair 8800 computer. I ordered it, assembled it and it worked great as soon as I powered it on for the first time. Over the next year or three I enhanced it with some additional memory, a homebrew parallel and serial interface and the Processor Technology video card. I wrote hand-assembled machine code to "boot load" my own little monitor via a modem to the mainframe computer on campus. This involved an automated log-in to my account, starting the listing of a hex file and then capturing and loading that hex file into the Altair RAM. Of course I had to switch a couple hundred bytes of machine code into the Altair whenever I needed to "reboot". I also wrote a terminal emulation program so I could then use it as a terminal to that same mainframe. Great fun and done on a shoe string because I was a very poor college student. > > Disaster literally struck out of the sky one day around 1979 when a very powerful thunderstorm hit and a lightning bolt literally blew the top off the power pole that fed the off-campus house I shared with three other students. I should have unplugged the Altair when the thunderstorm arrived, but I didn't want to have to take 15 minutes to reboot it. Stupid! Anyway the power surge killed the machine. It would still light up but it wouldn't do anything approaching normal operation. I did replace a number of the chips in the weeks that followed, but I couldn't afford to do a proper job of it. > > Well, graduation came, then a job, then an IBM 5150, and then other computers over the decades and now the Altair has been stored in a box for almost 35 years. I always meant to fix it someday but never got around to it. But now that I'm semi-retired from a career in computers I'm finally getting around to it. So a couple months ago I finally got it out of that box and started doing a bit of research and I'm so happy to see all the love that people have for these old machines. > > One of the first things I learned was not to trust the original power supply. So I went out and got a couple switching power supplies from MeanWell, mounted them up in the chassis, and leaving the old supply physically in place, removed it electrically and replaced it with the new supply. > > I popped out all the boards, and turned it on. I'm getting all the proper voltages in all the proper places, including regulated +5.13 on the display board. With the CPU in I get the proper regulated voltages on the CPU card: (-5.25 on Pin 11, +11.69 on pin 28, +5.00 out of the regulator). > > I've started working on the front panel and I've already identified two inverters with the same logic state on each side of the gate, on two different chips. So I know I need to replace those. > > Any and all suggestions welcome. > > (I've seen the very good article at http://s100computers.com/My%20System%20Pages/Debugging/Debugging%20for%20beg inners.htm) > > Eric O > > -- > You received this message because you are subscribed to the Google Groups "N8VEM-S100" group. > To unsubscribe from this group and stop receiving emails from it, send an email to n8vem-s100+...@googlegroups.com. > For more options, visit https://groups.google.com/groups/opt_out. > > --- Douglas Goodall, http://goodall.com Note: I don't use messenger, or skype, or facebook, chat programs in general. Having always-on open communication links through massive public servers I don't have control over seems like too much of an invitation to be infected by a virus or bot. It is bad enough that my Mac wants to stay in periodic contact with Apple's cloud. Skype was tempting before Microsoft bought them. There have been too many examples of remote session links being abused by vendor employees. Even "back to my mac" makes me nervous. There was a recent episode where Apple cooperated with a social engineer and compromised someone's entire electronic persona. If you want to speak with me, calling me on the phone works well, and you don't have to wonder if the electronic mail got through or not. When I say "Hello, this is Doug", you know who you are talking to. Just in case you were curious. -- You received this message because you are subscribed to the Google Groups "N8VEM-S100" group. To unsubscribe from this group and stop receiving emails from it, send an email to n8vem-s100+...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out. -- You received this message because you are subscribed to the Google Groups "N8VEM-S100" group. To unsubscribe from this group and stop receiving emails from it, send an email to n8vem-s100+...@googlegroups.com. For more options, visit https://groups.google.com/groups/opt_out.
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