Let's try to get the SWTPC 6800 computer working

well hello everyone and welcome back to adrian's digital basement this is part two in the south west technical products corporation 6800 computer system video series it's sitting right here on the bench behind me and boy is that name a mouthful to say i'm just going to call it the southwest 6800 computer or southwest or anyways yes if you haven't seen part one i really recommend you watch that first it's sort of an introduction to this computer which me personally i pretty much knew nothing about when i got this thing so watch that first and in this video i'm going to dive into seeing if i can get this computer even working completely unknown the state of it i have no idea if this thing works or not so let's try to get this thing up and running and see if i can we'll do anything with it so without further ado let's get right to it [Music] first let's talk about the bare minimum that's going to be required to get this computer working i do this because it's totally unknown the condition of all these cards so it's best to eliminate any that we don't need to have in here in case those are going to cause an issue besides the chips on the cpu card need to work properly and let me just plug that into the front here the console board needs to work as well but so do these chips on the motherboard and i'm a little worried that a couple of them on here have been socketed so perhaps there have been some issues with this address decoding as well luckily these all appear to be normal 74 ls logic except these two ics here which are 88 35 so maybe those are some kind of bus transceivers or something like that 74 ls logic would be easy for me to find but those on the other hand if these are bad i'm gonna have to order some online it looks like some of the other boards use 88 series chips as well this is a ds-8833 here from national semiconductor and on a floppy card here there's some 8602s and up here we have some amd 9324s you might be asking like what's going on with all these weird part numbers back when this machine came out there was no standard for ttl logics which is what we consider 74 ls stuff now 74 ls existed but all these competing ones also existed they're all ttl so they can talk to each other and in a lot of cases the the ics are actually interchangeable with each other like there will be an equivalent part from one of the other standards but not always and i think on my apple 2 repair video i ran into a case where i had a bad bus transceiver on the motherboard and i had to build a little adapter to use a 74 ls part in place of whatever the original part was because the pin out was not compatible so i'm just going to keep my fingers crossed right now that these special parts are working and this machine is going to just fire right up after this long okay so for this serial console card is my understanding that it needs to go into this slot here which is slot one and there is a key by the way on all of these slots so you can't accidentally plug it in the wrong direction because of that key and even these serial connections here have a little key on them as well and actually before i plug in the motherboard power connector i'm just going to leave that off to the side here i'm going to plug the mains power cord in here and i'd like to use the oscilloscope here on the bench just to make sure that those unregulated supplies on there aren't too ripply so let's power this up there it is we have a nice glowing neon light inside the power button there which sort of flickers but that's that's okay that's like a character and if i touch this right here to that pin 100 millivolts per division and we're barely seeing anything that is just really really good and then right here on the power connector there's two more unregulated outputs we have this one here which is plus 15.2 volts and this one minus 15.1 so that means that these diodes here are working properly and incidentally i used 35 volts rated caps originally they were 25 volts but plenty of headroom on those two caps to do filtering there on those legs now i'm just curious if i shut the power off here minus 15.1 when i turn this off is it falling we're still getting minus 15.1 well are those caps like so capacious that they're able to keep providing that power or is this power supply working even when the power switch is off let me pull the cord out of the wall no cords out of the wall and we're still getting 15.5 volts these two nichikon caps i guess have such low esr that they're just able to keep putting power out okay let's check this uh these other rail here uh yeah it's 8.59 and it's not really falling either so all of this here doesn't have a bleed resistor on it so even when the power is off with no load it just sits there well i don't feel like connecting the motherboard to this power supply even though it's off right now so i'm going to connect this 8 ohm resistor across this large cap here so let's see if that should do the trick yes we are almost to zero there and let's look at the minus 12 here and it's been a few minutes but we are still at 15.1 volts there on the minus 12 and yeah 15 on that one as well so let's use this big resistor here and drain this down all right so first i'm going to add a little deoxit here because uh this connector is very crusty just because this computer has not seen the best conditions we'll plug that in and actually before i power this up i just taken the two cards out i'm gonna power it up with the motherboard connected i'm just gonna check this voltage regulator right here all right getting 5.13 so that's good let's turn that off it drops very slow but there we go dropped pretty quick okay back in with the cpu card now these front slots are not specific it can go into any one of them they're not mapped to like a particular slot number like these back ones now what i'm going to do is take the oscilloscope and i'm just going to hook onto this first pin right here which according to the legend on the front of the case which you can't see that is data bit 0. and let's hit the power and take a look at the oscilloscope and let's see if we see activity uh that doesn't look good that's not what i would expect to see let's try data bit 1. that is what i would expect to see what is going on on data bit 0 though oh yeah okay so i don't think this computer is gonna work i'm gonna power the machine on and i'm just gonna go through each one of these pins with the probe here so there's zero which is definitely not working one is good two is good three is not looking good i mean that might be fine it's pretty low four looks normal five is normal six is normal seven is good as well and now let's check out the address lines i'm just going to go through them all really quickly and point out any that look bad all right these are what i found that looks suspect so far let's start with uh databit zero so there it is it never goes above that and i assume there's supposed to be highs there i mean i'm positive there is so something that's holding that low and then here's data bit three and it looks very similar to zero where it's sort of stuck low all the time and when i hit reset kind of drops away for a second and zero will do the same thing here's address line 15. it's always stuck high it's possible the cpu is only ever writing to stuff in the higher part of memory because of course the cpu card has that static ram on it and the rom which are all up in the upper half of memory which means i think address line 15 will always be high but then here's address line 13. now that doesn't look right notice how it's kind of like dropping down here and that just doesn't that doesn't seem right so i think 13 is bad and then 12 is always stuck down here at ground and when i reset the machine it does go high but i feel that the driver on the cpu is probably not working because that should be going all the way to high so i need to look to see if there's buffers on this card here the cpu card or what's going on exactly because clearly something is is wrong there all right so luckily there's good documentation for this machine and the schematics are available but luckily it's such an easy design here i don't really need them to to know what's going on exactly so these pins right here or on the motherboard here are the data bits and remember i saw that data bit 0 and 3 looked screwy well these two chips here they seem to be buffering all of the data bits right there so there could quite well be a fault on these and these are ds-8835n all right so there it is a tri-state quad bus transceiver doing four bits at a time for each one of these cards so we're seeing that data bit 0 is looking like it's not working what i need to do is figure out on here this is data bit zero right there goes to that pin i need to find the equivalent pin on the cpu and we'll take a look at that on the oscilloscope one of the cool things is because i can stick this into any of the slots here then i can probe the cpu quite easily with the oscilloscope here so i can just get to the address lines and the data bus lines right on the chip without it being totally inaccessible like in a chassis alrighty so there's the cpu on screen i have the cpu pin out as well so why don't we probe pin 33 here all right so there's data bit zero now you notice it's up here at four volts that's fine but you see this part here that's in the middle that could be floating that just be the bus could be floating right there the cpu could be trying to talk to for instance the card that's on here like the serial card and maybe it's just not getting back what it expects and that's the floating part and then these are low it's being driven low by the cpu or one of the other parts on this board let's zoom out a little bit yeah so that could be normal floating let's take a look at the other pins that's zero there's one yeah see it's doing it as well so that's just floating i think that's normal that's two that is three so three on the cpu looks totally fine but three on that bus transceiver like if we go down to the uh bus down here so let's see here's pin zero oh wait that looks fine now wait a second what could it be a bad connection on this motherboard i mean i have it in a different slot now yeah okay so zero is fine uh one two and three and three is fine as well okay i guess we had a bad connection there well with that said let me go through and check all the other connections now all right we're on address line 15 and everything looks high but i remember i said that that could be completely normal and address line 13 now looks completely normal as well look at that and address line 12 was the other one that was problematic and that one looks perfect as well so everything appears to be working here just when i said that these connections these connectors on the motherboard were robust and not troublesome it was acting up but you see a little troubleshooting by just checking the signals on the card showed me that indeed actually things were working now one thing i can do to try to help the situation is take a little bit of sandpaper and just lightly sand down all of the pins on these connectors here just to kind of rough them up and get some of this oxidation off i think that will help the situation a little bit i'm gonna go do that right now all right took a little bit of sandpaper here and i just went over the pins on the main the 50 pin connectors and then this one right here for the console card just to clean that off and then i used uh some deoxit on a little cotton swab and just wiped off all the the grit that was left over and yeah a lot of uh gray oxidation came off hopefully that fixes the situation okay cpu board back in i'm gonna put it into one of these sockets further away here so i can access it like that and then let's plug in this serial console card see if we can get this thing coming up okay i'm probing data line zero let's turn this on again we have activity folks this is exciting and the nice thing is we see some changing activity on the data bus lines that would imply that the cpu is actually doing something like it's running some code or something like that well on the back of the machine here is the serial port and the lab pc actually has an rs232 port on it so i'm going to connect right up to this so we're not even going through a usb thing and i'm going to connect to com1 300 baud let's hit open and let's see what happens ah we have nothing i'm gonna shut the power off and i see that it's connected to pins one two three and there's a couple things that are looped on there and if we take a look at the pin out right here pin one of course is ground which matches up with what i see happening here and that is definitely the ground on this little card and pin one is transmit and pin two is receive let's let's use the oscilloscope here and we'll hook onto these pins and see if we see any characters being transmitted when i turn on the power all right so we're getting about negative 10 volts on here even though the system is off right now and that is just coming from those capacitors there so when we turn this on okay it definitely went low so it was these two pins here that were connected let me see if i can find the schematics for this little board and see if we can see how this is supposed to work okay a little googling around i found the manual for the mpc serial control interface and here it describes what each of those pins do on the connector so r0 is the rs232 output ri is the input voltage activated input which conforms to specific is the input to use when trying the computer system all right and on the board here it was r1 that i was seeing i don't know negative 15 or so volts but r zero i wasn't seeing anything on and usually with rs-232 i think you should be seeing a negative voltage on both and basically the computer on one end will drive it to ground and then the other computer will do the same to transmit back okay then it goes on to say how to configure the card for rs232 operations so you need to have a jumper between the t1 and the tc pins so with this thing connected so a jumper between t1 and tc okay that is done okay you also need to connect the c1 or ci pin to the co pin ci to see oh okay those are looped all right and scrolling down here here are the schematics that i was looking for so let's see the ro looks like it goes between plus 5 volts and minus 12. all right so the multimeter is here let's power this on first i'm going to check that r0 what are we getting on r0 r0 right here oh we are getting negative 14 volts and the other one r1 or ri okay this is the one that is not getting anything now wait a second uh let's check the schematics okay our eye is right to here so i gotta try to find on this page so that's this it goes into ic5 i have to look that up it's probably like an open collector all right with the power back on our eye is uh oh it's still zero you know what maybe the whole time the problem is my connection from the bench pc maybe that's what's going on here i'm pretty sure this is actually connected and working but maybe it's not so one thing i know for sure though is ro is the output so i'm going to bring up the oscilloscope again which we've obviously lost connection to power it off and on there we go yes okay there we go working system okay power it on all right do we have any activity here so this should be pulled up to uh i don't know like five volts or something when the computer tries to when this thing tries to transmit and i'm assuming when it powers on or hit reset it should do something so i'm going to put single shot on again but there is absolutely no activity i just realized of course that one of the problems might be i have it in the wrong slot like i'm assuming it goes into this slot because i think i've seen pictures where it's in this slot it doesn't actually say in this manual where to install it but it does say here that you have to finish the motherboard first so let's go check out the motherboard assembly instructions maybe it tells us where to put this card and in the motherboard installation instructions here's the information on the i o ports remember how i said that these slots here were mapped physically like each slot is mapped into the address space and i think we can see that right here i o port 0 and i o port 1 control interface and that would indicate slot 1 right here and then there's 2 3 4 5 6 and 7 which you can see are all of these different i o ports so wherever you plug a card in it's going to be mapped into the equivalent i o port space here so yeah control interface definitely it goes into slot one but either way it does appear that this card's not doing anything and the only thing i don't know for sure is that when you turn on this computer do you just get like a little prompt or do you get nothing until you push enter that's what i'm not positive about okay so what do i know so far this is in the right slot the voltages seem to be okay on here so i started taking a look at the schematics for this control board now there's a couple chip select lines that go to the 6820 here there's the i o line and i did probe that with the oscilloscope and it's definitely being selected and that's the the select logic on the motherboard but there's also this enable pin here which goes with the clock phase here so this this input right here phase two and when i looked at that pin on here which is going to pin 25 i didn't see anything at all nothing so over on the motherboard schematics i love how this has like a bunch of handwritten notes on here whoever built this thing uh so this is what's coming from the system bus here or the cpu card right here and it goes into ic5 here and that comes out it goes to some other stuff and it gets buffered again and that generates that phase signal on this bus and i'm definitely not seeing anything it's on the connectors or on the card so there's something going on right here now let's start right here at ic5 now when we scroll down we can see which ic is what because it's not marked on the board ic5 is this one right here and it's this one right here in this socket so i have the multimeter connected to that phase two clock on the lower part of the motherboard where the cpu card is should be going to pins two and three on this chip right here now on the motherboard if i touch it there's two there's three we're definitely good but on the chip itself it's good on pin three but it's not on pin two and that's really crusty and corroded looking and that socket might just be bad or the chip is bad so i'm gonna pop that chip out and we'll take a look at it so i don't think necessarily the chip is bad but the socket might be especially just because of corrosion or whatever okay all right so it's looking a little crusty on the pins here so why don't we use the sandpaper which i had handy and we'll sand this down a little bit these fiberglass brushes can also come in handy just to try to polish things up a little bit so this is looking a little better the pins are a little mangled now because i've bent them up on the sandpaper let's pop a little deoxit into the socket there looks like it's got gold contacts in there this could be a problem under the motherboard as well i've tried to polish these pins as much as i can the chip is just in really rough shape now let's check the continuity yep okay there we go so we're getting continuity now i might as well remove this chip here and do similar process at least put deoxide on the socket yeah it's looking a little crusty as well all right i sanded the pins a little bit on this pop this back in here just checking that i put them in the right way all the notches are facing that way that's good okay back to the schematics for the serial card so i'm looking at pin 25 on this board right here so one two three four five okay i'm on it and i'm going to turn the power on alrighty now we're seeing that signal there so i'm going to clip the oscilloscope right here on the output let's see if we see anything i'm going to single shot this and we'll turn this on okay there we go everyone it is working when i hit single and we hit that we see it there it is so let me go back a little bit we'll hit single again i'll hit the reset button look at that i don't know if that's actually like what we should be getting i don't really see enough activity there oh wait no there we go i forgot this is like 300 baud so it's super super slow so i'll single shot again there we go everyone look at this i think this is actually working right now oh okay let me uh get to turn off the power i'm going to unhook the scope and we'll get the serial cable again which i'm not sure this is actually working because i still wasn't seeing the correct input voltages but i don't even know what the serial port on this computer does exactly okay power on okay we got nothing all right so this is the serial cable from the back of the computer and i can confirm that pin three on here is actually at like minus 12 volts or so which makes sense because pin three on the computer which is the input is also well it should be driven down to that by the computer right now what i don't know is that when i plug this adapter in i don't know if this adapter works because i don't even know where i got this so let's check what's happening on this so i should see pin three low and you know what i don't i see pin too low so it's almost like this adapter here is a no modem adapter and an and it converts from nine pin i really forgot my serial basic while recording the video there are two types of serial equipment dte or data terminal equipment or dce data carrier equipment it's basically the terminal is your computer and the carrier equipment is the equivalent of the modem that's back in the day when modems were supplied from the phone company if you're going to connect two dte devices together basically two computers you're going to need a null modem adapter we go back to putty and we load com1 at 300 bits per second and we open and we turn the power on oh yes look at that we got something let's get the keyboard to hit enter on this okay we're not getting anything here why is that what character are we seeing i'm gonna change the font make it a little bigger and easier to see okay we're seeing an asterisk and you know what that's what the manual said we're supposed to see when we powered on if we hit reset we should see it again and we do okay transmit is definitely working but when i hit keys on the keyboard i am not getting anything all right i've loaded up the old terra term program here this thing is ancient for like when did this 1998 oh no 2019 wow the font that's using that looks like it's a windows 3.1 app all right let's see the serial port settings here we go 300 bits per second i know it's eight uh bit no parity one stop bit if you're running at 300 baud here all righty let's uh turn the computer off and let's turn it back on again okay we got a star oh m m zero zero zero zero all right this is showing us ram this is actually working i can't believe it okay so there's no ram installed down there um so how do i exit out of this i think i push a random key no space i am shocked that this is all working all right back on the motherboard pdf we have ram right here at 8000 and then we have the rom here at e thousand so let's take a look at that rom here e thousand there we go i guess this is the rom here this is what we're seeing now obviously i can i can't just type stuff in here to change it because this is the rom right but i certainly can in that ram now i don't know what part of the ram is being used so if i hit a space does that exit out yes it does okay m and let's look here a thousand eight thousand okay so 89 i mean i don't know what a bite i want to change in here because you know could break this thing okay well anyways i need to look at the manual to see how this machine language monitor actually works but but for sure this machine's working it's taking commands we're able to look at the rom and the ram it's freaking working so far the only problems with it wow was that dodgy socket that i just had to deox it and then we had the problem where the slot down here just wasn't making good contact with the cpu card so when i took it out and moved it here it just magically started working so that is a testament to the reliability of this old stuff this machine is from 1977 it was left out in the moisture and whatever that made this transformer go bad and other than those two capacitors i had to change out this thing freaking works it just freaking works that is so exciting that is so unbelievably exciting well that's going to be it for this video i am so excited to get this computer working and there were no actual faults it was just dirty contacts which isn't really that surprising considering how much kind of filth was inside that machine when i opened it up and clearly it's been exposed to the elements and that doesn't do good things for the contacts especially the ic legs and stuff like that but luckily with a little sanding or a little deoxit or a combination of both you can get things working again without having to replace any components and that was definitely the case on this machine so in the next video i'm going to be looking at the rest of those cards that came with this thing to see if those ram cards work and the disk controller and of course there's another parallel port card as well you can't really do very much with this thing without those ram cars because all it's got right now is i think 128 bytes of ram which is just enough for the monitor rom to run but what is kind of cool is that there are some memory tests that i saw online that can run inside of that 128 bytes so you don't need functional ram to actually check that your ram cards are working well other than that 128 bytes which appears to be working properly so get subscribed so you don't miss any future updates on this machine next up i want to thank my patrons their support means so much to me and you see their names scrolling up the side of the screen if you want to become a supporter or a patron you can do so at the link in the description below i'd love to hear from any viewers who have used one of these machines in the past i posted part one early access to my patrons and i had several people comment that they had one of these machines when they were younger or used one in university or there are some of my patrons that actually still own one of these machines although they've been in storage for so long they have no idea if they work or where they are really are anymore so yeah really like i said in part one i had honestly never heard of this machine i never saw a picture of it and i didn't know that there was a lineage of 686809 machines that all stem back to this computer so just very very cool anyhow thanks very much for watching thumbs up if you like seeing all the usual youtubey stuff and that's gonna be that so stay healthy stay safe and i will see you next time bye [Music] you

Channel: Adrian's Digital Basement

Views: 74,271

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Id: R5rhDzP0srk

Length: 28min 58sec (1738 seconds)

Published: Sat Apr 09 2022