Exploring 1950’s Computer Logic with the Bendix G-15!

[Music] thank you hello and welcome back and today we're working on this big blue beautiful Behemoth right here this is a Bendix g15 and it is a full-fledged vacuum tube computer from the 1950s his on loan from system Source Museum up in Maryland and if you haven't heard of system Source go check them out immediately it is a bucket list level Museum that just has a staggering array of machines one of my personal favorites is the univac that is the size of a New York City apartment and if you uh play your cards well they might even let you get hands on with a few of them and if you do go give Bob CJ and Brendan and all the guys over at system source of high five for me because they're all absolute Legends but what are we going to do with this machine today well in the previous episode we kind of just took a cursory look so you know we're taking stock of where we were and what we what kind of mountains we had to climb and well before we can even start climbing those mountains we got to figure out what we're working with yes we know it's a Bendix g15 but think of this kind of like carbs from the 1960s a 1964 and a half Mustang is slightly different from a 1965 which is again different from a 66 which again is very different from a 67 and the Bendix is just like that every year there were new different changes that were going on things to fix problems that they maybe came across or upgrade the functionality and so we are serial number 238 and of the 400 that were made there were several different models and revisions and a rata that were fixed along the way and this one has been modified as we saw from some of the rewiring that was going on inside so we need to take stock of what it is we have we do know that this is a numeric machine but it very well could have been modified to become an alphanumeric machine and this is an incredibly important distinction to figure out because it determines what typewriter we can use with it and so that is the first question I want to try and answer is this numeric or alphanumeric and in the process we'll figure out what the difference between numeric and alphanumeric is and in order to do that I think we need to identify what cards we have and what each card does the manual only shows that there's about 11 different cards but I've already found one or two that are pretty special and unique from these 11 that are in the list here so I think in total there's going to be maybe 15 or 16 completely unique different cards that we're going to need to take a look at so we're going to slide this big boy out pull these side off these doors are just going to keep getting in the way so I'm going to pull these doors off so we don't ding them up we're going to pull every single card out make a list of where it's located within the door so we can then cross-reference that against the documentation we have and start to figure out what kind of modifications or where within that year Spectrum ours sits and then we'll figure out whether it's alphanumeric or numeric and we're going to go through all of it one by one all right let's wheel this big boy out and the new casters work great but a thousand pounds is a thousand pounds and that's just heavy no matter which way you slice it but let's pop open the side cover here so we can actually remove the cover and it's held in place with about 20 screws all down the edge the screws go through the hinge and door and there's a plate on the back that they thread into and after removing the last screw Freedom now we just got to set this somewhere safe so we don't damage it and next let's start the Journey of making a list of where every single card is all of the cards are actually keyed but this is good practice just in case and it allows us to learn a bit more about this particular systems layout once I've got everything written down it's time to remove every single card from the machine and I'm going to lay them all out so I can get a nice good look at them and uh don't don't pay any attention to my bare feet here I was maybe getting a little too comfortable in this room laid out before us is all of the logic in the Bendix g15 and I did this for two main reasons the first is that I thought it would look cool and it does it looks very very cool to see it all laid out like this but the second reason was that I wanted to get a sense of scale when all of these cards are plugged into the machine it's kind of hard to wrap your head around how much is actually going on but when you lay it out like this you can see exactly how many vacuum tubes are in use how many diodes there are on all of the diode cards over there it's pretty wild to see how dense that thing really is and how far it spreads out and also this isn't all of the vacuum tubes in the machine there's tubes up in the paper tape reader there's a bunch of tubes down in the power supply and around the rotating drum so there's a whole lot more tubes still left in the machine but the core functionality the core logic is controlled by these right here and my initial goal was to spread it out and go yay this looks awesome let's take a look at each individual card in more detail and we're going to do that but I think I'm going to clean these up and put them back in the machine first before we take a look at each card in detail simply because it takes up so much space I don't have room to set up a table to look at each unique car card in detail I'll start with cleaning the diode cards first and I'm just going to use a clean paint brush for this wiping all of the dirt and grime off of both the front and back of the cards and this was indeed quite time intensive but I found it to be pretty therapeutic and with all the diode cards clean I'll then move on to the tube cards and again I used a paintbrush for the PCB on the front and back then I took a rag and soaked it in water and used that to wipe down the handles and the tubes again quite time intensive but I just had the tunes rolling and jammed out while getting some good work done next up let's clean up the inside of the doors on the machine itself starting with a good vacuum and you can see there's quite a bit of dirt and debris collecting on the bottom of the doors here and a good bit of that came from the door seals which have deteriorated into a nasty mess we'll remove the remains of the seals and using a razor will clean up the worst of the goo these outer side panels still had seals all the way around but I'm already feeling way better with these cleaned off and we'll find a suitable replacement for these seals to glue back in in the future next let's wipe off the more stubborn dirt and I'm using Windex and a rag here as Windex is usually really safe to use on painted surfaces but the printed text is making me a bit nervous so I cut out some masking tape to protect the text so now I can clean a little more judiciously next we'll clean up the support rails that run horizontally and we'll follow that up with a wipe down of the separation panel between the tube and diode module then we'll try to clean up the seal area as best as we can by wiping it down repeatedly and make no mistake this machine is huge cleaning took quite a bit longer than I was expecting but it was absolutely worth it the machine is really starting to shine up wonderfully next we need to work on one of the door latches so I'll move all the screws holding the latch mechanism in place and you gotta be kidding me this piece was installed before the back plane and it's impossible to remove while the back plane is still in there so it just can never be removed again I'll just have to work on it in place which is terrible for filming so let's work on the latch hook itself then I'm going to spacer it out slightly so the door latches a little cleaner and now that it's all back together yeah that's working exactly like it's supposed to next up let's Trace out the modified wiring we'll plug an alligator clip onto one side of a modification wire and then we'll hook it up to one probe of our multimeter next I'll try to follow the wire over to the other side and once I have kind of a general area I'll just tap it with the probe to confirm a connection finally I'll just write down what connects to what so we have a full list of modification coordinates and surprisingly there weren't actually that many modification wires even though it looks like a lot it made tracing them out really not that bad it mostly just required a lot of walking from one side to the other and back again but here's a list of all of the modifications I'll put this list in the description below so if you're curious as to what's been changed you can check it out and now that we've cleaned the doors and traced out the modifications let's get all of these beautifully cleaned cards back into the machine and we'll start with the diode cards and I just did this by grabbing all of one type like the D1 card here and started populating them into the appropriate sockets I'll of course cross reference against my notes but I can actually see the keys in the sockets which makes it very easy to follow along and plug the d1s into the Right Slots and if you look closely in this particular shot you can actually see the key on the bottom of the socket and when plugging in just one type of card like this you can get a really interesting look at how seemingly random they seem to be placed this is all of course dictated by the logic wiring on the back side but it still looks quite interesting however once all the d1s are in place I'll grab the d2s and go through and fill in the spots and here it is with all the diode cards populated looking nice and clean and up next are the tube packages just like the diode packages I'll look at the keys and the sockets to plug the appropriate pack into the right place and I'll start with the CF ones which are the red handled tube packages these are by far the most common package used in the entire machine which is interesting because CF stands for cathode follower meaning that these are just buffers the large majority of logic in the machine is actually just buffering signals up next we'll start getting the gold packs in which are flip-flops then we'll do the blue packs which are buffer inverters finally we'll get the black and green packs in which are right and read amplifiers and there it is fully populated and looking absolutely gorgeous all right now that we've got everything clean and I can actually touch the cards without feeling like I need to wash my hands immediately afterwards let's actually take a look at what each of the cards does and we're going to start with the simplest card possible and that is the D2 card here and you can see it is dead simple it is just seven diodes and they're not actually connected up to each other in any way and then it has uh 210 K ohm resistors coming off of 100 volt that go to some outputs V and L I have absolutely no idea what these resistors are used for they're just there in case the designers needed them I guess but the diodes are going to be used to make or Gates sometimes you need a two input or a three input or a five or six input or a gate and having the diodes set up separately like this gives the designers the ability to kind of pick and choose and build whatever or Gates that they want now there are some interesting things about this card and the first is that it is a dual sided PCB but the connectors are not the cartridge connectors on the back plane are really only single-sided you only get a single post coming out of it so that kind of limits the total number of connections that you can have per card another interesting thing to notice about this card is that at some point in this card's life probably in the mid 60s it had one of the original diodes go bad and it was replaced with this one in 270 and that's really interesting because the one in 270 is new enough for us to find a data sheet on it which means that if we need to replace any diodes in the future on any of the cards we can potentially use this as a good starting point now moving on to the next diode card that I want to take a look at that's going to be D1 and now we're getting into a little more complex logic but not much more so we're just building uh diode and Gates so essentially you have plus 100 volts coming through a 110 K Ohm resistor and it goes through diodes to a low voltage but if you put a higher voltage into the back side of that diode it blocks the path to that low voltage and if you block all of the paths that it has the pull up resistor that 110k ohm resistor will pull up our output again a really pretty dead simple card and again we can see that this one has had two diodes replaced right here and right here and both of these are also one and two seventies now moving on to the next diode that I want to take a look at the next diode card that's going to be the D3 diode card and it it's actually just very similar to the D1 diode card that we looked at it's just building and Gates the only difference is that we have some input duplicated across multiple and gates for example input s here is feeding the top and gate and the third and gate down and input B is feeding the second and gate the third and gate and the fourth and gate so this kind of complex interaction between inputs and outputs on the and Gates gives a pretty interesting results coming out the other side and the designers can choose how they want to use that and we'll go on to actually our first tube pack that I want to talk about and that is the buffer inverter which looks like this little one right here now the two packs are differentiated by their handle color and this one is colored blue and it has three dual triodes on it and that's because there are actually two buffer inverters on this particular package other than that it's just a three tubes and a whole lot of resistors so what exactly is going on inside of this one well we've got the schematic right here but this schematic is a little difficult to parse because it's got two buffer inverters on it and actually if we look at the next package here this one right here this is a gold colored package and this is the Flip Flop and again it's got two flip-flops on a single package but again the the schematic is a little difficult to parse because there's a whole lot going on so let's move on to a different schematic here this is the schematic out of the theory of operation manual I'll put a link to this manual in the description below it's fantastic if you want to know more about the Bendix g15 but this shows the buffer inverter and the flip-flop and it only shows half of the package so it breaks it down to be a lot simpler and easier to understand starting with the buffer inverter we'll move that package back over there it's actually really pretty simple the input comes into a pretty standard inverting amplifier and then the output comes out of that into the input of a standard run-of-the-mill inverting amplifier the output of that feeds into the input of the other half of that triode and it is also an inverting amplifier so we've got three inverting amplifiers going on here where's the buffering coming from well if we take a look at the voltages we have a plus 160 volts coming in at the top here and then the two cathodes are tied together and they go to negative 160 volts through another 3.9 K ohm resistor but the such tiny resistors mean that the output coming out of there is going to have a lot of oomph behind it it's going to have quite a bit of current so that's their buffer they're not actually doing a cathode follower buffer type thing like we usually do on this package they do do that a little later and we'll take a look at some of those here in a second but let's take a look at the flip flop the gold package here and it's set up like a pretty standard flip-flop you've just got cross-coupled anodes to grid but the grids have an extra interesting connection that goes through a couple of diodes there is two diodes that connect together and go to the clock but that clock isn't going to do anything as long as our set or reset terms are not set so you can see the set and reset terms need an actual voltage signal on them and it's interesting because the little arrow here shows that voltage signal as being 0 or minus 20 volts and I say that's interesting because that's really all you need for a grid in general zero volts will take the tube into saturation minus 20 volts will put the tube into cut off so what Bendix does here is a really fascinating they diode clamp the outputs of every package to zero or minus 20 volts this does a couple of things it sets up the output perfectly for the input of the next stage but also the tube is actually swinging far past that which means that it's swinging past our zero or minus 20 logic higher low very quickly this allows them to get the speed of the machine even faster it's really brilliant and that diode clamping is not actually done on the flip-flop or the buffer inverter package so let's take a look at our next package here which happens to be the diode clamp package this diode clamp package just holds the two diodes that clamp it to zero or minus 20 volts and then it has the resistor that's coming off of our large B plus which would be plus 160 volts so it's a really dead simple package but that plus 160 volt resistor has to be pretty beefy so there's one more diode package that I want to take a look at and it is this one right here this is a pretty unique package this is the clock clamp package and I say this is unique because there's only two of these cards in the entire system there's one on each door but it's essentially doing the same thing you can see it's just a collection of diodes that are clamping the Reed clock the right pulse or the shift pulse or the clock pulse to a specific level now this card does have some electrolytic capacitors on it these two up top here that are gudmon or gudemon or however you pronounce them I think they're probably okay most of the capacitors in the system are this manufacturer or Sprague and Sprague was really high quality so I imagine that good mod is also decently high quality but these two electrolytics down here are very skunky looking they are really nasty so we're gonna have to replace those but fortunately we can see on the schematic here these are actually just smoothing caps so we'll just hop on Mauser and order the correct axial capacitors and get those replaced in the two cards that we have because all four of those are really skunky uh now we're going to move back to a tube package next and I mentioned that they did actually have some cathode followers going on this is the cf1 package so cathode follower one you can see that there are four cathode followers on here the anode of each triode is tied directly to plus 100 volts the cathode goes to -160 volts through two 30k ohm resistors so there's only 15K of resistance between the cathode and -160 volts that's why these uh 30k resistors here are very beefy but that's also only for these top two cathode follower buffers up here the ones below it are a little different they've got 24K ohms so they've got a little more resistance on there so they aren't doubling them up and then we just take our outputs directly off of the cathode this is straight up a plain Jane cathode follower buffer I use a design very very similar to this on my vacuum tube computer that I'm building only I do it at much lower voltages but that's pretty pretty simple except that in some cases that's not quite enough so they actually made a unique package called cf3 and if we bring it over here you can see it's pretty much identical to the cf1 except that the cathode resistors are much larger physically they're actually smaller themselves they're only 15 000 ohms and well you can see there's eight of them so they're paralleled that'll knock it down to seven and a half thousand ohms each well it's half of what it is on the other one now there is one more type of cathode follower buffer that they used and that is the cf2 here which has the silver handle and it's a little different in that the cathode resistors are not included on here you can see that the cathode is coming straight off and out but there's really not that many cf2s in the entire system there's like five or something like that so this is not a very common package and I'm not really sure why they built it this way moving on to the next package we get into some pretty difficult stuff this is now the read amplifier which has a green handle on it now this one is quite complex and I haven't quite fully wrapped my head around it so I'm probably wrong about some of this stuff but the first half of it is at least moderately simple we've got a Transformer coupled input coming into one half of our double triodes up there after that it looks like it goes into a set reset flip flop which is really interesting they've stuffed an entire flip-flop on the read amplifier here I don't know this package is very much so beyond me it's fascinating but it's going to require a lot more time of me sitting down and wrapping my head around it so instead of sitting here bumbling let's move on to the next package and that's going to be this package with the black handle here which is the right amplifier so this package actually runs two right heads and this package with the previous package the read amplifier are the main amplifiers that connect up to the rotating drum memory and so this package is going to be writing new data onto that rotating drum memory and so we get an input coming into the grid of v2a here this looks like a cathode follower buffer and the output of it goes to CR2 and that is creating an and gate with cr1 which is a right pulse so we have to have an input coming in and we have to have a right pulse hitting at the same time to affect the grid of V1 and then this 6197 Pinto to either conducts or goes into cut off and that sends a transient through our right head coil and that is what is flipping the magnetic flux on the drum the final package that I want to take a look at actually has a green handle but it is is not a read amplifier this is a completely unique package and there is only one of them in the entire machine and it is the read clock package and this actually looks a lot like an amplifier out of a radio in that it's like the output driver Stage of it the two big huge pin toads that are on here are actually totally in parallel so you have an input coming into the grid of both of these the anodes are tied together and the anodes go to plus 250 volts through a Transformer that's this big yellow transformer that we see over here and then this Transformer is changing our high voltage low current signal to a low voltage High current signal and so it's wild it's really fun to see kind of radio amplifier technology going on here in this Bendix g15 but it makes sense all right I think we're getting a pretty good handle on the logic of the machine even though one of the amplifiers is still totally baffling me but there was one big question that I wanted to answer at the beginning of the episode and that is is this an NC machine or an ANC machine and this becomes very important when it comes time to plug the typewriter into the machine so what is NC and ANC well NC stands for numeric control and ANC stands for alphanumeric control what's the difference between these two well numbers and letters this machine is a 5-bit machine at least the input output side of it and when I hear five bit I think oh nice but doe you could do a full keyboard with digits on a 5-bit bado code but nope no no that's that's not how this works that five bits is split up in a completely unique way this fifth bit here is an indicator as to whether we're doing an input for value or an output to display value or whether we're doing a command so if this bit is set these four bits represent the value that we're inputting into the machine or the value that we're outputting into the typewriter to print on the paper if you only have four bits that only gives you 16 characters and this machine is a hexadecimal machine and those 16 characters are 0 through 9 and U through Z wait what U through Z hexadecimal should be a through F right well apparently back in the 50s they used U through Z instead of a through F so that's another wonderful mental conversion that I'm gonna have to try and internalize but if this fifth bit is off now we're doing a command and there are specific keys on the keyboard that you can press that tell the machine to do something specific and so that's how the five bits is moving in and out of here but if you only have four bits to represent a value you can't represent anything other than hexadecimal or zero through nine in U through Z if we want to actually type something like say hello or type some text to the user we need to do alphanumeric control and so in order to do that the machine actually sends 10 bits to the typewriter it it doesn't do this gracefully what it does is it sends two five bit chunks of data in Rapid succession into a massive relay box that then latches those five bits into 10 bits that parallels them into the typewriter itself and when the typewriter sends something back in alphanumeric level it sends 10 bits into that relay box that then latches them and then shoots them back out to the machine five bits at a time so it becomes very important because this typewriter is an ANC typewriter which means it has to plug into that specific box which I happen to have Bob sent that box over but if this is not an ANC machine then that box doesn't talk to this machine which doesn't send stuff to the typewriter and we've got some other head scratching to do so is this an ANC machine and right off the bat no uh ANC machines came with an additional little light right up here and this one doesn't have it however the early numerical machines did often get converted to ANC machines there was a whole procedure written for it and sometimes in the field Engineers would do that upgrade the customer to a newer typewriter and they'd be Off to the Races so is this machine a numeric machine that's been upgraded now that's a much more difficult question to answer and for that we're going to have to dive back into the documentation as I mentioned at the beginning things changed on an almost yearly basis so it's really kind of difficult to find the right documentation for the specific machine that you have this is serial number 238 but we can at least get a little bit of an idea here so looking at this page here it's a list of signals that pertain to alphanumeric and numeric operation we can see that over here on the typewriter coupler but we have two signals that are not making their way over to numeric so these are alphanumeric specific Sigma channels so if we can trace these back to the actual schematic we might be able to find if we're missing those packages on our machine so this is the type signal and the as signal so let's start with trying to find them in the schematics and uh well it printed out incredibly small but if you blow it up here this is a list of all the signals within the machine and where they're located within the schematics so as here and type signal over here and those are on schematic 3D 288 and 3d290 however we don't actually have those schematic Pages no we have to go to the bottom right corner of this page and there's a little conversion chart that goes from that number to the actual schematic diagram number this is ridiculous I don't know why they did this but 3D 288 is 3d590 and 3D 290 is a 3d592 so there we go uh now we know which pages to look for so if we go to 3D 592 and we look for the as signal we can actually find it up here in this flip flop uh and this flip flop is B3 and it goes to a 54. uh now if we look at the actual machine both B3 and a54 are present and fully wired up so does that mean that we have the alphanumeric machine because we have a alphanumeric specific signal existing within our machine that's interesting all right let's check one more the type signal that was on the next schematic over here and looking at this schematic right off the bat we can see that we have the poop signal I absolutely will not apologize for the toilet humor that made me literally laugh out loud when I saw it but if we look below the poop signal we can find the type signal here and this is module j51 and again if we look at the machine we actually have j51 populated and fully wired up so we have our two alphanumeric signals present and accounted for so we have an alphanumeric machine right um well maybe maybe not uh there's one more thing I want to check those signals have to actually make it out of the machine and into the typewriter so let's check that right quick if we look here the type signal is pin 33 on plf1 that is the big Canon connector on the back and if we crane our head very carefully inside pin 33 is blank so the type signal exists within the machine but it never makes it out the back so I think we have a numeric machine but we have alphanumeric signals don't really know what's going on there's been a lot of interesting modifications done to this machine and it's not exactly following the documentation but judging by that Canon connector on the back I think the machine is essentially a numeric machine and they may be using the other flop on that flip-flop board for some other purpose within the machine that I'm just not seeing here yet well there we have it we were on mostly a fact-finding mission today and we mostly found out that we have a lot more fact-finding to do this is a very interesting machine with a lot of interesting modifications done to it I did get an entire list of all the modification wires that were added inside the doors and it's going to take a little bit of research to figure out what those are but we did figure out a ton about the actual logic cards that plug into the door and we also managed to get every single one of them clean it looks beautiful in side of this machine now I want to clean the outside next and well it has this kind of unique crinkle paint on the outside and so I'm going to need to do a little more research on how best to clean this without damaging it and with successfully cleaning it I used Windex on the inside I'm not sure how that's going to work on the outside so I might just use a brush and some soapy water and see how that goes but if if you have any ideas on how best to clean this crinkle paint leave them in the description below I'm really curious and maybe you can help me get this thing looking absolutely beautiful and brand new fortunately we can bring the power up on this machine in two different stages we can bring the AC up while leaving the DC off that allows us to check a bunch of stuff without getting into worrying about sprayed capacitors and selenium rectifiers and all of that so that is our initial Focus bringing the AC up confirming that all of the fans come up confirming that the drum spins up and then ultimately we want to confirm that we have a valid clock signal on that drum so that is our goal uh for the short term and then when we get into the long term we'll start dealing with bringing the DC up and then start trying to figure out how to get the machine doing something interesting from there and chief among that is going to be figuring out the connection between this machine and that typewriter because if this is a numeric machine and that is an alphanumeric typewriter we've got some head scratching to do but that's all going to happen in the future so I want to thank you guys so much for watching and I hope to see you next time

Channel: Usagi Electric

Views: 103,806

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Id: q9IxVi24R5Q

Length: 35min 28sec (2128 seconds)

Published: Sun Jul 23 2023