• Please review our updated Terms and Rules here

Build your own PDP 8I, Part 4..

The summer Vintage shows are all over now and it's time to get back to cold-weather projects. I exhibited the LD-12 at VCF Midwest but unfortunately it didn't work as well as it did in California. Part of the Midwest show is an annual review of current PDP-8 projects and I gave a short update on the LD-12 - see the video of our "Family of Eight" panel here - https://www.youtube.com/watch?v=0sbgQrzcEpI ; the LD-12 segment starts at 32:00 but I encourage you to listen to the whole discussion for more cutting-edge 12-bit activity.

Meanwhile, I'm sure DaveR is hard at work, finishing up the Front Panel. We're all looking forward to hearing more from him Real Soon Now.
 
Hi Jack,

Unfortunately, DaveR is working on his day job, and has to try and find a problem with a 1980s card we are trying to manufacture. We have a glitch on some of the address lines that shouldn't be there. Unfortunately, during the course of debugging, we have ruled out everything as a possible cause :-(! Back to the drawing board. We must have missed something somewhere.

I agree though, cold weather = computer projects!

I did take one of the LD12 cards to Cambridge the other week. One guy did ask an interesting question that I wasn't able to answer though. The original PCB had sockets for an extra four RAM chips per word and (what appears to be) a spare 74181 socket. Was the LD12 ever made into a 16 bit machine at some point and (if so) what machine instruction set did it implement?

A good question I Thought!

Dave
 
Nothing as simple as that I am afraid :)!

We had a brainstorming session and put together a plan this afternoon.

It is not generated on-board (we have disconnected all of the devices wired to the track and the glitch is still there).

It is not generated off-board (we have performed a transaction on another device with a similar - but not identical - address and no glitch is present).

The PCB track is only a couple of inches long from the backplane edge connector to the ICs and there should not be enough energy induced to see the 1.7 V glitches!

Removing the data bus buffers on the card causes the glitches to go away.

I am, therefore, looking for a direct or indirect link from the data bus to the address bus. I have measured resistances - but everything is in the MOhm range (unpowered).

We have also checked ground loops (between the oscilloscope/ logic analyser and test system) and a load of other things.

Highly confusing, but we must have missed something obvious!

Dave
 
Since a bit more than a year has passed, I'm wondering if there's been any progress on this project?
Specifically, has there been any update to the wiring list for the main board? Has there been any progress on the front panel board?

Thanks, in advance, for posting any news - or even lack thereof.

smp
 
Front panel both yes and no. Let me explain...

I have managed to obtain some accurate measurements of the front panel and have had to re-layout the PCB. This is the third time if have redone the PCB for the LEDs. So, progress is being made - albeit slowly. I have obtained all of the components for the front panel from Mouser, so I have been slowly designing new footprints to suite.

Manual both yes and no...

I have a load of photographs and text to enter into the documentation from my current build.

I have also started to work on the various wiring lists for the registers (AC0, AC1, etc. etc. etc.). I am comparing my netlist for each signal from KiCAD with the documented wiring lists from the original LD12 documentation. Discrepancies are being chased down. There are a few differences (mainly to do with the serial port(s) that are part of my schematics) and a few 'fixes' to the errors that appear to exist within the existing LD12 wire-lists (not many though).

Unfortunately, the project has come to a bit of a standstill at the moment due to the current situation. I find myself working on other activities that we need (e.g. live video streaming for our Church services). I am working on the wire-lists "on paper" so I will have to transcribe the paper to WORD at some point in time.

Hope that clarifies the position.

It's going to take a while yet I am afraid.

Dave
 
Hi Dave,

Thanks a million for this status update. I tried to send you an e-mail some time ago, but I believe it did not go through - I must have fouled it up somehow.

Slow progress is still progress. I would be most interested in what you think are the best wire lists that you can muster, for both the mainboard first, and then the front panel whenever you get it into a publishable state.

Right now, I have the KiCAD schematic you shared with me long ago for the LD20 mainboard, and your draft & uncontrolled document version 0.00 Draft A, describing how to wire up the mainboard.

I will probably not start wire-wrapping anything until you announce that you think it's in a good state. I remain interested in building up this project, so please do share your updates when you can into this topic.

Thanks again. Please keep me in mind for any info that you may have.

smp
 
Hi,

Oops!

I have just realised that I hadn't uploaded the latest manual (Draft B) onto my google drive...

It has more than doubled in size since Draft A.

A few more bits and bytes are there now :)!

In terms of the netlist - all I am doing is importing the text netlist from KiCAD (that is in the manual already) and 'massaging' it based on the fact that most of the GND/VCC and SRAM pins are tracked into the PCB and using the original LD12 wireless from the retrobrewcomputers website. The LD12 wirelist gives me the best route from A-B-C etc. and then I am manually adding my additional pins into the list in the best way.

Dave
 
Hi Dave,

Glad to see this project is still sustaining a little bit of your attention :>) . I'll grab the new manual and take a look. Let us know if we can do anything to help out.

Best,
Jack
 
jackrubin said:
... Let us know if we can do anything to help out. ...
Click to expand...

Yes, indeed!
If there is something that needs to be checked, or any other paperwork kinds of things (that I am capable of handling) please let us know.
I'd be very glad to help out wherever I can.

smp
 
daver2 said:
... I have managed to obtain some accurate measurements of the front panel and have had to re-layout the PCB. This is the third time if have redone the PCB for the LEDs. ...
Click to expand...

Hey Dave, as I am preparing to acquire all the wire-wrap connectors for my board (whew! they certainly will be expensive!) I am wondering if you might have a recommendation for the two 2x50 pin connectors for the Front Panel?

Your advice here would be greatly appreciated.

smp
 
I do in fact. I have already purchased and assembled mine - next release of the document for details and photographs... But I will upload a couple of photographs now to my google drive for you so you can see the result.

I bought the following parts from Pololu:

4 off 1802 - wires with pre-crimped terminals. These are multicoloured (50 pieces each of 10 colours per pack). You may decide you want all the same colour or whatever...
1 off 1844 - wires with pre-crimped terminals. These are also for the temporary jumper leads (see item 1900 below).
4 off 1037 - 2x20 pin female headers.
4 off 1025 - 2x5 pin female headers.
2 off 1917 - 2x10 pin housing (5 pack).
2 off 1992 - 2x20 pin housing (5 pack).
4 off 2748 - 2x20 pin female header.
4 off 1035 - female header set for Arduino shields.
1 off 1900 - 1x1 pin connector housing (25 pack). I think I was going to use these for temporary jumper leads - but they may be the wrong size for the wire-wrap spills. EDIT: Just tested them again (to make sure) and they are too small to fit on the IC socket wire-wrap spills.

There are two (2) types of female headers - those that have leads that can be wire-wrapped (well, sort of). These are for the LD12. And the other type (solder) that are for the front panel.

I found a had to use a bit of sandpaper on the female headers to get them to sit end-to-end. Their definition of stackable and mine are not the same thing!

EDIT: IMG_0441 is the top view (the male headers are just to keep all the individual female headers in line whilst I solder them to the LD12 PCB).
EDIT: IMG_0447 is the bottom view (with a temporary 5V supply connected to test out my DEBUG LED display).

In this post is weeks of research and experimentation :)! The wires with pre-crimped terminals and the housings interconnect the LD12 to the LD12FP. They just plug in. I decided to purchase pre-crimped terminals to give it a more professional look and I just couldn't face the idea of assembling all of those cables...

Working on the IC VCC and GND interconnection table this weekend. And then I shall start working on the interconnection tables for the MA, PC, IR, MB, AC, MUX and ALU registers and busses next. Each of 12 bits...

Dave
 
Last edited:
I have just uploaded a minor update to the construction manual onto my google drive.

I have added the wiring table for the VCC / +5V pins.

Is this format of the table suitable? A bit of constructive criticism required before I crack on and start adding the 0V wiring table and then the many happy days of adding the rest...

Dave
 
Good morning, Dave,

Thanks very much for this. I pulled out the few pages of the table you added - I can't say which pages precisely because I have to convert to US letter size - and the table looks great. It conveys what you are looking for clearly, IMO.

Since I've printed out your entire Draft B, I didn't print it all out again, but I just marked where the table was inserted, and printed those few pages.

I'm looking forward to anything more that you have to add! Thanks very much!

smp
 
So,

I am going to change the crystal from 4.9152 MHz to 5.0688 MHz. I think I was trying to be too clever and came up with a solution that was ideal for 19k2 (0.0% error) but not for any other BAUDRATE!

I have also been working on the front panel PCB this weekend. You can see my next attempt here if you want a look...

https://drive.google.com/drive/folders/1vDuYyG8gLeZHAY4cQXbUq37ToabqLK40?usp=sharing

Still not happy though - but getting happier! I am about to make the footprints for the switch snd potentiometer - so I will be able to see how much physical room I have in the gap on the PCB. If I have sufficient space, I may move the buffer IC and 555 clock back to where the original was.

Now I have seen the graphic rendition of the PCB tracking on the back (I haven't uploaded that one though) I can see a few optimisations I can make to where I have put the decoupling capacitors and a couple of tracks. I would like to keep as many tracks off the top of the PCB as possible - so I can keep that as a ground plane.

Getting there...

Dave
 
Did a few more tweaks before giving up for the night.

Re-routed the VCC power to the buffer ICs as I wanted to do it. Had to re-route the odd signal line to suite.

https://drive.google.com/drive/folders/1vDuYyG8gLeZHAY4cQXbUq37ToabqLK40?usp=sharing

The new footprint is a partially completed rotary switch. I need to get the locating hole in the correct position yet and add some text (M/S/F). I just created the outline and the correct size of holes at the moment.

I will do the potentiometer next and then the 1/8" jack socket for the logic probe power connection.

I then have the power rails from the J1 and J2 connector to add, some of the 'flying components' and pads for the switch/button wires that need to be added and a ground plane on the top surface.

There is no bottom solder resist or bottom silk screen at the moment.

I think all of the LED tracking (including current limiting resistors and buffer/driver IC's) is all there though.

EDIT: Just got rid of the extraneous decoupling capacitor I saw...

Comments / suggestions?

Regards,

Dave
 
Last edited:
Updated the PCB again...

Added all of the footprints now (first pass).

The text for the "speed switch" (M, S and F) is a bit small. I will increase it to be the same size as the other text on the panel.

Also need to 'beef up' the +5V and +12V rails to the LEDs and ICs.

That will be tomorrows job.

Getting there slowly...

Dave
 
Hi Dave,
I would also suggest to maybe reroute the board so that the LEDs and switches are at the top and the ICs at the bottom.
This may improve useability of the system because it is easier to toggle the switches than when they are right at the bottom.
You could also reduce the overall hight considerably unless the other boards dictate the height and there is nothing gained by squashing the design.
Best regards
Tom Hunter
 
You must log in or register to reply here.
Back
Top