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Me and my pdp-8/L

Arcadetech04 said:
Wow, terrible photos.
I’ll try this one.
Click to expand...
I thought the photos were fine, when viewed full size.

Most of that doesn't actually look that bad, to me. I've worked on machines that were much worse. I'd consider using a brass (not steel) brush to buff off the corrosion, then remove as much of the dust as possible. Many modules became functional again after this treatment. (The corrosion seems to be mildly conductive.)

That won't be enough if the pin has rusted open, obviously.
 
pbirkel@gmail.com said:
The TI steel-pin rust curse strikes! An EPROM-based PTR replacement would be quite nice. Fit it into an empty double-wide slot in the rear?
Click to expand...
If I can find a PTR card or a schematic, I can definitely wire wrap a circuit. Shouldn’t be that hard. After all, all you really need is an eprom, 4 74ls93 and a sequencer. Then all you need is a ribbon cable connecting to the interface.
I built one a long time ago using a user, 2764 eprom and some ICs.
Wonder if I still have buried somewhere.
 
That can just be dirt and gunk, I found buying a small 7400 series chip tester is *GREAT* for the M113's and 7474's. However most things that blow up are the NAND gates (they're used as drivers and thus take a lot of load) and the flip flops.
 
Yep, ordered 100 SN74LS00, 50 SN74LS74 and 50 SN74LS40

Next question….
Does anyone out there know how to access any form of a buss or maybe I/O buss for an 8/L ?

Like add something more than a Teletype or a Paper Tape Reader.
Something like a Chuck E Cheese Robot or Dot Matrix Sign. That would be fun.
 
You may want to confirm that the 74LS family is acceptable for replacing all of the dead 74 family chips you find. My recollection is that there are some cases where only a plain 7474, for instance, will work. Factors include switching speed and the actual internal circuit of the flip-flop, in the case of the 7474. On other flip-flops, the outputs are buffered, but I think there's one case where DEC drives the outputs of a 7474 directly since the output on that is not buffered. Vince can probably recall where I'm remembering this from.

The Posibus is not a trivial bus to interface to, but it's of course quite possible. I recommend studying any of the I/O devices from DEC in order to learn more about the bus. If you're not looking to do any data break (aka direct memory access), it's much easier.
 
antiquekid3 said:
You may want to confirm that the 74LS family is acceptable for replacing all of the dead 74 family chips you find. My recollection is that there are some cases where only a plain 7474, for instance, will work. Factors include switching speed and the actual internal circuit of the flip-flop, in the case of the 7474. On other flip-flops, the outputs are buffered, but I think there's one case where DEC drives the outputs of a 7474 directly since the output on that is not buffered. Vince can probably recall where I'm remembering this from.

The Posibus is not a trivial bus to interface to, but it's of course quite possible. I recommend studying any of the I/O devices from DEC in order to learn more about the bus. If you're not looking to do any data break (aka direct memory access), it's much easier.
Click to expand...

There is a case using a 74x175 device in the PDP-8m (and maybe 8e IDK) console logic where not all 74x175's are created equal. Only non-buffered output types will work in the circuit.
Discussion here: https://forum.vcfed.org/index.php?t...failure-eg-loadaddress-does-not-work.1245533/
where I again had to fix the exact same problem I saw back in 2006 with my PDP-8m console board.
 
Hmm…
Thanks for the info. I would think that a buffered output would add a bit of reliability to prevent feedback triggering of the flip flops.
Maybe I’m wrong??
 
Arcadetech04 said:
Hmm…
Thanks for the info. I would think that a buffered output would add a bit of reliability to prevent feedback triggering of the flip flops.
Maybe I’m wrong??
Click to expand...

Actually that was the whole trick behind using a part with a non-buffered output. By forcing the output low you could flip the state of the flip flop.
The PDP-8m console logic switch design (load address, exam, deposit, etc) requires these specific type of 74x175 with non-buffered outputs for the debouncer logic to work correctly.
IMHO it was a complete hack by some designer that never should have passed a design review. Simple RS flops built with 74x00's would have been a better solution.
 
Arcadetech04 said:
Yep, ordered 100 SN74LS00, 50 SN74LS74 and 50 SN74LS40

Next question….
Does anyone out there know how to access any form of a buss or maybe I/O buss for an 8/L ?

Like add something more than a Teletype or a Paper Tape Reader.
Something like a Chuck E Cheese Robot or Dot Matrix Sign. That would be fun.
Click to expand...
Careful with LS versus standard 74xxx parts as LS parts have substantially less fan-out when driving standard 74xxx.
 
Arcadetech04 said:
Does anyone out there know how to access any form of a buss or maybe I/O buss for an 8/L ?
Click to expand...
The Posibus is there, and is easy enough to interface to. Mainly it's physically cumbersome, conventionally with 3 double sided paddle cables for bus in, and 3 more for bus out, and that's for the simple non-dma case. There's a memory 'bus' too, but you'd have to build the mmu to go with it.
 
Arcadetech04 said:
Well this sucks.
As I was checking the M series flip chips, I notice rust and green corrosion on quite a few of the ICs leads. So much so, that my meter shows low or shorted leads.
So now, I am pulling every flip chip and the ones that have any corrosion will rebuilt from the ground up.
Anyone know just how many DM/SN7400 ICs are in a pdp-8/L?
Damm near 100 ICs.
Yea, some learning curve. But, I’ll fix each and every board. Just need some time.

I’m also now looking for a paper tape reader.

Or, if I can’t locate one, I’ll build an eprom version using some 2764 eprom in an old game cartridge.

Store all my programs on EPROMs.
Click to expand...
Well, I ended up with replacing about 1/3 of all IC's in the machine. And not only the IC's where a problem, I had to replace quite some transistors since the legs rusted of. Espescially the TO-5's that where mounted with a plasic spacer between the board and the package.
Mine 8/L had been stored in a garage with a leaking roof... Even the case was rusty.
 
Yes, I came across the 'flipping the state of the device by playing with the output' trick by accident when casually looking through the schematics one day (like you do). I was both horrified and admired the 'smarts' at the same time!

Most parts should be interchangeable, but beware the ones that aren't...

Dave
 
Yeow, good thing i mistakenly ordered a pile of 7474 chips.
Almost sent them back.

Ok, another thing, is there anyone out there that make unpopulated flip chips or can make custom boards based on my schematic?

I have a few boards I would love to be made. Now, before we get into gurber files, I should tell you the following.

I lost my home in the Camp Fire, November 2018. Lost everything.
Living now in a warehouse with no internet other than my cell phone.

Still waiting on getting plans for new house. And it’s taking forever.
Ugg….
 
Sorry to hear about your predicament.

Here is the ultimate website for the question you are asking: https://so-much-stuff.com/pdp8/flipchip/Mxxx.htm. If you go to a higher level, you get the other flip-chip modules as well as the M-series: https://so-much-stuff.com/pdp8/flipchip/flipchip.php.

PDF schematics, datasheets, Eagle PCB and schematic files.

However, they do come with a "buyer beware" warning if you use them!

Thanks for the very helpful website and the effort in creating it all Vince :)!

Dave
 
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