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Errors reading paper tapes

Mal

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I've recently brought a bare bones PDP-11/10 back to life and am enjoying getting to know PDP-11 assembler (PTS) and paper-tape BASIC.

I'm having some reliability issues reading in paper tapes from a Model 33 Teletype. This shows up as checksum errors when reading absolute binary tapes.

I get the same reliability issue when reading the tapes on a PC from the ASR33 (using a current loop to RS232 interface). The frequency of read errors is about 1 in 1000. If I read the same tape multiple times then (ignoring the leader/trailer) I get differing byte counts, checksum and CRC results. The reader enable circuit is working fine.

One of the issues seems to be that the source tape (which is an original DEC tape in seemingly undamaged original condition) has some irregularity in the spacing of the tractor-feed holes. For example, sometimes the pitch between two frames is significantly larger than normal. So what happens is that when this section of tape arrives at the tape reader, the reader attempts to read the tape slightly prior to the actual location of the frame (perhaps 0.020" ahead of the actual location). The result is that the ASR33 reads a NULL (because there are no holes there), rather than the correct byte. It then skips the character it should have read, and accurately reads the next character.

Here's a link to a photo showing the irregular spacing on the tape. Note that the 5th frame is positioned "late" and is almost touching the 6th frame.

I'm getting this problem on multiple tapes. Typically I get about 15 misreads in a 15,000 byte tape.

So the questions I have are:


  • Was this a common problem "back in the day" with the ASR33?


  • Is there an adjustment/fix for it?

Thanks.
 

RSX11M+

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Well, that is an error in an original DEC paper tape. (Fan-fold I'd think)

It was probably within tolerance for other reader types to read, but apparently your ASR-33 chokes.

Frankly, those tapes are so ancient and getting so valuable as collectibles, I would not advise repairing it mechanically - which we would not have hesitated to do back in the day. [Think of it as a misprint dollar bill] Instead, I would read it into the PC, correct the errant data electronically, and make a new tape if I wanted a working copy.

In fact, I would only use the originals to make copies and then put them away. I haven't seen a source for blank fan-fold in decades so you'll probably have to use roll instead - if you can find that.(?)

BTW - I doubt you'll stumble across any, but your ASR-33 can probably handle Mylar tape too. Funny thing - those would be most rare of all as a collectible.

Oh - repair... just making the holes where the ASR-33 expects them will probably fix it. Don't bother to fill anything in. You can temporarily overlay the area with another section of tape to use as a guide. Mark the original, thru the apertures in your guide, then cut or punch the original correctly. (but I think you could do it by eye - it's only two holes) Leave the index hole alone. Even if this works - I'd make a copy and use that from now on.

A more clever person than I, could probably use the ASR-33 to re-punch the original for that character. I wouldn't take the chance without practice first... a lot of practice.


Ok - so here's a fun thing... where can one buy blank paper tape any more???
 
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Chuck(G)

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I note that the sprocket hole is also offset by the same as the other holes. A good possibility is to have someone with an optical tape reader process the tapes for you. Note, for example, that DEC states that the PC04 and PC05 are "feed hole strobed", which tells you why those offset holes didn't matter to customers who had one of those.
 

smp

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I'm having some reliability issues reading in paper tapes from a Model 33 Teletype. This shows up as checksum errors when reading absolute binary tapes.

I get the same reliability issue when reading the tapes on a PC from the ASR33 (using a current loop to RS232 interface). The frequency of read errors is about 1 in 1000.

<...snip...>

I'm getting this problem on multiple tapes. Typically I get about 15 misreads in a 15,000 byte tape.

Hi,

You are already getting some answers that are far better than I could do...

When I read your post, I thought back, all those many years ago, to when I was loading a PDP-8 with *mylar* tape via an ASR-33 paper tape reader, and how long it took and how much fun it was... WHEW!

Anyway, my first thought was that your experience of error rate is pretty much what I remember, too. That mechanism was clunky and error prone. But, it was all I had to work with at the time. At one point, I remember that I had to unroll the tape and lay it out all the way down a hall to allow it to feed perfectly straight into the reader or else I would have a problem if the tape skewed a little bit to one side or the other during loading. So, for what it's worth, my long in the past experience supports what you are experiencing, about 1 error in 1000 bytes read.

Also, FWIW, I am also in agreement with RSX11M+. Make copies and store away those originals, if at all possible.

smp
 

NeXT

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haven't seen a source for blank fan-fold in decades so you'll probably have to use roll instead - if you can find that.(?)
When I lived in Vancouver there were a few specialty paper companies who were willing to cut up an entire 4' wide roll of paper to make 47 rolls of unlubricated paper tape. Of course you had to pay for the nearly empty roll of paper and the tooling but even if you kept five rolls and sold the rest for $15 each you could pay off the bill and if demand for rolls at the least is that high I'm sure you could quickly make the money back.
 

RSX11M+

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... Note, for example, that DEC states that the PC04 and PC05 are "feed hole strobed", which tells you why those offset holes didn't matter to customers who had one of those.
Agreed - absolutely. I should have said it before - a more general fix is different reader. The ASR-33 is itself a collectible. You'd hurt it's value to modify it to be more tolerant, even if you could. I'll look at mine tonight to see if there's anything simple I can think of, and suggest it in another post.
 

Chuck(G)

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In the past, I've seen projects that use 9 phototransistors and the sprocket holes for clocking--basically a "pull through" setup with no automated feed.

Here's such a project

If you didn't want to make the elaborate guide to compensate for the wide photo-transistors, you could go the SMT route or even use bits of fiber optic cable to spread things out.
 

NeXT

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I found that those packages of LED bars are perfectly spaced for paper tape.
bar.gif

They provide an excellent light source however I have had absolutely no luck finding small enough phototransistors. At this point I'm close to buying eight regular phototransistors and trimming them into the blade like packaging to fit side by side.
 

1944GPW

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The reader article that Chuck pointed to is a nicely engineered solution if all you have are large phototransistors. A piece of rod perspex cut into pieces would be useful as light pipes here.
There was a possibly even simpler reader described in the September 1979 issue of BYTE titled 'Inexpensive Optical Paper-Tape Reader' (three page article and schematic). It was fabricated from a bent piece of sheetmetal, an automotive lamp, some small phototransistors and an Intel 8212 I/O chip and hooked up to a parallel port for a total cost of around $20 in components (for that time).
See ftp://helpedia.com/pub/archive/temp/Byte/
I've considered building this version for quite some time for my own 11/05, after picking up some DEC paper tapes with PAL-11, BASIC and a few other programs circa early 1970s off eBay a while ago.

Steve.
 
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Mal

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Thanks for the responses - much appreciated. It is interesting to read that 1 in 1000 errors are somewhat normal for the ASR-33.

Yes, the ASR-33 is recently oiled. My aim is to read these tapes for archiving purposes, then make working copies (of some) for occassional use. I've got a hundred or so tapes, so I'm keen to find a solution. Many of the tapes can already be found elsewhere online, but I'm keen the archive the full set eventually.

Eventually I plan to built a small standalone virtual tape reader/punch that will interface to the PDP-11 via current loop and store the tape files on an SD card. If anyone is interested in participating in the functional or electrical design, please let me know. I figure that will provide a way of loading tapes at perhaps 9600 baud, and I can still use the real (copied) tapes for demonstration purposes. As much as I enjoy the smell, noise and heat that the ASR-33 and PDP-11 generate in my home office, a more family-friendly option appeals to me as well.

If a PC04/PC05 was available, I'd grab it. Failing that, I think I'll have to explore an optical solution. I've got an OAE OP-80A that has been attached to a keyboard by its previous owner. Inside the keyboard there are some custom electronics that presumably convert parallel to serial. It's in pretty rough shape, but getting it going might be my best alternative at this point.

Thanks also RSM11M+ for the link to westnc - I've been looking for oiled tape.

Many of my tapes have an "SA" and "RA" address printed on them. Are these "Start Address" and "Restart Address", or do they mean something else? I've studied the source code for an early version of the absolute loader (V005A of 1970), but that hasn't helped me to yet understand the meaning of SA and RA.
 

MikeS

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I've got five or six (motorized) PPT readers (and a few perfs) somewhere; if you've got (or can make) a duplicate of one of the problem tapes to send me it might be an interesting project to revive one and see how well it does. To my regret I threw out most of my paper tapes a few years back, many of them very rare Burroughs firmware and utility/test tapes... :(
 

NeXT

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If you can find me a paypal accepting source of cheap SMD phototransistors I can crank off a few prefabricated phototransistor assemblies for anyone who wishes to build the rest of the reader.
 

Roe

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I have an somewhat broken PC05 with unibus controller (the controller seems to work perfectly); if you wanted to take on the project, I could ship it to you, you could attempt to fix it, if successful, you could archive all your tapes and send me back a working PC05...
 

tradde

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I have an somewhat broken PC05 with unibus controller (the controller seems to work perfectly); if you wanted to take on the project, I could ship it to you, you could attempt to fix it, if successful, you could archive all your tapes and send me back a working PC05...
Excellent plan. Some day I might have a system with a PC05. But not right now. I do have a few left over fan fold boxes of paper tape that fit into the PC04/PC05.
 

tingo

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If you can find me a paypal accepting source of cheap SMD phototransistors I can crank off a few prefabricated phototransistor assemblies for anyone who wishes to build the rest of the reader.

Which reader? I'm interested.
 

woodchips

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I remember the ASR33 as being unreliable reading paper tape, but why? A telex machine is the same and they work fine. Doesn't the ASR33 use pins to sense the holes in the tape? Might just be sticky.

Source of paper tape, for the amount a hobbyist will use almost any paper will do, doesn't have to be mylar or oiled. I was looking at cutting down till rolls to make tapes, only 100' or so long but should do for most uses. Other source of decent paper is thermal paper, can get that in quite large rolls.

For phototransistors use the T1, 3mm, package with a little light filing. They will need some black paint to stop light leakage though.
 

Chuck(G)

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I remember the ASR33 as being unreliable reading paper tape, but why? A telex machine is the same and they work fine. Doesn't the ASR33 use pins to sense the holes in the tape? Might just be sticky.

Take a look at the tape photo--note the irregularly spaced frames, particularly the irregular sprocket holes. That's what's causing the problem. After encountering the odd spacing, eventually the tape reader gets the feed sprocket back into the right place and everything works.

This wasn't a real problem for DEC systems with optical readers, since the sprocket hole is used to strobe the frame data.
 
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