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:
Thanks.
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.