[GreenKeys] [External] Re: anyone know the details of this unit?

[Jones, Douglas W]

From: W2HX <w2hx at w2hx.com> -- Sunday, March 30, 2025 7:31 PM
> I could use it with a 12 bit machine and write out one word two 6-bit bytes at a time.

That is possible, but:

> have a PDP-8/E and I'm pretty sure that is how they did it. 

No, all the PDP-8 paper-tape support software assumed 8-bit punch.  RIM and BIN formats were the most widely used for loading software.

They used the low 6 bits of each 8-bit word to encode the 2 6-bit halves of a 12-bit word, most significant halfword first.  In both BIN and RIM formats, the bit 7 was used to signal the first 6-bit halfword of an address.  In RIM format, each word of data was preceeded by its address, so it took 4 lines of tape to store one word in memory.  This made the loader very simple.  In BIN format, addresses were followed by any number of consecutive data words and a checksum was maintained.

Most PDP-8 software that worked with text used 7-bit ASCII with the high bit set to 1 (Mark parity) and mostly it was upper-case only.  Some software did compress 7-bit ASCII down to 6 bits, packing two characters per word and sacrificing some of the little-used characters ( for example, ^ and ~) as shift codes.

                  Doug Jones