Reading Tape Images

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	by Bill Degnan - 08/19/2026 17:00
The tapes were read forward and backwards. The output from the TU56 was sent through an analog to digital converter into a laptop's serial port. We used a program to receive the signals and save them onto the laptop. The program displayed the waveforms as they were being recorded, which was useful for confirming that the signal was not under/over voltage. Two dumps of the same tape were made to verify that there was no data loss. DEC's TU56 tape units were used by different models of Digital PDP minicomputer over a 10 year span. To determine the correct DEC family, we opened the file in a HEX editor. We could then look for the signs that the tape was from a PDP-8 (12-bit) data or PDP-11 (16-bit) or PDP-10 (18-bit). Because all data is written to the TU56 the same way, we can determine the model by looking for words (bytes) of 12, 16, or 16 bits. 12-bit words have more 00's in the data, 16/18 have less. simH software allows a person to emulate antique computer environments including printers and media peripherals like tape drives. With this software it would be possible to load up the tape in its native environment. Example - PDP 11 tapes We loaded the simH emulation program and set up the necessary configuration for a Digital PDP 11/40 computer with TU56 tape drive attached. The PDP 11/40 is a mid-1970's DEC minicomputer that is a good starting point for reading PDP 11-formatted TU56 tapes. There are a number of operating systems used on a PDP-11, but the most common at the time as RT-11 so we started with that one. We loaded this OS using simH and inspected the tape contents. Within the emulation we ran a simple DIR command to reveal any files saved to the tape. One can use simH to capture the session and generate a text file log of the transactions used within RT-11 to investigate the contents of the tape in its native environment. This is exactly what would have appeared on the terminal screen of a person back in the mid 1970's with a real PDP 11/40 and TU56 tape drive to read your tape. WHAT's NEXT? If we knew more about the tape we could target our work. We can explore further by loading other operating systems to see if the non-file data can also be read. It is possible that some of the tape was used to compile a FORTRAN program and used temporarily as storage for that purpose, but not written as a file. It would have been more like a "RAM dump". We can look to see if there is a program that can read whatever produced the files we find, to help determine its purpose. We can use simH with alternative PDP 11 operating systems such as RSTS or BATCH-11 to see if more is revealed within that environment. We could also attempt to convert the binary data and decompile chunks of the raw data. Reply