info-vax@ucbvax.ARPA (07/11/85)
From: Charles Hedrick <HEDRICK@RUTGERS.ARPA> There have been a couple of confusing messages about tape interchange between 20's and VMS. Let me try to clear up what is actually going on. The easiest way to write a tape that VMS can read is to use the DEC-20's ANSI labelled tape facility. In order to do this, you have to make sure of the following: - make sure that the tape is initialized as an ANSI labelled tape. Both VMS and the 20 can initialize tapes. However on the 20 the default is not ANSI labels. So if you want to initialize it on the 20, you would want to specify ANSI. (The default is TOPS-20 labels. These are a variant of ANSI, so it is possible that VMS could read them. But the recommendation is to use vanilla ANSI.) - to write a file onto the tape on the 20, you must make sure that the tape is opened in 7 bit or 8 bit mode. If it is a standard DEC-20 text file, this means you want to use the COPY command with a subcommand "byte 7". We use a Pascal program to write files onto labelled tapes. It makes sure that all the options are right, and handles details such as producing legal file names. (ANSI tapes don't allow full 73-character file names.) You should then be able to read the tape on VMS simply. You should always be able to read an ANSI tape on the 20. You should not have to do anything special with byte sizes to read a VAX tape, unless something has changed since the last time I tried it. Here are answers to several misconceptions in various messages: - if you use the COPY command, you will get a strange encoding FALSE. With a labelled tape, you get PDP-10 encoding only for binary files. Text files are done in an industry-compatible way. The tape driver knows whether the file is binary or text by the byte size you specify when you open the tape. This is why you have to specify BYTE 7 to the COPY command. (The default is byte size 36, which the tape driver interprets as meaning a binary file.) Note that things work very differently depending upon whether the tape is labelled or not. With an unlabelled tape it is true that COPY always uses the PDP-10 encoding (unless you do magic). - most DEC-20's can't write ANSI tapes, because only the old TM02 controllers support ANSI ASCII mode. FALSE. Labelled tapes are handled internally in industry-compatible format. All controllers handle this. The documentation says that labelled tapes are handled *as if* in ANSI ASCII mode, but in fact this is software-simulated. - ANSI labelled tapes refer only to a header. The data in still in DUMPER format. HALF-TRUE. It is true that you can put anything you want to on a labelled tape, including DUMPER format data. But if you use the COPY command with the proper options, your data will be in an industry compatible format. -------