[Simh] SIMH tape images to real tapes

[Larry Baker]

On May 17, 2014, at 9:26 AM, <simh-request at trailing-edge.com> <simh-request at trailing-edge.com> wrote:

> I don't remember exactly how QIC tapes work, but it rings a bell that 
> they might have been different.

Quarter Inch Cartridge (QIC) tapes had 4 tracks recorded serially in one direction.  The device driver had to rewind the tape when each track reached the physical EOT (I think it was a hole in the tape) and continue.  A tape mark is a tape mark.

We built the first portable 16-bit digital seismograph in the late 1970s (that we still use!).  It uses an Intersil 6100 CMOS PDP-8 CPU and QIC cartridges, but we write them in serpentine format to keep up with the data rate.  (Can't wait for the rewind.)  That required a head with dual write gaps; standard QIC tape transports have heads with only one read/write gap.  (The erase gap is in the middle of the two.  In 7-track and 9-track tape drives, there was a separate erase head.  That was fine, since the tape was always written in one direction.  In small cartridge tapes, the erase head became an erase gap on the same head used to read/write, to save space I imagine.)  I modified the RSX MT driver to handle the serpentine format for a custom UNIBUS controller.  I remember when it was delivered that the microcontroller firmware timing had to be adjusted because the 11/70 was faster than the PDP-11 they used to develop the hardware and it was not always responding to device "register" read-write or write-read sequences properly. :)  That was quickly cured.  Other than that, I think we have had no problems with it.

We (Gary Maxwell) wrote the O/S for the seismograph as well.  (The O/S delivered by the contractor we used was unusable.)  We used the DECUS PAL PDP-8 RSX cross assembler.  (Gary rewrote the symbol table handling to use hash tables which made it usable.  I rewrote it from MACRO-11 to DECUS/Microsoft/DEC C, CPAL, so it runs anywhere now.)  Pretty clever to move 16-bit data around on a 12-bit CPU.

We still have instruments in the field.  (Our second version of the instrument has a larger capacity tape and is played back on a PC system with a custom ISA-bus controller.  Most of our inventory is the second version.)  We play back tapes on an LSI-11/73 RSX-11M-Plus system with a QBus-to-UNIBUS adapter, all completely controlled by a VAX/VMS system over DECnet/Ethernet.  When our techs playback tapes, they log in to VMS and run a program that opens a DECnet remote task object on the RSX system that runs the program that actually reads the tapes.  I don't think they have ever logged in to RSX.  I maybe do once every decade or so when we have to move the equipment.  (We've been downsizing for years.)  The VAX and Alpha VMS systems and the LSI-11 just run and run (except for the CD-ROM on the Alpha, which is a PC drive that we have had to replace several times).  I think the LSI-11 system has a mix of hardware from several OEMs, including Emulex RQDX-emulating SCSI II disk controllers connected to CDC Wren 766 MB disk drives partitioned by the controller into four logical drives so RSX could use drives that big.  I think the QBus-to-UNIBUS adapter is an Able Qniverter.

It has all been running for decades.  DEC made rock solid hardware, as did their hardware add-on OEMs.

Larry Baker
US Geological Survey
650-329-5608
baker at usgs.gov


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