National Multiplex data cassette tape drive

From: Tom Jennings <>
Date: Wed Aug 25 13:26:54 2004

On Tue, 2004-08-24 at 17:09, William Maddox wrote:
> Years ago, I owned a data cassette drive made by a
> company called National Multiplex, presumably long
> defunct. ...

Ugh! I had one on my SWTPC, around 1975? It was the first time I got
ripped off buying computer gear.

For the SWTPC, it came with an EPROM with "extended commands". You ran
it via "G 1234" where 1234 was some address in the EPROM. Many Nat'l Mpx
commands jumped back to the MIKBUG ROM. Kludge!

That was merely annoying -- the cassette drive and format was the rip
off. It was just a gutted audio five-pushbutton type piece of junk. They
took all the guts out, and put in basically a gate to drive the write
head, and an opamp or something to read.

THe worst part -- the motor had a trimpot in series with it, and that
was the sole motor speed control -- and! -- tapes written at one speed
adjustment could not be read at another speed!

Of course I turned the pot, my fault. But just tapping it was enough. It
was total crap.

The 'encoding method' (sic) was just a bunch of "C" characters or
something written as a header, followed by some dumb ad hoc block info,
data, and a checksum. There might have been some attempt at bit sync but
I long forgot. I went all through their ROM code and it was total crap
(obvious to me, a two-year-experience programmer, duh).

PLUS, I paid some exorbitant price for it (from a BYTE ad or something).

The only good thing I'll say about this worthless piece of total crap is
that it spurred me to write my own software. I got an old Pertec 7-track
drive (minus formatter), and with a SWTPC parallel port card, wrote a
nybble-banger controller (four data bits, a clock track, a strobe bit,
plus a few tracks left over!), with all the usual filemark/tapemark
stuff, high speed search, an index, loaded programs by name, and wasn't
dependent on tape speed! (The first program I wrote that used interrupts

(I had to load the tape driver/exec via paper tape with MIKBUG!)

I am very glad they went out of business, good riddance to bad rubbish.

(Though today it's a different story, it's worth keeping for historical
purposes, I agree.)
Received on Wed Aug 25 2004 - 13:26:54 BST

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