Cartridge Drive or Stringy-Floppy?
This article describes the fun that I had interfacing the “Electronic R/W
System Model 25-300” to my Nascom 1. The peripheral itself, whilst certainly
electronic and certainly a read/write system, is probably better described as a
cartridge drive, although I have seen it described in some magazines as a Stringy
Floppy; a completely misleading and unnecessary name.
I ordered the unit by telephone from Breda, Holland one Wednesday morning
after having all my technical queries answered immediately in excellent English and
the unit arrived the next day by post (I must admit though that I live in Holland).
For 330 Guilders (about 73 pounds and dropping as Mrs. Thatcher pulls the pound
through the ceiling) I received a deck and control board. For a further 80 Guilders
(work it out for yourself) a box of cartridges (wafers) of assorted lengths from 5
feet (20 seconds recording) to 50 feet (200 seconds recording). These cartridges
consist of an endless loop of tape with an End of Tape (EOT), which is also the
beginning of tape (BOT), marked by a piece of tin foil.
Unlike some pieces of kit, this drive came with very adequate documentation
but no case and no mounting brackets. Any capable metalworker with nothing more than a
small lathe, milling machine and borer could knock up a suitable mounting in a few
weeks. Anyway, I hacked and puffed and blew and finally mounted it in such a way which
I do not feel like exposing to public ridicule. I feel more confident on the grounds
of electronics and program design and will hastily move on.
The drive demands very modest +5V and +12V supplies and in return offers the
following interfaces on a small edge connector :–
|Start Motor||EOT Detected|
|Fast Speed on Motor||Cartridge is Write Protected|
|Enable Writing||Serial Data from Cartridge|
|Select the Deck|
|Serial Data for Recording|
The beauty of the “Select the Deck” input is that when it is held false
everything in sight goes into a high impedance state and many decks may be connected
The hardware interface wins no prizes for complexity but is very adequate and
is shown on another page.
The LED’s are necessary because once the cartridge is in place it is
impossible to see whether you remembered to take the write protect tab off. The 100K
resistor pulling up the Read line to the UART serves not only to make the pulses nicer
but it is essential because when the deck is not selected the Data Out line goes high
impedence and picks up all sorts of rubbish. And what does the Nascom do when it is
idle? It picks the rubbish up and displays it at great speed on the screen making it
impossible to type a command in to select the deck again to stop the monitor picking
up the rubbish .... I will be honest and admit that this resistor was not part of
the original design.
After a simple interface and a few test programs I felt ready to fill my
second 2708 socket with the Hobbs uncopyrighted Cartridge Operating System; surely an
impressive title. Here I tempered good design techniques (I design computer systems
all day as well) with the knowledge that it had to all fit onto 1K bytes plus whatever
I could save by taking Dump and Load out of Nasbug. The original 4 layer design has
reduced to three layers :–