SWTPC AC-30 Assembly Instructions Manual

Assembly Instructions AC-30 Audio Cassette Interface ©
Introduction
Cassette tape is one of the most flexible and least expensive means of
mass data storage for computer systems. When compared to paper tape readers
and punches, you'll find that although the paper tape readers can be made
rather inexpensively, the punches cannot. Paper tape systems are typically
slower and the punched tapes cannot of course be repunched and used over and
over again, as you can with cassettes. Disk systems on the other hand offer
significant advantages over cassettes but 'are still too expensive for many
applications, and for most hobbyists. Even those lucky enough to have a
disk system still need a more universal 'medium for exchanging programs. —
Although there are several commercial digital cassette tape decks on the
market today, recording techniques vary and they are of course much more ex-
pensive than the average audio cassette unit. As could be expected most hob-
byist computer system mass data storage, designs have been based on the audio
cassette recorder. ee
The use of inconsistent: recording techniques among 'the various manufact-
urers makes it impossible for example to record a program, or data tape on a
SWIPC 6800 Computer System and play it back on a MITS 680 Computer System.
In order to coordinate manufacturer design efforts, and exploit the most ef-
fective recording technique, BYTE Magazine of Peterborough, New Hampshire,
03458 held a symposium in the Fall of 1975 in Kansas City in an attempt to
establish a recording standard for the storage of digital data on audio cas-
sette recorders. The standard which was adopted has been tested and fully sup-
ported by Southwest Technical Products Corporation. It appears to be the best
compromise between economy and reliability. Although complete details are
contained in the Feb., 1976 issue of BYTE Magazine, the recording philosophy
is to record data serially using the standard UART format at 300 baud (30
characters/second). Marks or logic ones are represented by recording a 2400
Hz sine wave on the tape while spaces or.logic zeroes are represented by re-
cording a 1200 Hz sine wave. With the proper circuitry this recorded data can
then be read off the tape and transposed into a self clocking UART based tave
system which will tolerate audio recorder speed variations of approximately
+30%. This figure is far better than that: of most other modulation techniques
and is a real advantage when you consider: the degree of worst case speed varia-
tion between inexpensive audio recorders. In addition we have speed
variations due to line voltage, battery voltage, wow and flutter, mechanism
wear, etc. Thus evolved the "Kansas City" standard. It should be noted that
the standard does not specify how the data is to be organized on the tape, so
there can, and probably will be some incompatibility among various manufact-
urer's units. This is however more of a software problem than a hardware pro-
blem and thus a little easier to resolve. —
Although the SWTPC AC-30 Audio Cassette Interface has been used extensive-
with the SWIPC 6800 Computer System and CT-1024 (TV Typewriter II) Terminal
'Maitem, it has been designed to be as universal and flexible a system as pos-
sible. If your computer's control terminal is interfaced to the computer thru
300 baud, RS-232 compatible serial interfdces with accessible UART type 16 X
baud rate clocks on both computer and: terminal, the SWTPC AC-30 Cassette Inter-
face Unit is simply plugged between the computer and terminal interfaces. This
is the ideal mode of operation since the cassette unit can take full advantage
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