Using The Cassette Tape Drive; Operation; System Care; Controls - RCA CDPl8S693 User Manual

9. Using the Cassette Tape Drive

The cassette tape interface enables the user to store
and retrieve programs or data on low-cost audio
cassette tapes. The UT62 monitor program, BASIC3,
the Editor/Assembler, and the PROM Programmer
all provide for cassette tape data storage. The
CDP18S652 Combination Memory and Tape I/O
Control Module has two separate cassette tape
channels addressed in software as tape 0 and tape 1.

Operation

The UT62 monitor program takes care of tape
housekeeping for the user. A tape read routine
converts ASCII serial data from tape into 8-bit bytes
and stores them in a read block buffer in RAM.
Similarly, a write routine fills a write block buffer, then
converts the data to serial ASCII, and stores it on tape.
Data on tapes are stored and retrieved in 384-byte
(lV -page) blocks. Between blocks, the software has
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the ability to stop the tape. The Editor/Assembler
needs this capability to enable the system to process the
data in chunks. The format of data is shown in Fig. 16.
A block of data occupies about six seconds of tape.
START BIT BIT BIT BIT
BIT I 2 3
0
A Z E R O B IT IS O N E C Y C L E O F A P P R O X IM A T E L Y 2 K IL O H E R T Z .
A O N E B IT IS O N E C Y C L E O F A P P R O X IM A T E L Y 800 H E R T Z .
Fig. 16—Data format for cassette tape drive
unit.
BIT BIT
BIT BIT
PARITY
6 7
4 5
The first block written stores an extra length ofleaders
(0Y) to provide assurance that the non-oxide portion
of the beginning of the tape is skipped. Each byte is
serialized into a 10-bit word consisting of a start bit, 8
bits of data, and a parity bit (odd). If a parity error is
encountered on tape read during a block, the system
issues the message:
PARITY BAD THIS BLOCK
Parity errors can be generated by a dirty or worn tape,
dirty tape heads, or improper level settings on the tape
drive unit for volume or tone. The tone controlshould
be set at maximum (10)for best frequency response.
The volume control also should be set at 10.

System Care

At regular intervals the user should clean the tape
heads with a cotton wetted with alcohol. Also, it is a
good precaution to discard worn tapes to avoid
dam aging the heads. For other tape-handling
precautions, see Appendix C.
The recommended cassette tape is Maxell or BASF
60-minute (30 minutes per side) tape or equivalent.
Tapes longer than 60 minutes are thin and should be
avoided. A 60-minute tape is capable of storing over
115,000 ASCII bytes per side. Tapes of shorter length
can be used and, in fact, are desirable for short
programs on both sides because they permit fast
rewind. Note that one minute of tape (after the
extended leaderforblock l)canstoreabout4000 bytes
of ASCII code. Also note that one hexadecimal byte of
address or data equals two bytes of ASCII.

Controls

The system controls the starting and stopping of the
tape drive unit through the "remote" jack. When the
remote is disabled (open), the tape drive motor is
inoperative. In this mode the tape control buttons
(play, rewind, record, etc.) have no effect. The UT62
monitor and the Editor/Assembler have a means of
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