Radio Shack TRS-80, Service Manual, Page: 13 / 44

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DETAILED TAPE FORMAT INFORMATION
tor equal to 1/2 volt. Since the other input is
biased at 1 volt, the comparator output is
switched to the high state. If the AC input from
the recorder is positive, diode D6 is turned off
and the input to the comparator will be at some
point greater than 1 volt, in which case, the
comparator output will be low. The input circuit is a zero crossing detector.
R12 is a termination resistor for the cassette
output. Resistors R16 and R17 are used to bias
one input of the comparator at 1 volt. The other
input is also biased at 1 volt by R15 and the
series combination of R14 and R13. If the AC
input from the recorder goes negative, diode
D6 turns on and sets the input to the compara-
the comparator. The final portion of the cas-
sette circuit is capacitor C7 which is used to
isolate noise from the cassette cable. The comparator output is open-collector so
pullup resistor R19 is provided to generate a
TTL signal. R18 is used to prevent oscillation of
Figure 2. Sample Data of Cassette Format
This is shown inverted to indicate possible phase inversion by the tape recorder.
0
0
1
1
0
0
0
0
1
1
1
1
1
1
0
0
REFERENCE SQUARE WAVE
COMPUTER MEMORY DATA
D/A OUTPUT
INPUT TO TAPE
OUTPUT FROM TAPE
INPUT TO THE PIA
DATA STORED
IN MEMORY
4.6v
0v
1v
0v
2.5v
0v
2.5v
5v
0v
The End of File block is a standard block with a
length of 0 and the block type equal to FFH. The
Namefile block is a standard block with a
length of 15 bytes (0FH) and the block type
equals 00H. The 15 bytes of data provide infor-
mation to BASIC and are employed as de-
scribed below:
1.
Eight bytes for the program name
2. One file type byte - 00H = BASIC, 01H =
Data, 02H = Machine Language
3. One ASCII flag byte - 00H = Binary, FFH =
ASCII
4. One Gap flag byte - 01H = Continuous, FFH
= Gaps
5. Two bytes for the start address of a
machine language program
6. Two bytes for the load address of a
machine language program
The standard MC-10 tape is composed of the
following items:
1.
A leader consisting of 128 bytes of hex 55
2. A Namefile block
3. A blank section of tape approximately
equal to 0.5 seconds in length; this allows
BASIC time to evaluate the Namefile.
4. A second leader of 128 bytes of Hex 55
5. One or more Data blocks
6. An End of File block
The block format for Data blocks, Namefile
Blocks, or an End of File block is as follows:
1.
One leader byte - 55H
2. One sync byte - 3CH
3. One block type byte — 01H = Data, FFH =
End of File, 00H - Namefile
4. One block length byte - 00H to FFH
5. Data - 0 to 255 bytes
6. One checksum byte - the sum of all the data
plus block type and block length
7. One leader byte - 55H