Rev. 1.0, 02/00, page 711 of 1141
Duty Detection Mode (VASS): VASS detection is carried out by the duty discriminator.
Software can detect index sequences by reading the duty I/O flag at each CTL pulse.
At each CTL pulse, the duty discriminator sends the result of duty discrimination to the duty I/O
flag, and simultaneously generates an interrupt request. The duty I/O flag is cleared to 0 if the
CTL pulse is a 1 (duty cycle below 44%), and is set to 1 if the CTL pulse is a 0 (duty cycle above
44%).
The duty I/O flag is modified at each CTL pulse. It should be read by the interrupt-handling
routine within the period of the PB-CTL signal. VASS detection format is illustrated in figure
26.58.
1
Tape direction
Written three times
1 1 1 1 1 1 1 1 1 1
M
S
B
L
S
B
L
S
B
M
S
B
M
S
B
L
S
B
L
S
B
M
S
B
Thousands
Header (11 bits)
Hundreds
Data (16 bits: 4 digits of 4-bit BCD)
Tens
Ones
Figure 26.58 VASS (Index) Format
Assemble (ASM) Mark Detect Mode: ASM mark detection is carried out by the duty
discriminator. If the duty discriminator detects that the duty cycle of the PB-CTL signal is 66% or
higher, it generates an interrupt request, and simultaneously clears the duty I/O flag to 0.
The duty I/O flag is updated at every CTL pulse. It should be read by the interrupt-handling
routine within the period of the PB-CTL signal.