Lexicon 960L Service Manual page 112

960L Multi-Channel Digital Effects System Service Manual
used to generate the AA0/, AA1/, AA2/, and AA3/ chip select signals. These address decodes are assigned
to:
AA0/ – I/O BUS (16 wait states)
AA1/ – T-MIX Host bus, Dual Port Ram, Boot PROM (8 wait states)
AA2/ – T-MIX devices DSP bus (3 wait states)
AA3/ – (Not Used)
DSP 56301 Interrupts
The 56301 external interrupts are assigned as follows:
IRQA – Word Clock Interrupt
IRQB – I/O Bus Interrupt (not used)
IRQC – Dual Port Ram Interrupt from Z80 1
IRQD – Dual Port Ram Interrupt from Z80 2
56301 SCLK Output
The 56301 SCLK output is used to supply a clock to the T-MIX devices at start-up through the mux (U45)
on sheet 15. This clock is needed at start-up because the TMIX_CKI signal may not be available when the
T-MIX devices are initialized. After the TMIX_512FS signal becomes available the 56301 switches the mux.
DSP 56301 Timers
The 56301 has three timers that use the pins TIO0, TIO1, TIO2. TIO0 receives the PREVIEW_CLKS signal,
which is a word clock signal that is sourced from any of the available word clock sources in the 960L. The
timer measures this signal and the result is used by software to determine if the selected word clock
frequency is acceptable. This clock is not necessarily the one the system is currently using. TIO1 receives
the TOT_WCK signal, which is the word clock the system is currently using. TIO2 is a watchdog timer
output from the 56301.
56301 Data & Address Buffers (sheet 3)
The DSP bus is used by the 56301 to access the T-MIX devices, the Z80s, and is the pathway to the I/O
bus. The DSP data bus interface to the 56301 uses 3.3V logic levels. The 56301 address bus also drives
3.3V logic levels. The 56301 control bus drives 3.3V logic levels, but control inputs to the 56301 are tolerant
of 5V logic levels. 5V/3V interface devices are required to interface the 56301 to the T-MIX devices since
these Asics have 5V CMOS inputs and outputs. The 5V/3V level translation is accomplished by using
Quickswitch QS2345 bus switches to attenuate 5V signals to 3V levels These devices are U10, U11, and
U12 on sheet 3. These devices will attenuate the 5V signals provided they are powered by a 4.3V source,
which is provided by an adjustable regulator (U2, sheet 14). The T-MIX devices must have 5V CMOS levels
on their input pins, so the 3V signals from the 56301 must be converted to 5V CMOS logic levels, This is
done using FCT2244 buffers (U4, U5, U6, U7, U8, U9, U13, U14 on sheet 3). Pull-up resistors are used on
the SAMP_DAT5V bus to set the data bus to a valid logic level when the bus drivers are tri-stated.
T-MIX #1 (sheet 4)
The Reverb Card has three T-MIX devices. These chips function as serial to parallel converters for digital
audio. The 56301 transfers 24 bit parallel audio samples to/from the T-MIX chips. T-MIX #1 shown on sheet
4 receives serial digital audio from audio input cards on the I/O bus. These digital audio streams are
formatted with eight audio samples per word clock. These audio streams can be probed on R126-R138,
and if a scope is triggered on word clock it is possible to see the audio samples in these data streams. The
assignment of these signals to the I/O cards in the system is under software control and changes
depending upon how the I/O bus is populated. The TMIX1_NSAC signal is the audio word clock interrupt to
the 56301.
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