Lexicon LXP-5 Service Manual

LXP-5 Service Manual
Circuit
Description
DAC logic signals are provided by the Lexichip to control the Sample and
Hold (SAH) and deglitch functions and conversion data latching. A Word
Clock signal (WC/) is used as a clock reference and indicates the beginning
of a conversion cycle.
On the falling edge of WC/, SAMP goes low in order to hold an input sample
for an A/D conversion. The internal SAR then applies a DAC data output to
DABO-15. This data is latched into conversion latches on the positive edge
of the CCLK1 signal. The DATA signal from comparator is then compared by
Lexichip to determine the next SAR output value. Sixteen SAR output
samples are required to perform an analog conversion. Since the later
samples produce smaller voltage changes on DAC output, less time is
required for DAC to settle, therefore, the time duration between CCLK1
pulses towards the end of the A/D conversion is reduced.
The 17th CCLK1 pulse after WC/ latches left channel data out to the DAC.
After sufficient DAC settling time, the left channel degiitch switch is disabled
by the high state of DEGO, applying a DAC sample to the left channel output
circuitry. The last CCLK1 pulse in the WC/ cycle latches right channel data
to the DAC. DEG1 goes high which applies the DAC sample to the right
channel output circuitry.
WC/
SAMP
CCLK1
□ EGO
u u uu uuuuuuuuuuup
If
DEG1
^ A/D CONVERSION
r : ^ RIGHT CHNL D/A CONVERSION LEFT CHNL D/A CONVERSION
- ►
Converter Timing
UART, I/O
Serial communications are handled by U3, an 82C51A UART 1C. U3 is UART
mapped into the Master Z80’s I/O space at locations 80h (data register) and
8lh (instruction register). The Slave PAL decoder provides it with a 2MHz
clock (UCLK). The MIDICLK input from the Lexichip is a 500kHz baud rate
clock. It is frequency divided by U3 internally to produce the 31.25kHz baud
rate used in MIDI serial communications.
The transmit and receive data connections (TXD.RXD) are applied to the
MIDI standard I/O hardware. The hand shake signal (DTR) is used to drive
the OVLD LED (LED2). The Master Z80 will set this bit high to light this LED,
indicating thatthe Audio signal is about to exceed, or has exceeded, the digital
clipping level.
3-17

Lexicon LXP-5 Specifications

General

Brand	Lexicon
Model	LXP-5
Category	Recording Equipment
Language	English