Lexicon LXP-5 - Page 47

Circuit
Description
Lexicon
Master Interrupt
MIDI Hardware
User I/O Hardware
The 31.25kHz WC/ signal is applied to the digital counter U13. Every nine
clock pulses, RCO (pin 15) will go high in order to force an interupt routine on
the Master Z80 processor. The processor will then access the UART to
examine the RXD port for any MIDI input data received.
The MIDI hardware utilized by the LXP-5 complies with MIDI specs. It incor¬
porates 5 pin, female DIN connectors for input and output connections (J1 and
J2). MIDI IN is optocoupled into UART R x D line for ground isolation
purposes. A switch (SW4) located on the rear panel allows the user to select
either the MIDI THRU or MIDI OUT function for J2. With SW4 in MIDI THRU
position, received data is sent back out through an inverter to a current loop
driver, Q2. In MIDI OUT position, TXD output from the UARTwill be connected
to Q2, allowing MIDI output functions to be performed. Ferrite beads FBI, FB2
and C8, C9 are used to reduce RFI from being radiated from the MIDI OUT/
THRU port.
Two tri -state buffers, U12andU19, provide input ports to the MasterZ80.The
101/ address decode enables buffer U12 allowing status of SW2 (Select
control) and SW3 (Adjust control) to be examined.
102/enables U19. SW1 (MIDI LEARN button on front panel) is connected to
a pullup resistor in RP2. When pressed, a low state is produced on pin 11 of
U19. An external on/off switch connected to the FOOTSWITCH input jack on
the rear panel will control the logical state of the FOOT/ signal. FOOT/ is
normally high due to its connection to a pullup resistor (RP2). R19 and Cl 6
on FOOT/ act as an RFI filter. CR7 and CR8 provide overvoltage and static
protection for the FOOT/ input. SINT/, which indicates the status of Slave
Processor interrupt, is monitored on pinl3 of U19. A low state indicates the
Slave Z80 interrupt has been asserted but not acknowledged.
The bi-color front panel LEARN LED is controlled by the status of bits 3 and
4, which are latched into U5 by a Master Z80 write-to-port address 104. Bit
3 high; bit 4 low will result in a green display. Bit 3 low; bit 4 high results in a
red display. When bit 3 = 4, the LED is off.
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