5. Main circuit explanation


1) Keyboard scanning

The CPU outputs 3-bit addresses in the range AO-A2 which are decoded by the address decoder HC138 (IC25, IC28). The output of this decoder goes through connector CN10 (first contact) and CN11 (second contact) to the keyboard matrix for scanning.

Keyboard matrix output goes from connector CN6 through the inverting octal buffer HC240 (IC16 HD specification) to be passed to the data bus DO-D7.


The main LSI chips are the CMOS gate array IC47 (MB-64H129) and IC60 (MB62H133), the wave-table 256K mask ROM IC45 and IC46 (HN613256), the ten TTL 64bit RAM chips for PAI & PAR (IC48 IC57; S189), the 8-bit D/A converter IC42 (DAC-08), decoder chips IC50, IC59 (LS244), KLM-662 IC16 (LS175), IC14, IC15 (LS138), as well as S/H analog switches (IC17 IC20; 4066) and OP AMPS (IC21 IC25; 072).


IC60 (MB62H133) is a 64-pin LSI with about 800 gates handling major aspects of the system including the CPU interface, timing generation, and adder.


2) Key on/off data and velocity data

Velocity data is computed using the CPU timer to measure the time from when the key contact leaves the first contact until it reaches the second contact.


KEY ON data is generated when the key contact reaches the second contact. Key off data is generated when the first contact is reached after leaving the second contact.


3) After Touch data

The ESK-901 keyboard's after touch unit (sandwich of metal plate, conductive rubber, and metal plate) produces impedance variations which K LM-759 detects as analog voltages over the range of OV 3.5V.

The voltage passes through the multiplexer 4051 (IC5) to the CPU where, via a DAC, it changes the control voltage for the effect.


4) DWGS system

This board contains the DWGS basic system. The purpose of this system is to get pitch and waveform data from the CPU bus and output a cyclic (repetitive) waveform of constant amplitude.


Oscillator operation


The PAI (phase angle increment) value and PAR (phase

angle register) value are added and the result is stored again

in the PAR. The PAR value is used as the wave table ad-

dress. The wave table stores different harmonic configura-

tion data for each octave on the keyboard.

IC47 (MB64H129) performs the processing needed to use

the PAR value as the wave table address.

Finally, data read from the wave table is converted to an

analog waveform by a D/A converter.

Time division multiplexing enables dual oscillator 6-voice

sound source capability.

Maximum simultaneous output of this system is 8 voices

x 2 oscillators.

Note: Given a sampling frequency of 50 kHz, PAI data

N = 2" x f/50 x 10‘ (where f is the pitch frequency)

is rounded to an integer value for N and converted

to a hexadecimal number.













74LS1S9 x 10 IC48-IC57


IC66 (62H133)
















HC138 + 4066 + 072