Memory Mapped I/O; Data Entry Knob And Footswitches; Keypad Decoding; Midi - Alesis D4 Service Manual

completing the reset process. The reverse process occurs anytime the raw supply falls below 7.2V.
This ensures that all functions that might cause data corruption are not functioning when the power
supply is unstable.

3.12 Memory mapped I/O

In order to easily control the vast number of hardware functions that the 8031 needs to access, a
system of memory mapped I/O is used. The basic idea is to make hardware functions appear to the
8031 as unused memory locations. That way all that the software has to do is write to a memory
location in order to send that information to a specific device such as the LCD, or ASIC. In the case of
the D5 many of the map decoding functions are incorporated into the D56IFT ASIC, reducing the parts
count relative to the D4.
74HC138 (U12) {D56IFT} performs the majority of the work in this circuit. Two things are
required before U12 becomes active. 1> A15 must be low (i.e. the 8031 is accessing the lower 32K of
address space). 2> The 8031 WRite line must be active (the 8031 is performing a memory write). A15
is used to directly control which function (memory or I/O) is active.
Once U12 is enabled, addresses A12-A14 are decoded by it, and the latch corresponding to the
value of the decoded address is strobed. At this point, data on the 8031 data buss is "written" into the
latch.

3.13 Data Entry Knob And Footswitches

The data entry knob and footswitches are read via the memory mapped latch U11 {U22}. This
allows the 8031 to directly read any changes in the status of these inputs.

3.14 Keypad Decoding

The keypad is handled through a simple polling matrix implemented through the memory
mapped latches U10 and U11 {U20, U22}. Each row is tested by the 8031, one at a time, via U10
{U20}. Any closed switches are read into U11 {U22}, and passed along to the 8031 for interpretation.

3.15 MIDI

The MIDI hardware is a standard implementation. MIDI out/thru begins at the 8031's TXD port
(pin 11) and is buffered with two inverters of U7 {U27}.
MIDI in consists mostly of the opto isolator (U8) {U23}, protection diode D4 {D6}, pullup R18
{R128}, and threshold resistor R19 {R129}.

3.20 DM3AG ASIC

The DM3AG ASIC is a complex LSI device, specifically designed for the purpose of playing
percussion samples. Obviously, the internal workings of such a device are beyond the scope of this
manual, however, a brief description of the important pins follows.
Alesis D4/D5 Drum Modules Service Manual 4 02/19/03