of Microprocessors

SOME REAL ApPLICATIONS 725
The analog demultiplexor in Fig. I9-7B has a unique feature that
allows the computer some control over the transition times of analog control
signals. For normal updating of the sample-and-holds, the top multiplexor
only is enabled, which gives an immediate voltage change on the large
(0.033 f-LF) hold capacitors. However, the actual control voltage is taken
across the small (0.0068 f-LF) capacitors, which with the I-MEG resistor gives
a time constant of 6.8 msec. When the control computer desires an
instantaneous voltage update, the bottom multiplexor is also enabled, which
charges both capacitors to the new voltage. Of course, slower transition rates
for envelopes are provided by writing successive values to the sample-andholds
in slow mode. Chroma software refreshes the sample-and-holds every
20 msec, which is 50 times per second. Because of the I-MEG tesistors, it
was essential to use EET input op-amp buffers. The demultiplexor is actually
on the voice boards, which greatly reduces the number of analog connections
otherwise required.
External Computer Interface
When it was released, one of the Chroma's unique features was a highspeed
parallel interface between its 6809 control computer and any kind of
external digital device. Its primary purpose was to connect to a Chroma
expa1Zdel~ which is identical to the Chroma except the keyboard is missing and
the control program is slightly different. Increasing interest in personal
computers by players, however, prompted full documentation of the interface
hardware and command language so that an external computer could record
keystrokes, play them back, and operate the control panel. The "Interface
Manual" even shows an Apple II computer on the cover. The philosophy and
command structure is similar to MIDI, which was discussed in Chapter 9,
but, being parallel, the potential data rate is much higher.
The interface logic for data out consists of an 8-bit latch, a data out
ready flip-flop, and an acknowledge input for a total of 10 lines. When the
Chroma wishes to send a byte to an exrernal computer or an expander, the
byte is written into the data out register, which also sets the flip-flop. The
external device responds by reading the data out lines and then sending a
pulse on the acknowledge line, which resets the flip-flop. Having been reset,
the flip-flop interrupts the Chroma's program, which then knows that it can
send another byte.
The interface logic for data in uses a tri-state buffer for eight more datain-lines
and some gates to sense input data ready and generate input data
taken signals. When an external device has a byte ready, it places it on the
data in lines and raises the input data ready signal. This signal then
interrupts the Chroma, which responds by reading the data through the tristate
buffer, which also pulses the input dara taken signal. The remote device
responds by dropping input dara ready until it has another byte to send.