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APPLICATION
EXAMPLES
that eliminate the need for the +
12
volt and -
12
volt power supplies normally required.
The +
12
volt power supply
was
eliminated by
connecting the emitter of
Tl
to
+5V.
While
this produces a signal that
is
not strictly
within the RS232C specification, it works
well
on interconnections of less than
10
meters.
This design also employs a useful trick to
eliminate a -12V power supply. Many people
have attempted to eliminate this supply by
driving the TRANSMIT-DATA line bet-
ween GROUND and
+5V.
Because of a
circuit switching element (Transistor T2), the
low-level signal
is
always a little higher than
ground and hence won't work with many
terminals requiring a negative voltage for a
LOW. This design, however, uses the REC-
EIVE-DATA line (presumably driven by a
true RS232C-compatible terminal) as a
source of a negative voltage.
This negative voltage (negative whenever
RECEIVE-DAT A
is
low) charges capacitor
Cl
through diode
Dl.
This circuit has been
verified to work when receiving any sequence
of characters, except BREAK.
BREAK causes a very long
"I"
on REC-
EIVE-DATA; TRANSMIT-DATA event-
ually exhausts the negative charge on cap-
acitor
Cl.
If
desired, a
-12
volt supply may be
connected to the junction of
Cl,
Dl
and R4.
This RS232C interface
is
driven by soft-
ware to provide the proper timing for
transmitting and receiving characters.
Multiplexed
System #2:
The
Vest
Pocket
Computer
Combining state-of-the-art microprocessor
components results in a usable computer
small enough to be carried in a vest pocket
(Fig.
4.2).
In only
15
square inches (3"x5"), this system
could contain a 2K tiny BASIC operating
system,
16K
memory for user programs, and
an
I/O
port. The port
is
designed to interface
to a terminal.
The system
is
designed with an
8088
CPU,
8755A
I/O
Port with
EPROM
and
21821
RAMs with 4 K byte density each.
The
21821
is
a new concept in
RAM
architecture, interfacing directly on the iAPX
86,
88
or MCS-85 multiplexed bus, respond-
ing directly to controls from the processor.
Contained within the
21821
is
a com-
plete memory system on a single piece
of
silicon.
DIP----
MDS
s
[J
rTl
A
2
4
:
D
D
L
SWITCH VDT !
D!
W 8088
8755A-2
~RESET
2
1
8
2
1
2
1
8
2
1
2
1
8
2
1
Figure 4-2. Vest Pocket
Computer
Component
Layout
4-5
2
1
8
2
1