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This circuit is a simple 8-bit
analogue to digital converter circuit which is connects
to PC serial port. The circuit is based on TLC548
A/D-converter chip (might be hard to get nowadays, Texas
Instruments makes this IC), which is an A/D-converter
with serial output. The output of the TLC548 is not
directly suitable for standard serial data reception, so
this circuit uses serial port handshaking lines in a
nonstandard way which enables the communication between
computer and converter chip to be implemented with as
few components as possible. The circuit takes all the
power it needs from PC serial port.
A generally asked question is "Can I
Connect a Serial A/D to My Computer's Serial Port?". The
answer to this question is yes, but there are simpler
ways to interface to a computer. To begin with, the
computer's RS-232 port uses higher voltage levels that
are incompatible with most ADC's TTL/CMOS logic.
Another problem is that the computer's
RS-232 serial port is asynchronous, which places a few
more requirements on the communication, including some
additional handshaking control lines. Also, with the
RS-232 serial port, you must set both ends of the link
to the same baud rate. The A/D converter's serial port
is synchronous; thus, its handshaking requirements are
minimal and it only requires one wire for clock and one
or two wires for data.
If you are truly determined, you can
bit-bang using the RS-232 port's handshake lines. This
circuit uses this control method. The voltage conversion
of signals from RS-232 port to IC input pins is done
using a combination of series resistors and the internal
input protection diodes in the IC. The output of this
A/D was able to directly output signals which seemed to
be detected by RS-232 port (the output of the chip does
not meet RS-232 specifications, but in practice still
worked with the PC serial ports tested).
Maximum sample rate: 40 000
samples/second (maximum rate of TLC548 chip)
Input voltage range: 0-5 V
Accuracy of A/D converter chip: 8
databits (+-0.5 LSB)
Computer interface: PC serial port
Interface protocol: Proprietary
syncronous serial interface (uses PC serial port as
general purpose I/O port)
The picture below is tha circuit diagram
of the circuit. The pin numbers of the connector are pin
numbers of 25-pin serial port connector. All diodes are
IN4148 or similar. All resistors connected to IC are 30
kOhm and their function is to protect the IC inputs with
the diodes against overvoltage. The 1 kOhm resistor, one
diode, 5.1V zener-diode, 10uF and 100nF capacitors make
the power supply which takes circuit power from serial
port Data Terminal Ready -pin.
Click Here to View Circuit
Circuit diagram of the serial port A/D
converter. This picture is available separately in GIF
format.
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3 pices of 30 kohm resistor
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7 pieces of 1N4148 diode
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1 kohm resistor
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5.1 V zener diode
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10 uF 15V electrolytic capacitor
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100 nF capacitor
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TLC548 A/D converter IC (may be hard to
get)
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25 pin female D connector
The following signal diagram tells that
signals are needed for controlling the TLC548 A/D
conversion chip. The chip needs chip selected (CS) and i/o
clock signal (CLK) to operate. The chip gives the
conversion results from data output pins (Dout). Those
signals are simple and easy to understand and can be
easily generated using simple software routine.
Signal diagram of TLC548 chip. This
picture is available separately in GIF format.
This type of signaling is quite common
in many A/D converters with serial outputs. There are
differences in different type of A/D converters, but you
can easily use the ideas used in this circuit for
interfacing other types of A/D converters to your PC.
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