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© 2002 Microchip Technology Inc.
DS00782A-page 1
AN782
TC7660 Powers RS-232 Data Loop Low Cost Adapter
The RS-232 input voltage spec is ±3V. The minimum input resis-
tance of the TC232, a typical RS-232 transceiver, is about 3kΩ.
Therefore, 1mA of current must be supplied, and R1 must be:
R
1
= = 2kΩ
For reliable operation the optoisolator should be biased to
saturation, so:
I
OPTO
= + = 2kΩ
Since the optoisolator’s current transfer ratio is only 20%, the LED
current must be:
I
LED
= = 33mA
For interfacing to lower-power devices a higher gain optoisolator
can be substituted. The 4N33 Darlington, for example, has a
current transfer ratio of 500%, which reduces input drive current
requirements to only 1.3mA.
For a cable length of six feet, the circuit operates properly up to
9600 baud. Unfortunately, high baud rates are not always useable.
This is because many computer prototyping boards seem to
have software serial-communications routines which are de-
signed for 100 baud teleprinters. These routines do not make use
of the handshakings signals which RS-232 provides. Unless the
serial communications routines are rewritten, lower baud rates
may be required for proper operation.
10k 5V
2kΩ 3kΩ
FIGURE 1: This low cost circuit converts TTL-level signals to an RS-232 level without the expense of a negative power supply. The
TC7660’s –5V output permits the optoisolator to swing to RS-232 levels at up to 9600 baud.
Author: Wes Freeman,
Microchip Technology, Inc.
INTRODUCTION
The introduction of single-voltage EPROMS and dynamic RAMs
has permitted designers to produce complete digital systems
powered by a single 5V supply. One area which has not yielded
to single-supply operation, however, is the RS-232 interface. If a
system must communicate with an RS-232 serial device, such as
a printer or another computer, a separate power supply is
required.
The circuit in Figure 1 provides an RS-232 driver without requiring
a second power supply. Originally built for downloading files from
an IBM PC to an IAM-65, the circuit is applicable to a wide variety
of single-board computers as well as single chip microcontrollers
such as Microchip's PIC16C745. In 100-piece quantities the
component cost is less than $3, and pc board space is only a little
more than is occupied by a 20-pin socket.Understanding the
circuit’s operation is easy. U1 is the CMOS TC7660 DC-to-DC
converter. It contains an oscillator and matrix of switches which
convert the +5V supply to –5V. The optoisolator converts the
TTL-level input current into voltage which swings between the
plus and minus supply rails, producing RS-232 compatible output
levels.
Resistor R1 determines the RS-232 output voltage swing. R1’s
value is determined by the input specifications of the receiving
device, current transfer ratio of the optoisolator, and the driving
circuit’s output current capability.
5V – 3V
1mA
6.6mA . 100%
20%
Single Board Computer
or Other Serial
Communications Device
GND
+5V
Serial
Data
Output
(TTL Level)
100Ω
10µfd
2
1
4
5
4N25
Optioisolator
R1
2kΩ
8
2
3
5
V+
GND
CAP+
CAP–
V
OUT
–5V
10µfd
4
Serial Data
Output
(RS-232)
3*
7
Signal
Ground
Transmitted
Data
RS-232
DB-25
Connector
* Configures the Single Board Computer
as DAA Communications Equipment (DCE)
TC7660
DC-DC
Converter
+
+
