Microchip TC7660S Manual

Microchip Ikke kategoriseret TC7660S

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Side 1/4

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:

R1 = = 2kΩ

For reliable operation the optoisolator should be biased to

saturation, so:

IOPTO = + = 2kΩ

Since the optoisolator’s current transfer ratio is only 20%, the LED

current must be:

ILED = = 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

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

4N25

Optioisolator R1

2kΩ

V+

GND

CAP+

CAP–

OUT

–5V

10µfd

Serial Data

Output

(RS-232)

7Signal

Ground

Transmitted

Data

RS-232

DB-25

Connector

* Configures the Single Board Computer

as DAA Communications Equipment (DCE)

TC7660

DC-DC

Converter

Produkt Specifikationer

Mærke:	Microchip
Kategori:	Ikke kategoriseret
Model:	TC7660S

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