DS2480 datasheet(22/26 Pages) DALLAS

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DS2480

042498 22/26

12V RS232 Figure 10c

START

1–WIRE BUS

RETURN

VDD

VPP

RXD

TXD

POL

1–W

GND

UART

µC

(5V)

STOP

DTR

VOLTAGE

REGULATOR

DS2480

12V

RTS

RXD

TXD

GND

SEE TEXT

POWER

STEALING

2.7 k

6.8 k

4.7V

BSS110

In the interface to a true RS232C system (

± 12V, Figure

10c) the power for the DS2480 is stolen from DTR and

RTS. The software must make sure that at least one of

these signals constantly provides the 12V operating

voltage. The 6.8 k

Ω resistor and the 4.7 V Zener diode in

the TXD line limit the positive voltage at the TXD pin of

the DS2480. The Schottky diode limits the negative volt-

age to 0.3V maximum. The Schottky diode in series with

the capacitor forms a parasitic supply to generate the

negative bias for the host’s receive channel. The posi-

tive signal is switched in through the P–channel MOS-

FET that connects to the RXD output of the DS2480. In

this circuit diagram the MOSFET switches the RXD line

to +5V, which normally is sufficient for RS232C

systems.

Switching to 12V is also possible, but requires a

P–channel transistor with a different threshold voltage.

The signal inversion caused by the transistor is com-

pensated through the DS2480 by using a value code of

100, 101, 110 or 111 for the RS232 baud rate setting.

UART DIRECT OPTO ISOLATED Figure 10d

START

1–WIRE BUS

RETURN

VDD

VPP

RXD

TXD

POL

1–W

GND

UART

µC

(5V)

STOP

SIN

DS2480

12V

HCPL–2300

OPTOCOUPLER

SOUT

2.7 k

6.8 k

2.7 k

DC TO DC

CONVERTER

The circuit in Figure 10d is essentially the same as in

Figure 10a. The main difference is the opto–isolation.

The characteristics of the opto–isolators are not very

critical. Using a different type, will affect the values of the

resistors that limit the current through the LEDs and bias

the photo transistors.

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