DS2480 Dallas Semiconducotr, DS2480 Datasheet - Page 22

• Manufacturer Part Number: DS2480
• Description: Serial 1.Wire Line Driver
• Manufacturer: Dallas Semiconducotr

+
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 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-
UART DIRECT OPTO ISOLATED Figure 10d
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
DS2480
042498 22/26
12V RS232 Figure 10c
OR C
UART
OR C
UART
5V
SOUT
SIN
CONVERTER
DC TO DC
2.7 k
1
RXD
GND
DTR
RTS
TXD
POWER
STEALING
2.7 k
START
START
OPTOCOUPLER
HCPL–2300
L
6.8 k
L
REGULATOR
VOLTAGE
BSS110
0
1
1
0
6.8 k
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.
critical. Using a different type, will affect the values of the
resistors that limit the current through the LEDs and bias
the photo transistors.
4.7V
M
M
STOP
STOP
2.7 k
SEE TEXT
S
D
5V
(5V)
12V
5V
12V
(5V)
V
RXD
TXD
PP
DS2480
V
RXD
TXD
V
GND
PP
DD
DS2480
GND
V
POL
1–W
DD
POL
1–W
1–WIRE BUS
RETURN
1–WIRE BUS
RETURN