DS2480 Datasheet, PDF (22/26 Pages) Dallas Semiconductor – Serial 1.Wire Line Driver

DS2480
+ 12V RS232 Figure 10c
UART
OR µC
POWER
STEALING
DTR
RTS
RXD
2.7 k
TXD
1µ
GND
L
START
5V
VOLTAGE
REGULATOR
12V
(5V)
V_DD
V_PP
POL
BSS110
6.8 k
S
D
SEE TEXT
RXD
1–WIRE BUS
1–W
DS2480
TXD
4.7V
GND
RETURN
0
M
1
STOP
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 kΩ 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
DC TO DC
5V
CONVERTER
5V
SIN
2.7 k
UART
OR µC
SOUT
6.8 k
12V
2.7 k
V_DD
HCPL–2300
OPTOCOUPLER
1
L
START
0
(5V) ^VPP
POL
M
STOP
RXD
1–WIRE BUS
1–W
DS2480
TXD
GND
RETURN
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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