Datasheet

DS2480B
Maxim Integrated ........................................................................................................................................................................................... 26
Figure 11c. ±12V RS232
SIN
(RXD)
SOUT
(TXD)
UART
or µC
Start
Stop
1
0
L
M
1-Wire Bus
Return
DTR
Power Stealing
GND
1-W
POL
VPP
VDD
RXD
TXD
DS2480B
RTS
See Text
2.7K
4.7V
5V
Regulator
6.8K
1 µf
BSS110
S
D
+12VDC Programming
*
* only one DS950x ESD protection device with 5V
DS9503
DS9503
In the interface to a true RS232C system (±12V, Figure 11c) the power for the DS2480B 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 6.8kΩ resistor and the 4.7V Zener diode in the TXD line limit the positive voltage
at the TXD pin of the DS2480B. The Schottky diode limits the negative voltage to 0.3V maximum. The
Schottky diode in series with capacitor forms a parasitic supply to generate the negative bias for the
host’s receive channel. The positive signal is switched in through the P-channel MOSFET that connects
to the RXD output of the DS2480B. 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 compensated through the DS2480B by using a value code
of 100, 101, 110, or 111 for the RS232 baud rate setting.