DS2480B
27 of 31
Figure 11d. UART DIRECT OPTO-ISOLATED
SIN
RXD
SOUT
(TXD)
UART
or µC
Start Stop
1
0
LM
5 V
DC to DC
Converter
*
1-Wire Bus
Return
GND
1-W
POL
VDD
VPP
RXD
TXD
DS2480
* only one DS950x ESD protection device with 5V
5V
12 V
HCPL-2300
2.7K
5.1K
2.7K
5.1K
DS9503
DS9503
The circuit in Figure 11d is essentially the same as in Figure 11a. 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.
Figure 11e. ±5 TO 12V RS232 OPTO-ISOLATED
SIN
RXD
SOUT
TXD
UART
or µC
Start Stop
1
0
LM
1-Wire Bus
Return
DTR
Power Stealing
GND
1-W
POL
VPP
VDD
RXD
TXD
DS2480
RTS
2.7K
HCPL-2300
5V
Regulator
1.5K
1 µf
HCPL-2202
5V
+12VDC Programming
*
* only one DS950x ESD protection device with 5V
1.5K
DC/DC
Convert.
DS9503
DS9503
The circuit in Figure 11e combines the true RS232C interface with opto-isolation. The energy to power
the LED in the TXD channel and to provide the positive voltage for the host’s RXD input is stolen from
DTR and RTS. The negative voltage for the RXD input is taken from the TXD line through a parasitic
supply consisting of a Schottky diode in series with a capacitor. The HCPL-2202 opto-isolator has a
totem pole output that allows switching in positive as well as negative voltage. The +5V are sufficient for
most RS232C systems. Switching in 12V requires an opto-isolator with different voltage characteristics.
In the schematic the HCPL-2202 opto-isolator is sourced by the RXD pin of the DS2480B. It can as well
be connected the traditional way where the DS2480B sinks the current through the LED. This, however,
causes a signal inversion that has to be compensated through the DS2480B by using a value code of 100,
101, 110, or 111 for the RS232 baud rate setting. Using other types of opto-isolators than shown in the
schematic will at least require changing the values of the resistors.
