DS2480 Dallas Semiconducotr, DS2480 Datasheet - Page 23

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

+
The circuit in Figure 10e combines the true RS232C
interface with opto–isolation. The energy to power the
LED in the TXD channel and to provide the positive volt-
age 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 a opto–isolator with different
voltage characteristics.
RS232 DATA TIMING RXD LINE Figure 11
RECEIVE DELAY TIMING Figure 12
5 TO 12V RS232 OPTO ISOLATED Figure 10e
OR C
UART
t
FR
START
TXD
t
RR
INPUT SAMPLING 8.5T
BIT 0
1
RXD
GND
DTR
RTS
TXD
BIT 6
POWER
STEALING
BIT 1
START
L
BIT 7
R
BIT 2
(DIAGRAM DRAWN FOR POL = 5V)
T
REGULATOR
R
0
1
9.5T
STOP
5V
R
M
BIT 3
STOP
5.1 k
10 T
POWER
SUPPLY
R
t
RESP
RXD
BIT 4
In the schematic the HCPL–2202 opto–isolator is
sourced by the RXD pin of the DS2480. It can as well be
connected the traditional way where the DS2480 sinks
the current through the LED. This, however, causes a
signal inversion that has to be compensated through the
DS2480 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.
HCPL–2202
(DIAGRAM DRAWN FOR POL = 5V)
HCPL–2300
BIT 5
1.5 k
12V
5V
START
BIT 6
2.7 k
BIT 0
BIT 7
V
RXD
TXD
PP
DS2480
t
IDLER
GND
V
DD
STOP
POL
1–W
1–WIRE BUS
RETURN
042498 23/26
DS2480