Transformer Driver for
Isolated RS-485 Interface
MAX253
V
IN
5V
C1
0.1µF 5
6
N.C.
V
CC
D1
1
ISOLATION
BARRIER
ICT:1.3CT**1N5817
C3
0.1µF
8
8
C2
22µF
IN
OUT
2
ISO 5V
C4
22µF
ON / OFF
4
SD
MAX253
D2
MAX667
SET GND SHDN
6
4
5
FS
GND1 GND2
2
7
3
1N5817
5 x 3.3kΩ
10 x PC417
*74HC04
T1
IN
74HC04
T2
IN
74HC04
T3
IN
74HC04
T4
IN
74HC04
T5
IN
5 X 3.3kΩ
74HC04
R1
OUT
74HC04
R2
OUT
74HC04
R3
OUT
74HC04
R4
OUT
74HC04
390Ω
1
2
6
5
4
8
V
CC
T1
IN
GND
3
T1
OUT
390Ω
7
T2
IN
T2
OUT
4
390Ω
15
T3
IN
T3
OUT
2
390Ω
16
T4
IN
T4
OUT
1
390Ω
22
T5
IN
T5
OUT
19
MAX205
6
5
4
1
2
390Ω
6
R2
OUT
R2
IN
5
390Ω
9
R1
OUT
R1
IN
10
390Ω
23
R3
OUT
R3
IN
24
390Ω
17
R4
OUT
R4
IN
18
390Ω
14
R5
OUT
*74HC04 OR EQUIVALENT
R5
OUT
SD
21
R5
IN
EN
20
13
**
SEE TABLE 2
4N25 LOWER SPEED, LOWER COST ALTERNATE OPTOCOUPLER CONFIGURATIONS (FOR DATA RATES BELOW 9.6kbps)
V
CC
1N5711
4N25
6 1N5711
6
4N25
3.3kΩ
3.3kΩ
390Ω
1
1
T
IN
ISO R
OUT
5
5
T
IN
390Ω
74HCO4
*74HC04
2
2
ISO
ISO
4
4
GND
GND
V
CC
ISO
R
OUT
Figure 5. Typical RS-232 Application Circuit
8
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MAXIM [ MAXIM INTEGRATED PRODUCTS ]
