DESIGN SHOWCASE
Standby current for RS-422 repeater is
less than 3碌A(chǔ)
The RS-422 interface is an excellent choice for commu-
nicating in noisy environments and over a distance.
However, when the distance exceeds the RS-422 capa-
bility for reliable data transfer, you must add a repeater.
The repeater circuit in
Figure 1,
which must operate
from batteries when no power supply is available,
draws less than 3碌A(chǔ) of current from a 3V supply.
Low standby power and true fail-safe operation are the
key features in this application. U1 and U2 drive their
receiver outputs (RO) high when the RS-422 inputs
are open circuited or terminated and undriven. An
incoming data byte on the differential inputs A1 and
B1 forces a transition on RO of U1, and a state
machine (right half of the Figure 1 schematic) is
latched ON by the falling edge of RO. The state
machine asserts a high level at U2鈥檚 driver-enable pin,
causing the incoming data byte to be retransmitted
from U2 at full RS-422 levels.
The state machine watches for transitions on RO.
When a data-byte transfer is complete (as indicated by
no falling edges within a time delay internal to the state
machine), the state machine resets itself in anticipation
of the next data byte from either side of the interface.
An incoming data burst (Figure
2,
top) is retrans-
mitted as outputs A2 (bottom trace) and B2 (middle
trace). U2 de-asserts those outputs 700碌s after the
final transition. Other delays can be implemented by
adjusting R1/C1 and R2/C2 as shown in Figure 1.
V
CC
D1
1N4148
R1
4.7M鈩?/div>
C1
220pF
8
7
R3
120鈩?/div>
A1
6
5
V
CC
B
A
GND
U1
RO
RE
DE
DI
1
2
3
4
1
2
U3B
74HC00
4
5
6
V
CC
C3
100nF
B1
1
CLK
D
V
CC
CLR
PRE
4
10
U3C
74HC00
8
D
CLK
PRE
CLR
13
11
12
11
Q
Q
U4B
74HC74
9
8
3
U3A
74HC00
3
2
Q
Q
6
5
U4A
74HC74
MAX3471
GND1
V
CC
9
10
GND2
C4
100nF
8
7
V
CC
B
A
GND
U2
RO
RE
DE
DI
1
2
3
B2
R4
120鈩?/div>
A2
MAX3471
6
5
V
CC
4
D2
1N4148
R2
12
4.7M鈩?/div>
13
C2
220pF
U3D
74HC00
Figure 1. This RS-422 repeater draws only 3碌A(chǔ) of standby current.
16
1
2
