3054
MULTIPLEXED
TWO-WIRE
HALL-EFFECT SENSOR ICs
Bus Driver
It is recommended that the bus be controlled
by microprocessor-based hardware for the
following reasons:
• Sensor address information may be stored
in ROM in the form of a look-up table.
• Bus faults can be pinpointed by the
microprocessor by comparing the diagnos-
tic response to the expected response in
the ROM look-up table.
• The microprocessor, along with an A/D
converter, can also be used to self cali-
brate the quiescent currents in the bus and
hence be able to easily detect a signal
response.
• The microprocessor can also be used to filter out random line noise
by digitally filtering the bus responses.
• The microprocessor can easily keep track of the signal responses
and initiate the appropriate action (e.g., light a lamp or sound an
alarm, and also pinpoint the location of the signal).
Optimally, the microprocessor is used to control bus-driving
circuitry that will accept TTL-level inputs to drive the bus and will return
an analog voltage representation of the bus current.
Interface Schematic
The bus driver is easily designed using a few operational amplifi-
ers, resistors, and transistors. Figure 4 shows a schematic of a
recommended bus driver circuit that is capable of providing 6 V to 9 V
transitions, resetting the bus, and providing an analog measurement of
the bus current for the A/D input of the microprocessor.
FIGURE 4
BUS INTERFACE SCHEMATIC
+15 V
1 kΩ
10 kΩ R
4
9V
Z
1
0.001
µF
20 kΩ
R
5
ADDRESS
5 kΩ
Q
1
RESET
5 kΩ
50 kΩ
R
8
50 kΩ
R
7
1
2
3
1
2
3
1 kΩ
Q
3
OP
1
BUS SUPPLY
50
Ω
R
6
Q
2
X
X
NC
SWITCH
100 kΩ R
9
BUS RETURN
ANALOG OUT
OP
2
100 kΩ
R
10
Dwg. EH-003A
ALLEGRO [ ALLEGRO MICROSYSTEMS ]
