MAX3157CAI View Datasheet(PDF) - Maxim Integrated
Part Name
Description
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MAX3157CAI Datasheet PDF : 13 Pages
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High CMRR RS-485 Transceiver
with ±50V Isolation
Block Diagram
DI
TXP
DE
RO
RE
RXP
VCC
OSC
POWER
Y
Z
YR
ZR
A
B
AR
BR
H/F
ISCOVCC
GND
C1L C2L
C1H C2H
ISOCOM
and amplitude are also determined by the resistance
between the remote ground and the ISOCOM pin. The
receiver input resistors will cause the isolated common
voltage to go to the mean voltage of the receiver inputs,
which will be a direct function of the remote ground
potential. The receiver input resistance and the capaci-
tors C1, C2, and C4 set up a time constant that limits
how fast the ISOCOM pin can follow variations in the
remote ground voltage. Connecting YR and ZR in half-
duplex operation, or AR and BR in full-duplex operation,
to ISOCOM results in a relatively low input impedance of
the MAX3157 receiver inputs (2kΩ). This allows for a
60Hz sine wave with a 50V maximum amplitude (see
Typical Operating Characteristics). If YR, ZR, AR, and
BR are left open, the receiver input impedance is 12kΩ
allowing up to 32 transceivers on the bus. To guarantee
the same low time constant under those conditions, use
a shielded cable with a 1kΩ resistor connected between
the shield and ISOCOM. Using a lower value for this
resistor is not recommended because this could trigger
a holding current in the internal ESD protection device if
the ±75V isolation limit is exceeded.
A single point hard-ground connection for the shield is
recommended.
Communication Between
Two MAX3157s
If two MAX3157 devices are used to communicate with
each other, one of the devices must have ISOCOM
connected to local ground. Failure to do so will result in
floating ISOCOM pins, with both devices trying to adapt
to the isolated ground of the other.
Chip Information
TRANSISTOR COUNT: 1309
______________________________________________________________________________________ 11
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