MAX14781E_14 Datasheet, PDF (17/19 Pages) Maxim Integrated Products – Half-Duplex RS-485 Transceiver with Polarity Correction

MAX14781E
Half-Duplex RS-485 Transceiver
with Polarity Correction
Automatic polarity detection and correction in the
MAX14781E is based on having non-changing voltages
on the A/B bus signals for longer than 20ms that
define the reference polarity. Fixing the A/B voltages
can be achieved during the idle intervals in master/slave
communication.
Passive Polarity Definition
To allow the bus to define A/B polarity, connect one
pullup/pulldown resistor pair to the bus to set the bus
status during the idle periods. It is preferable to locate
the resistor pair in the bus master, as shown in Figure 11.
Active Polarity Definition
Alternatively, the bus polarity can be defined actively by
the bus master. For this technique, connect differential
termination resistors on the bus, preferably in the bus
master module, as shown in Figure 12. The termination
resistor ensures that the differential (V_A – V_B) voltage
during bus idle times is zero. The MAX14781E fail-safe
feature ensures that the receiver outputs are logic high
and that the detected polarity is not changed during the
idle time. This active technique requires the bus master to
the A/B signals to a constant DC condition for over 20ms.
The master can do this by driving its DE high for longer
than 20ms while DI is logic high.
RO
R
RE
DE
DI
D
VCC
A
B
MISWIRED!
MASTER
A
B
MAX14781E
Figure 11. Passive Polarity Definition
RO RE DE DI
SLAVE
A
B
A
R
RO
RE
B
DE
D
DI
SLAVE
RO RE DE DI
SLAVE
www.maximintegrated.com
Maxim Integrated │  17