Datasheet

LTC2870/LTC2871
25
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applicaTions inForMaTion
The TIA/EIA-485 and TIA/EIA-422 standards do not
definitively specify the polarity of the driver input (DI) or
receiver output (RO) that correspond to the bus polarity
(Y/Z/A/B). This has caused some contention among man-
ufacturers of early devices, with some using one polarity
and others using its opposite. However, today nearly all
manufacturers, including Linear Technology, agree on the
convention of A−B and Y−Z being positive for a logic high
on DI and RO.
Because the LTC2870 was not designed to be a direct
replacement of the LTC1387, the polarity of the RS485/422
signaling was updated to the more modern convention.
For LTC2870 signal compatibility with the LTC1387 or
LTC1334, if the polarity of the RS485 cannot be reversed
in the software, exclusive-OR logic gates can be used
at the driver input and receiver outputs. Tie one of the
logic gate inputs to RS485/RS232# and the other to DY
or RA. When RS232 mode is selected, the signal is passed
without inversion but in RS485 mode the output signal
becomes inverted.
Logic Loopback
A loopback mode connects the driver inputs to the receiver
outputs (non-inverting) for self test. This applies to both
RS232 and RS485 transceivers. Loopback mode is
entered when the LB pin is high and the relevant receiver
is enabled.
In loopback mode, the drivers function normally. They
can be disabled with outputs in a Hi-Z state or left enabled
to allow loopback testing in normal operation. Loopback
works in half- or full-duplex mode and does not affect the
termination resistors.
RS485 Cable Length vs Data Rate
For a given data rate, the maximum transmission dis-
tance is bounded by the cable properties. A typical curve
of cable length vs data rate compliant with the RS485/
RS422 standards is shown in Figure20. Three regions
of this curve reflect different performance limiting fac-
tors in data transmission. In the flat region of the curve,
maximum distance is determined by resistive losses in
the cable. The downward sloping region represents limits
in distance and data rate due to AC losses in the cable.
The solid vertical line represents the specified maximum
data rate in the RS485/RS422 standards. The dashed lines
at 20Mbps show the maximum data rates of the LTC2870
and LTC2871.
DATA RATE (bps)
CABLE LENGTH (FT)
28701 F19
10k
1k
100
10
10k 10M 100M1M100k
LTC2870/LTC2871
MAX DATA RATE
RS485/RS422
MAX DATA RATE
Figure20. Cable Length vs Data Rate (RS485/RS422
Standard Shown in Vertical Solid Line)
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