LTC2870_15 Datasheet, PDF (25/42 Pages) Linear Technology – RS232/RS485 Multiprotocol Transceivers with Integrated Termination

LTC2870/LTC2871
Applications Information
receiver input, B is the positive receiver input, Y is the
negative driver output, and Z is the positive driver output.
The TIA/EIA-485 and TIA/EIA-422 standards do not defini-
tively 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 manufacturers
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.
10k
1k
LTC2870/LTC2871
MAX DATA RATE
100
10
10k
RS485/RS422
MAX DATA RATE
100k
1M
10M
DATA RATE (bps)
100M
28701 F19
Figure 20. Cable Length vs Data Rate (RS485/RS422
Standard Shown in Vertical Solid Line)
For more information www.linear.com/LTC2870
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