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S768DAV – S7768DAV User Manual
A
A: RS485 A
PPENDIX
A.1
I
NTRODUCTION
The S768DAV is configurable for both full-duplex (four-wire) and half-duplex (two-wire) operation. It can
be used for interfacing to systems adhering strictly to the RS-485 specification and for use with systems
that use a modified, "fail-safe biased" RS-485 bus.
A.2 C
ONNECTION
Use high-quality twisted-pair wiring, and make sure all connection points are clean and tight. A loose
connection on one of the wires can appear to function, yet cause intermittent errors: DATA LEDs may be
flashing as signals pass through the system, but those signals will be corrupt.
A.3 C
ONFIGURATION
The S768DAV multi-protocol data (MPD) units are designed to work with virtually any RS485 system.
Unfortunately, some systems operate on a "modified" version of RS485 that use a fail-safe biasing to pull
up/down their bus during a tri-state condition. In a standard RS485 system, when a driver on a properly
terminated bus goes into tri-state, the voltage between the differential outputs should be less than
200 mv.
(This is considered "standard offset" in the product instruction manuals.) A differential output tri-state
voltage this small can cause some non-standard systems to latch up, because they are designed for
much larger, "fail-safe," offsets.
To be able to interface to such equipment, the S768DAV MPD units offer two more "offset" level modes.
In most cases, the S768DAV unit should be configured for "standard offset" operation.
When the system is operating properly, the DATA IN and DATA OUT LEDs are off when there is no
communication (tri-state) and flash when data is being sent or received. If the link is not functioning
properly (LEDs will most likely not turn off), change the DATA SELECT switch on the unit to a higher
offset mode.
First try 1 V, then 2 V. If switching the mode switch does not prove effective, the offset level may have to
be emulated at one end of the system by using pull-up/down resistors on the data connector. The
S768DAV has a +5 V bias pin and ground pins on the connector for this purpose. Contact the equipment
manufacturer's technical support for recommended resistor values and configuration.
A.4 T
ERMINATION
RS485 systems need to be properly terminated in order to work reliably. Two terminating resistors are
used on each RS485 bus, at the farthest ends of the link.
These resistors should be attached to the data/audio connector on the back of the unit if the S768DAV
link is at one end of the bus. If the terminating resistors are left out, the tri-state condition will not be
detected, and the bus might lock up. If there are too many terminations on the bus, signal levels might
drop too low, or driver circuitry might fail. The standard value of terminating resistors for RS485 is 120Ω.
11-0768DAV-B/ December 2003
N
PPLICATION
OTES
RS485 Application Notes
27
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