9
DIGITAL VIDEO AND BIDIRECTIONAL DATA SYSTEM
ENABLED Indicator
This LED has three states: green indicates a valid mode
has been selected, red indicates an invalid mode (spare or
test mode) has been selected, and flashing red/green indi-
cates that NO mode has been selected.
RS-485 APPLICATION NOTES
The V732 is configurable for both full-duplex (4-wire) and
half-duplex (2-wire) operation. It can be used for interfac-
ing to systems adhering strictly to the RS-485 specification
and for use with systems that use a modified, "fail-safe
biased" RS-485 bus.
Connection
Use high quality twisted-pair wiring and make sure all con-
nection points are clean and tight. A loose connection on
one of the wires can appear to function, yet cause intermit-
tent errors; data LEDs may be flashing as signals pass
through the system, but those signals will be corrupt.
Configuration
Vicon V732 units are designed to work with virtually any
RS-485 system. Unfortunately, some systems operate on
a "modified" version of RS-485; they use failsafe biasing to
pull up/down their bus during a tri-state condition. In a
standard RS-485 system, when a driver on a properly ter-
minated bus goes into tri-state (inactive), the voltage
between the differential outputs should be less than 200
millivolts. (This is what is considered "standard offset" in
the product instruction manuals.) A differential output tri-
state voltage this small can cause some nonstandard sys-
tems to latch up, since they are designed for much larger,
"fail-safe," offsets. To be able to interface to such equip-
ment, the Vicon V732 offers two more "offset" level modes.
That is, the maximum differential input voltage that can be
applied where it will be perceived to be in tri-state. Of
course, the equipment manufacturer does not disclose this
information. In most cases, the Vicon unit should be con-
figured for "standard offset" operation. When the system is
operating properly, the DATA IN and DATA OUT LEDs will
be off when there is no communication (tri-state) and they
will flash when data is being sent or received. In the case
where 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 sim-
ply switching the mode switch does not prove effective, the
offset level may have to be emulated at one end of the sys-
tem by using pull-up/down resistors on the data connector.
The V730 has a +5 V bias pin and ground pins on the con-
nector for this purpose. Contact equipment manufacturer's
technical support for recommended resistor values and
configuration.
Termination
RS-485 systems need to be properly terminated in order to
work reliably. Exactly two terminating resistors are used on
each RS-485 bus, at the furthest ends of the link. These
resistors should be attached right on the DATA IN/OUT
connector if the Vicon 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 may lock up. If there are too
many terminations on the bus, signal levels may drop too
low, or driver circuitry may fail. The standard value of ter-
minating resistors for RS-485 is 120 ohms.
MAINTENANCE
There is no operator maintenance other than keeping the
units clean.
TECHNICAL SUPPORT
For installation assistance, or in the event that a unit should
fail or if the cause of a problem cannot be determined, con-
tact Vicon’ technical support at:
TOLL FREE: 800-34-VICON (800-348-4266) or
PHONE: 631-952-CCTV (2288)
