RS485 APPLICATION NOTES
The V730 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 RS485 specification
and for use with systems that use a modified, "fail-safe
biased" RS485 bus.
Connection: 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 func-
tion, yet cause intermittent errors: data LEDs may be flash-
ing as signals pass through the system, but those signals
will be corrupt.
Configuration: Vicon V730 units are designed to work
with virtually any RS485 system. Unfortunately, some sys-
tems operate on a "modified" version of RS485; they use
failsafe 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 (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 V730 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 config-
ured 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: RS485 systems need to be properly termi-
nated in order to work reliably. Exactly two terminating
resistors are used on each RS485 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 terminating resistors for RS485 is 120 ohms.
TEST MODE
Test mode allows the end user to verify the operation of the
copper receiver/driver circuit in a V730, as well as the fiber
connection from one V730 to another. Using the test mode
is simple:
1. Switch the V730 to be tested into position F (TEST
MODE) on the rotary switch. At this end only, wire the con-
nector as shown in Table 9.
2. At the receiving (or opposite) end, switch the V730 to
position 9 on the rotary switch.
In the test mode, the transmitting unit should behave as fol-
lows:
A. Enabled LED is RED, indicating that a valid data format
has not been selected.
B. Data OUT LED is slowly flashing between amber, green,
and off. This indicates that the test mode is generating an
output pattern and sending it out on copper.
C. Data IN LED should mimic the DATA OUT LED. This
indicates a good, proper loopback connection, and proves
that the data transmit/receive circuitry is working properly.
D. Level/Loss may be red or green - indicates received fiber
signal strength.
The receiving unit (position 9) should behave as follows:
A. Enabled LED is GREEN, since a valid data format is
selected.
B. Data OUT LED should slowly flash amber, green, off.
This indicates that the fiber path from the unit set for TEST
MODE is reliable.
C. Data IN LED should be OFF, since there is no input cop-
per connection made.
D. Level /Loss should be green to off (but not red), indicat-
ing that sufficient optical power is being received.
After the test has been performed at one end, swap switch
positions and connectors to perform the test on the other
end. Having done this, you have proven that the copper-in-
to-fiber-to-copper-out conversion is working in both direc-
tions.
LED OPERATION
The V730 has 5 LED indicators that are very useful in
describing the current state of operation, as well as the cur-
rent status of data flow and fiber optic signal strength.
These indicators are LEVEL /LOSS, VIDEO IN (or OUT),
DATA IN, DATA OUT, and ENABLED. They function as fol-
lows:
LEVEL/LOSS: This LED is useful for indicating the relative
optical signal strength at the fiber optic receiver. When suf-
ficient optical power is being received, the LED shines
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DIGITAL VIDEO / DATA FOR NOVA™ CONTROL SYSTEMS
