STEP 8 -
Loopback Test
• Configure for RS-485 4-wire.
• Jumper terminals B to D and C to E.
• Connect a PC to the RS-232 port.
• TD and RD LEDs are ON when power is applied.
• Using hyperterminal or a similar program, connect
to the appropriate COM port. Turn off hyperterminal
local echo.
• Transmit data. The same data should be returned.
When data is sent and looped back, the TD and RD
LEDs blink on and off indicating data flow.
STEP 7 -
Bias and Termination
The circuit can be biased using the built-in 2k Ohm
pull-up and pull-down resistors. This is controlled with
DIP switch position 5. The default setting is ON (bias
resistors in).
When an RS-485 network is in an idle state, all nodes
are in listen (receive) mode. Under this condition, there
are no active drivers on the network. All drivers are
tri-stated. Without anything driving the network, the
state of the line is unknown. If the voltage level at the
receiver’s A and B inputs is less than ±200 mV, the logic
level at the output of the receivers will be the value of
the last bit received. To maintain the proper idle voltage
state, bias resistors must be applied to force the data
lines to the idle condition.
If termination is necessary on the receive lines, a
built-in 120 Ohm resistor can be switched in using DIP
switch position 6. In most cases, termination is not
required. The default setting is OFF (termination out).
Termination is used to match impedance of a node to
the impedance of the transmission line being used.
Termination increases load on the drivers, requires
installation complexity, changes biasing requirements,
and makes system modification more difficult.
Generally, termination should only be used for long
distances. “If in doubt, leave it out.”
Summary of Contents for ICD400A
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