Theory of Operation
2-3
Logic Convention
Shielded Twisted Pair
Custom Cable
A cable containing two wires which carry all digital
data on the SDN. The two wires are twisted together
to minimize interference from magnetic fields, and
encased in a braided shield to limit interference
from electrostatic fields. The cable is custom
designed with carefully controlled impedance and is
well shielded to guarantee noise immunity and
ensure system performance. It is necessary to use
only Agilent supplied cables in SDN systems.
LAN Unshielded Twisted
Pair Category 5
A cable containing four pairs of wires. One pair
carries all digital data on the SDN. The wires are
twisted together to minimize interference from
magnetic fields. UTP, unlike shielded twisted pair, is
not encased in a braided shield.
Differential
Two wires carry the same signal, with opposite
polarity. These two wires are designated as positive
and negative. For a digital one (1), the positive wire
is at a high voltage and the negative wire is at a low
voltage; vice versa for a digital zero (0). The voltage
between these wires (high to low, 1 to 0) is
approximately 3 volts (1.5V minimum, 4V
maximum).
Termination
In order for differential signals to remain clean, the
length of the cable must appear infinite to the
circuitry. The cable’s impedance is matched by 120
ohms of resistance connected between the two data
wires at both ends of the branch (at the ACC and at
the instrument). Termination is contained in each
instrument.
Data Wires (SDC or XSDC)
Positive (+) Wire
Pink colored wire
Negative (–) Wire
Blue colored wire
High State (1) (True)
Pink wire potential is positive with respect to blue wire
by at least 1.5V
Low State (0) (False)
Pink wire potential is negative with respect to blue wire
by at least 1.5V
Positive Transition
Transition from low state to high state
Negative Transition
Transition from high state to low state
Term
Definition
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