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Appendix - A
Glossary of Terms:
Bit Rate:
This describes the number of transitions from low-to-high or high-to-low that take place
over a period of time (measured as bits per second) on a given communications system.
While the communications system can vary from satellite uplinks, fiber optic cable, etc.,
in this context the transmission medium is RS-485 with twisted wire cable.
Cable Impedance:
Characteristic impedance that a cable of infinite length (theoretical) will present to a high
frequency signal propagating down the path. This is not something that can be measured
with a simple ohmmeter across the leads. Placing a termination resistor of equal value
(with reference to the cable impedance) at the far end serves to effectively absorb high
frequency incoming signals, thereby preventing reflections.
CNI:
Honeywell designation for their product line known as the “Cellular Network Interface”.
Differential Signaling:
A communications method where a pair of wires are used to transfer information without
reference to a signal common. In the case of RS-485, if a person was to place an
oscilloscope across the two wires of the transmitter output while a 120 ohm load was
being driven, it would be found that the signal toggles b4 volts polarity and -4
volts polarity. Due to the fact that a signal common is not used as the return path, any
injected noise will be common to both wires, and therefore tend to cancel out.
Isolation:
In this context, isolation refers to a design method where the RS-485 circuit signaling and
power are coupled via capacitors and transformer to the other electrical circuits. In
general, products having this type of construction are more resistant to electrical spikes
and surges since the isolated circuit can ‘float’ to as high as 1000-2500 volts depending on
the design. One trade-off for this robustness is a reduced data transfer rate, since it is not
possible to exchange data at 10M bps over the isolation channel. The RS-485 adaptor board
incorporates isolation to maximize survivability in the field.
Line Contention:
A condition where two or more transmitters attempt to ‘talk’ on a shared cable at the same
time. For obvious reasons, this is to be avoided, although no permanent damage to the
transmitters will occur. Software protocol is responsible for collision avoidance as there
are no ‘handshaking’ control lines.