Specifications

Appendix C - Glossary of Terms RS-422 And RS-485 Applications Ebook
Page 126 of 137 Manual Documentation Number: <DocNumber>
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Terminal Blocks
Connection terminals which provide electrical contacts for tinned stripped
wires, with some means of tightening or loosening the connection to the wires.
Terminal blocks typically have a hole to insert the wire, and a screw to
tighten/secure it. The hole can usually accept one or two wires. Some surge
protection devices use a terminal strip, usually called a barrier strip. It accepts
wires with a three-quarter wrap around the screw or lug.
Termination
Connecting a resistance at one or both ends of a cable to prevent reflections is
called termination. The proper termination resistance is equal to the
characteristic impedance of the cable. Reflections traveling up/down the line
can alter the signal and cause data errors.
Typical RS-422/RS-485 cables may require a 120 ohm termination at each end
of a cable run if reflections cause a problem. Termination (when needed) is
normally installed on the receivers at each end. Some converters/cards provide
selectable termination using a jumper or switch, others provide extra terminals
in parallel for connection of termination.
Common termination values for electronic devices varies by the type of device,
75 ohms for video or RF antenna, 300 ohms for twin lead, 50 ohm for coax,
and other values of pull-up/pull-down termination for SCSI bus devices.
Transceiver
A combination transmitter and receiver. Sometimes used to refer to an RS-485
driver and receiver device combination when bridged (internally or externally)
to transmit and receive on the same communications line.
Also used to describe a radio transmitter/receiver combination.
Tri-state
An output that can assume any of three states. Two states are normal low-
impedance: logic 1 or logic 0. The third state is a high-impedance, or off,