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Multi-Drop:
The most basic RS-485 configuration will typically have two transceivers separated by a
length of cable. When additional receivers, transmitters, (or both) are added at
intermediate points along the main cable run, then the system is known as ‘multi-drop’.
Node:
Refers to a receiver, transmitter (or transceiver) that has been attached somewhere along
the path of the main RS-485 cable.
RS-422:
A predecessor to RS-485; some degree of compatibility between the two does exist.
Companies such as Texas Instruments, Linear Technology, and others have application
notes available at no cost that address the topic of compatibility in great detail.
RS Prefix:
An obsolete prefix that still finds common usage, the letters stand for ‘Radio Sector’.
References to RS-232 and RS-485 are used throughout this text.
Termination Resistor:
A resistor placed at the far end of a far end of a transmission cable, intended to absorb
energy from incoming signals. Due to the high impedance of the receivers used within
the RS-485 system, a termination resistor is often necessary to prevent undesirable
reflections from the cable end.
Unit Load:
EIA-485 defines a single unit load to be 12k ohms for a receiver element, with each
transmitter having the capability of driving 32 unit loads. If a receiver has a higher
impedance than 12k (as is often the case), then the unit load value assigned to that
receiver will be a smaller number. If all the unit loads were all 0.5 for example, then the
transmitting element would be able to support 64 receiver nodes.