NOVUS RS422 Basics - Manual (Page 2)

RS485 & RS422 Basics
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Unterminated networks are low power, low cost, and simple to build. The disadvantage, of course, is that data rates must
be quite slow or cable length must be short for the network to operate reliably. A network up to 100 m long and
communicating at 19,200 bps is expected to be reliable even when no termination resistors are installed.
A parallel termination offers excellent data rates but is limited to networks that only have one driver. The driver must be
located on one end of the network and the termination resistor must be located on the far end. This is the case for each
pair of a RS422 or full-duplex RS485 network.
The third termination technique is a bidirectional termination, which offers excellent signal integrity. With this technique,
the line drivers can be anywhere on the network. The disadvantage is power consumption. This technique is probably the
most reliable RS-485 termination technique.
The last subject related to termination is what to do with unused conductors in a data cable. Unused conductors will self-
resonate and couple noise into the data wires. If the unused cables are left opened, they will resonate at all sorts of
strange frequencies. If they are grounded at one end, they will resonate at L/2 (“L” is the cable length). If they are
grounded at both ends, they resonate at L/4. The best method for minimizing energy on an unused conductor is to
dissipate the energy as heat. In short, terminate both ends of the unused conductor to ground with resistors (a
bidirectional termination). The resistors should be equal to the characteristic impedance of the line. Other option is to use
a cable with no extra wires.