VEGA VEGACOM 557 Siemens 3964 and 3964 R procedure with RK 512 - Manual (Page 34)

34 VEGACOM 557 Siemens
The interface RS 422 transfers the data as
voltage difference between two correspond-
ing cables. Signal earth as grounding is not
required. One pair of wires is required for the
transmitting as well as for the receipt signal,
consisting of an inverted and a non-inverted
signal cable. Possible common-mode inter-
ferences cause a symmetric shift of the volt-
age level and cannot deteriorate the useful
signal.
Thanks to the higher interference immunity
compared to RS 232, distances up to 1200 m
and high data rates up to 10 Mbits can be
reached. The interference immunity is also
visible on the permissible voltage levels: with
an output level of the transmitter under load
of ±2 V the receiver components accept a
level of ±200 mV still as valid signal.
Special feature of the RS 422 is that it allows
the unidirectional connection of up to 10 re-
ceivers on one transmitter. With higher trans-
mission rates and/or large distances, a
termination (adaption of the wave resistance)
is necessary and a galvanic separation of the
transmitter/receiver components is absolutely
recommended.
The interface RS 485 means an extension of
the RS 422 concept to a bus-compatible
system, whereby the physical differences
are neglicible.
The bus system can include up to 32 partici-
pants, i.e. 1 master and 31 slaves. A protocol
ensures that at any time max. one participant
is active as transmitter, whereas the others
are switched passively. For transmission and
receipt only one cable pair is required, which
is used in alternate cycle. With 10 Mbits/s as
data rate and 1200 m as max. distance, the
data correspond to these of the RS 422 inter-
face.
Supplement
To be neutral against (inevitable for large
distances) potential shifts, a galvanic isolation
of the transmission/receipt component is
recommended. A termination is generally
necessary, independent on data rate and
distance.
Partly also interfaces TTY (also called 20 mA
or Current Loop) are used. The data are
transmitted by switching on and off a current
of 20 mA in a cable loop in the cycle of the
data bits. This interface however is not sub-
jected to a standardisation so that the use
must be projected in detail. With galvanic
separation, distances up to 1000 m with data
rates of 300 … 9600 baud can be transmitted
safely.
Conclusion
Main features for interfaces acc. to RS 232
are:
- reduced cable lengths
- low data rates
- only point-to-point connection
Main features for interfaces acc. to RS 422
and RS 485 are:
- large cable lengths
- high data rates
- basis for bus systems