Vega VEGACOM 557 Profibus FMS Bedienungsanleitung PDF herunterladen (Seite 51)

VEGACOM 557 Profibus FMS 51

Supplement

A parity bit can follow the last data bit which

is used for detection of transmission errors.

The parity bit ensures that with

- EVEN parity always an even number of bits

- ODD parity always an odd number of bits

is transmitted.

A so-called handshake can be used to re-

lease or interrupt the data transmission.

Hardware handshake:

The receiver controls via its handshake out-

puts DTR or DSR the handshake inputs CTS

or DSR of the emitter.

Software handshake:

The receiver emits special characters to the

transmitter and with this, controls the data

transfer.

Although there is no standard for the inter-

faces RS 232, RS 422 and RS 485, they can

be considered as a standard because they

have gained acceptance as an industrial

standard.

The interface RS 232 transmits the individual

bits of a character as a sequence of defined

voltage levels via one cable.

Therefore:

- a voltage level of -15 V … -3 V corre-

sponds to a logic "1"

- a voltage level of +3 V … +15 V to a logic

"0".

Both levels are related to a common signal

ground (GND). The permissible ohmic load

must be more than 3 kOhm, the permissible

capacitive load can be max. 2500 pF.

The main characteristics of an RS 232 inter-

face are

- reduced cable length (max. 15 m to

9600 baud)

- low data rates (max. 19200 baud)

- only point-to-point connection

For industrial applications, the provided

handshake signals are usually not neces-

sary. In such case, the handshake inputs are

simply connected to the handshake outputs

of the same instrument (release level).

The interface RS 422 transfers the data as a

voltage difference between two correspond-

ing cables. Signal earth as grounding is not

required. One pair of wires, consisting of an

inverted and a non-inverted signal line, is

required for the transmitted, as well as for the

received signal. Possible common-mode

interferences cause a symmetric shift of the

voltage level and cannot deteriorate the use-

ful 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 can also

be seen on the permissible voltage levels:

with an output level of ±2 V from the transmit-

ter under load, the receiver components still

accept a level of ±200 mV as a valid signal.

One special feature of the RS 422 is that it

allows the unidirectional connection of up to

10 receivers on one transmitter. With higher

transmission rates and/or large distances, a

termination (adaption of the wave resistance)

is necessary and a galvanic separation of the

transmitter/receiver components is highly

recommended.

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