VEGA VEGACOM 557 Profibus FMS - Manual (Page 52)

52 VEGACOM 557 Profibus FMS
The interface RS 485 represents an exten-
sion of the RS 422 concept to a bus-compat-
ible system, whereby the physical
differences are negligible.
The bus system can include up to 32 partici-
pants, i.e. 1 master and 31 slaves. A protocol
ensures that at any time a maximum of one
participant is active as a transmitter, while all
others are switched to a passive state. For
transmission and reception, only one cable
pair is required, which is run in an alternating
cycle. With 10 Mbits/s as data rate and
1200 m as max. distance, the data corre-
spond to those of the RS 422 interface.
To ensure immunity against (inevitable for
large distances) potential shifts, a galvanic
isolation of the transmission/reception com-
ponent is highly recommended. A termination
is generally necessary, independent of data
rate and distance.
In some cases, the TTY interface (also called
20 mA or Current Loop) is also used. The
data are transmitted by switching on and off
a current of 20 mA in a cable loop in the cycle
of data bits. This interface, however, is not
subject to any standardisation and thus must
be planned in detail. With galvanic separa-
tion, distances up to 1000 m with data rates
of 300 … 9600 baud can be transmitted
safely.
Conclusion
Main charactersitics of interfaces acc. to RS
232 are:
- reduced cable lengths
- low data transmission rates
- only point-to-point connection
Main characteristics of interfaces acc. to RS
422 and RS 485 are:
- greater cable lengths
- high data transmission rates
- basis for bus systems
Table: Comparison of important interface
data
Interfaces RS 232 C RS 422 A RS 485
Transmission asym. symmetr. symmetr.
Number of drivers 1 1 32
Number of receivers 1 10 32
Transmission distance 15 m 1200 m 1200 m
max. transmission rate 20 KBit/s 10 MBit/s 10 MBit/s
Emitter
Permissible driver
output voltage
±25 V -0,25…6 V -7…12 V
Driver output signal
- without load
±15 V ±5 V ±5 V
- with load ±5 V ±2 V ±1,5 V
Driver load 3…7 k
Ω
100
Ω
54
Ω
Receiver
Input voltage ±15 V ±7 V -7…12 V
Sensitivity ±3 V ±200 mV ±200 mV
Input resistance 3…7 k
Ω
4 k
Ω
12 k
Ω
Diagram: Distance –– Transmission rate
Supplement
10 km
1,2 km
1 km
100 m
30 m
15 m
100 1 k 10 k 20 k 100 k 1 M Bit/s 10 M
RS 422-A
RS 485
RS 232-C
Distance
Transmission rate