Siemens SIMATIC PC 670, Equipment Manual

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Connecting and Switching on the Computing Unit
Release 04/02
3-18
Panel PC 670 Computing Unit, Equipment Manual
3.7 Using the Computing Unit in a SIMATIC S5 Network
Point-to-point connection
This section describes the options for connecting your computing unit to a
programming unit or an S5 programmable controller in a point-to-point connection.
A point-to-point connection is possible by connecting the computing unit to another
programming unit or a PLC via:
S
a V.24 connection
S
a TTY connection
Configuring notes for interfaces with current loop (TTY, 20 mA)
Different criteria have to be taken into account for reliable operation of a
connection. The maximum data transfer rate (baud rate) depends on the required
distance, the type of cable, the pin assignment of the interface and external
interference.
Rules
To reduce interference by a favorable choice and correct connection of the
cable, you should comply with the following rules:
S
The shielded cable used must have a low line resistance
(< 130
W
/ km) and a low capacitance (< 90 pF/ m). Twisted-pair cables are less
susceptible to inductive interference. A low line resistance result in reduced voltage
excursions and shorter charge reversal times; the line resistance decreases with
increasing conductor cross-section for the same length of cable.
S
The shorter the data transmission link, the higher the maximum possible transfer
rate.
S
If there is an active transmitter and an active receiver at the same end of the
transmission link, the sequence of access priority to the transmission circuit must
be taken into account in order to achieve the longest possible transmission link.
S
Signal lines and power lines must not be run together. Signal lines must be installed
as far away as possible from sources of strong interference (for example, 400 V
3-phase power cables).
S
The active TTY interface with a 12 V open-circuit voltage has been tested on a
100 m long cable at a transmission rate of 9600 bps in an environment with normal
levels of noise (field strength < 3 V/ m). If a LiYCY 5 x 1x 0.14 shielded cable is
used, reliable transmission is possible over a distance of up to 100 m. The AS511
driver was used for testing (only one transmitter at a time).
Note
The interference field of the source of interference is reduced by the square of the
distance.