User's Manual

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FR 13053 108 B
11/27
CE DOCUMENT EST LA PROPRIÉTÉ DE BALOGH IL NE PEUT ETRE COMMUNIQUÉ À DES TIERS SANS AUTORISATION ECRTITE PRÉALABLE.
THIS DOCUMENT IS BALOGH PROPERTY.IT CANNOT BE DISCLOSED TO ANY PARTY WITHOUT PREVIOUS WRITTEN AUTHORIZATION.
7. INSTALLATION AND CONNECTION PROCEDURES FOR THE
ASYNCHRONOUS LINKS
7.1. Cable to be used for serial link RS422 or RS485
1. In a "clean" electromagnetic environment - twisted pairs, without screen.
2. In a "noisy" electromagnetic environment - twisted pairs, individual screen for each pair
The cables are electrically characterised by :
- characteristic impédance (Z0 in ohms)
- distributed capacity (C
L
in pF/m).
- distributed resistance (R
L
in ohms/m)
For short cable lengths, normal cables are satisfactory.
For cable lengths greater than 1000 metres, high quality cables (low C
L
and R
L
) should be used for all
baud rates.
7.2. Line termination
For a simplex link, the termination (if present) should be placed at the receiving end of the line.
For a duplex link, the termination (if present) should be placed at the each end of the line.
For baud rates less than 1200 bauds, no termination is necessary. For baud rates greater than 9600
bauds and line lengths greater than 1000 metres, a resistor equal to the line impedance (120 ohms) is
usually necessary. For cases in-between, there is no clear-cut rule and depends on individual
installations (combination of baud-rate, line-length, cable quality, emitter/receiver characteristics).
7.3. Electrical connections
For an RS-232 link , wiring up is straightforward, the TX and RX lines of both equipments are
connected together.
For a differential link (RS-422 or RS-485), the polarities are not always clearly defined. Normally the
“+” line is at a high level at rest and is active low. For the “-” line, the opposite is true. This is the
case for the differential interface for the HYPERX readers. However if the differential signals are
generated by a converter acting on RS-232 signals, then the “+” line can be at a low level at rest and
active high. In this case, the “+” line of one equipment must be connected to the “-” line of the other
equipment.
Connection of 0V
Whether this is necessary or not, depends on the installation. If host and reader are distant with
different local ground potentials, then an RS-232 link may not work if the 0V references are not
connected. However connecting them will cause ground currents to circulate. In general, for large link
lengths, a differential link should be used. This also tolerates a large common-mode voltage
difference.
7.4. Networking
7.4.1. Topology
The preferred topology is the bus.