wtw TresCon UNO A111, Operating Manual

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TresCon UNO Appendix
8 - 3 ba43134e08 05/2017
8.3 RS485
8.3.1 Hardware
General information In principle, the RS485 interface is an RS232 interface that operates,
however, with difference levels that are not vulnerable to interference.
This enables error-free data transmission over greater distances (up to
approx. 1 km).
A commercially available level converter is required to operate data ter-
minals such as a printer or terminal. A 2-wire line (+ potential line) is
required to realize data transmission.
Communication is handled in so-called "half-duplex" operation, i.e. the
same lines are used for transmitting and receiving (in contrast to 4-wire/
"full duplex").
Up to 31 instruments can be added (BUS operation in the Slave mode)
to this line. Each instrument is given an address (1-31). Each address
may only be allocated once. The address 0 is not offered as an instru-
ment address; in SLAVE operation it acts as a broadcast address.
RS485 converter The RS485 converter must fulfill the following prerequisites:

Electrically insulated (computer / BUS)

adjustable record or automatic record recognition

integrated or switchable closing resistance 120 Ohm (100 - 130
Ohm)
Computer Any IBM-compatible computer with a serial interface can be used - from
a 386-processor upwards. The interface must be configured to "9600
bd / 8 bit / no parity".
Wiring and connections Use a twisted 2 wire or 3 wire cable with at least 0.22 mm
2
diameter, a
characteristic impedance of 100 to 130 Ohm and a capacity of max. 60
pF/m. For greater distances, it is advisable to include a third stranded
wire (GND) to compensate for any differences in potential and, thereby,
increase the signal-to-noise ratio.
The connections are marked as A, B and GND. The individual terminals
in the instrument must be connected with the correspondingly labeled
connections on the converter.