80
5.9. RS-485 Interace
The digital programmable PCE-P30U transducers are equipped with
a serial interface in the RS-485 standard to communicate in computer
systems and with other Master devices. Asynchronous character com
-
munication protocol MODBUS has been implemented on the serial in
-
terface. The transmission protocol describes the methods of information
exchange between devices via a serial interface.
5.9.1. Serial interface connection
RS-485 standard allows direct connection of up to 32 de
-
vices on a single serial link with the length of up to 1200 m
(with the baud rate 9600 b/s). In order to connect larger number of devices,
it is necessary to use additional intermediate-and-separating
systems
such as
PCE-
PD51 made by
PCE
S.A. Connection diagram is
presented
on the Fig. 3. In order to obtain correct transmission, it is
necessary
to connect the lines A and B in parallel to their
equivalents in other
devices. Connection should be made with
a shielded cable. The
c
able shield should be connected to the
protective terminal as close to the transducer as possible (the
shield should be connected to the protective terminal at one
point only).
GND line is used for additional protection of the interface line in case
of long connections. In such a case, GND signals of all devices on
RS-485
bus should be connected.
To obtain a connection with a PC, an RS-485 interface
card
or an appropriate converter, e.g.
PCE-
PD51 or
PCE-
PD10, is required.
The method of connecting devices has been shown on fig. 29.
