80
80
5. . RS-4 5 Interface
The digital programmable P30U transducers are equipped
with a serial interface in the RS-485 standard to communicate in com-
puter systems and with other Master devices. Asynchronous character
communication protocol MODBUS has been implemented on the serial
interface. The transmission protocol describes the methods of informa-
tion exchange between devices via a serial interface.
5. .1. Serial interface connection
RS-485 standard allows direct connection of up to 32
devices 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 PD51 made by LUMEL 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 cable 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. PD51 or PD10, is required. The method
of connecting devices has been shown on fig. 29.