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DHB-324

6. INTERFACE RS-485

DHB-3xx programmable digital meters have serial links in RS-485

standards for the communication in computer systems and with other

devices fulfilling Master function. An asynchronous communication

character protocol MODBUS has been implemented on the serial link.

The transmission protocol describes ways of information interchange

between devices through the serial link.

6.1. Connection Way of the Serial Interface

The RS-485 standard allows to a direct communication of 32 devices

on a single serial link of 1200 m long. For the connection of a higher

quantity of devices, it is necessary to apply additional intermediate-

separating systems.

The leading of the interface line out is given in the meter user’s manual.

To obtain a correct transmission, it is necessary to connect lines A and

B with their equivalent in other devices. The connection must be made

through a shielded wire. The shield must be connected to the protection

terminal in a single point. The GND line serves to the additional protection

of the interface line at long connections. One must connect it to the pro-

Fig. 10. Connection way of the RS-485 interface

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