TABLE OF CONTENTS The RS-232C interface system is used most frequently for series data transmission in measurement systems. However, this standard imposes many significant limitations on building measurement systems.
The most important are:
The limit of the 15-m length of the wire line-connecting devices;
The low transmission rate, up to 20 kbps;
The number of digital devices included in the system, two.
Therefore, several serial interface systems were produced and standardized, with properties better than the parameters of the RS-232C. The RS-449 and the RS-530 standards are used most frequently; they determine the functional and mechanical parameters of serial interface. The parameters of electrical circuits for those two standards are included in the RS-423A, the RS-422A, and the RS-485 interfaces. In the RS-423A, the RS-422A, and the RS-485 interfaces the signal line immunity to noise is considerably higher since differential transmission circuits are used. In the RS-423A interface, the transmitter output is defined as asymmetrical (signal "hot" wire has reference to ground), and the receiver output as differential. The RS-422A and the RS-485 interfaces have fully differential transmission circuits.
The RS-449 serial interface standard includes the functional and mechanical parameters of the interface system designed for remote data transmission with or without a modem. A 37-pin connector and 35 lines in the interface bus were defined for that standard. In the RS-449 interface, an additional 9-pin connector is assumed in the case of installing the secondary channel, which is, however, used...
