A primary consideration in the field is that the data transmission lines are cheap and reliable. This reflects not only the type of cable used in the interface but also adopted. Therefore, despite the high transmission speeds that can be obtained with a parallel interface as IEC-625/IEEE-488, where 16 lines are required to send 1 byte of information is very costly to install this level.
For this reason, the standard interface for the field is serial. Low installation costs (fewer cables and connectors), longer lines and transmission more secure, more than offset lower transmission speeds. The following describes the serial interfaces found in field applications.
- Current loop 20 mA
It originates from the telegraph where required reliable communication over long distances. Work with a current of 20 mA is switched on and off according to the rate of transmission. Therefore, each terminal has two loops, one for transmission and one for reception. The current loop 20 mA is used as an alternative to RS-232C interface. Its greatest advantage is its insensitivity to interference, so that is used when long distances are required or when they are strong electric fields in the vicinity of the transmission line.
- RS-232C INTERFACE
It is used to interconnect two devices via a multi-wire cable, usually a computer with a peripheral device or modem. The standard complies with electrical and physical requirements for serial transmission of bits. Define recognition signals for the control of standard equipment for telephone lines and modems. Electrically the system is based on positive and negative pulses of 12 V in which the data is encrypted.
Mechanically, the standard RS-232C connectors have 9 or 25 pins. The main signals carrying data from one terminal to another are handled by the lines "transmit data" and "receive data". To enable the transmission, it requires a third line that carries the common reference potential. The other lines, which need not be present, carry information on the status of communication terminals. The modems are controlled by signals "request to send" and "clear to send", the availability of a block of data into a computer by "data set ready" and the ability to receive the block from one computer to "data terminal ready ".
- RS-422 INTERFACE
It covers only the electrical and physical requirements for transmission. Use symmetrical differential signals and allows transmission speeds of up to 10 Mbits / s. At the far end of reception, the difference between voltage levels is used to decode the signals s, with the largest positive difference corresponds to "0" and less than "1". The advantage is that if an external field acts on the line, both signals are influenced at the same time. The difference in the signal is maintained with the exception of individual noise of each line, substantially the same. This makes it possible to build longer lines than for the RS-232C. Furthermore, since the interference effects are small, are possible higher speeds of transmission.
Because of the differential lines and the availability of appropriate drivers, this interface is applicable not only for extensive transmission paths, but also for serial bus structures. Despite being conceived primarily as point to point system, can operate up to 16 devices with a single transmitter.
- RS-485 INTERFACE
Specifies requirements for electrical and physical symmetric transmission data (similar to RS-422) across multiple devices. Up to 32 devices acting as transmitters or receivers can be connected to a two-wire cable is a real bus operation. The addressing and responding to commands must be resolved by software. The maximum line length varies from 1.2 km at a speed of 93.75 kbit / s up to 200m at a speed of 500 kbit / s.
This nterfaz uses three logic states "0", "1" and "non-data" (no data), the latter is used for control or synchronization of data flow. This interface is often found in the field. When using twisted pair shielded cable, ensuring a reliable and affordable communication.
- IEC 1158-2 INTERFACE
International interface is intrinsically safe fieldbus, the data signal in a non-return to zero, are coupled with a clock signal and sent as a current or voltage signal through the transmission medium. 1 and i 0 are formed by a phase change when you have the middle bit, two states in the absence of data is generated when there is no phase change.
The preamble nterfaz uses a synchronization and adds a start delimiter and an end to the transmitted data. Unambiguous nature of delimiters and strict monitoring of the timing of signal transmission ensures a very safe at high speeds and over longer distances. The interface can safely provide power for up to 10 devices, can accommodate more if they have external power.
Then you have a comparison of the interfaces mentioned.
Table 1 - Communication interfaces
