Interfaces - ITU-T V.24/V.28 or EIA RS232C interface

Application:

The interface is a serial duplex (+/-) asymmetric surface, which is used primarily for connecting the data terminal equipment (DTE) and modem (DCE). (The so calles serial interfaces of the IBM-PC compatible computers (COM1: COM2:) are in this category, too.) Indication of the two sides of the interface:

The interface is asymmetric, its two sides are built up differently.

The interface, which can be considered nowadays as a classical one, is also specified by several organizations dealing with international standardizations (ITU-T, EIA, ISO). According to the identifiers of recommendations, standards, interface is usually called ITU-T V.24/V.28 or EIA RS232C. Other names of the interface:: I2 interface, DTE-DCE interface. Developed version of the interface: ITU-T V.36 or EIA RS449.

Elements of the standard describing the interface:

Services:

Interface topology:

Data changing wires:

V.24 recommendation defines the set of circuits of the standard digital DTE-DCE interface of the modems. Set of circuits consists of circuits of binary data ( D ), controlling signals ( C ) and clock signals ( CK ). Modems use only a subset of the whole set of circuits of the recommendation. Functional completeness of the used set of circuits is the condition of the A DTE-DCE communication. Here we will show the most frequently used subset. Usage of further lines is less and less frequent, because with development of the microelectronics the software control of modems will take the place of the hardware control. Recommendation is equivalent with the RS 232C standard of the EIA. RS 232-names of the lines can be memorized more easily, since they are abbreviations relating to the function of the line.

ITU-T Id RS232 Id ISO 2110 pin number Function DTE-DCE direction Type
101 FGND 1 Protective ground --
102 SGND 7 Common ground --
103 TD 2 Data transmission -> D
104 RD 3 Data reception <- D
105 RTS 4 Data request -> C
106 CTS 5 Ready to transmission <- C
107 DSR 6 DCE ready <- C
108 DTR 20 DTE ready -> C
109 DCD 8 Data channel received signal detector <- C
113 TTC 24 Transmitter signal clock ( from DTE ) -> CK
114 TC 15 Transmitter signal clock ( from DCE ) <- CK
115 RC 17 Receiver signal clock <- CK

Connecting RS-232 on RJ-45:

ITU-T Id RS232 Id RJ-45 pin number Funktion DTE-DCE direction Type
101 FGND 1 Protective ground --
105 RTS 2 Data request -> C
104 RD 3 Data reception <- D
109 DCD 4 Data channel received signal detector <- C
102 SGND 5 Common ground --
103 TD 6 Data transmission -> D
108 DTR 7 DTE ready -> C
106 CTS 8 Ready to transmission <- C

Electrical characteristics:

Mechanical characteristics:

Based on the recommendation of ISO 2110: Connection: 25 pin D-type connection

Frame formats:

Format of signals going through the interface - framing - is not the specification of the interface, but the DTE-DCE. For framing of data we usually use the following three simple methods: