RS-232C

In 8251A (USART), we discussed how serial communication takes place. The TTL signals output by a USART, however, are not suitable for transmission over long distances, so these signals are converted to some other form to be transmitted. In this section we discuss device used to send serial data signals over long distances.

• RS232 is the most widely used serial I/O interfacing standard.
• However the I/O voltage levels are not TTL compatible. In the RS232, a 1 is represented by –3 to –25 V, while 0 bit is +3 to +25 V, making –3 to +3 undefined.
• For this reason, voltage converter such as MC1488 and MC1489 are used to convert the TTL logic levels to the RS232 voltage levels and vice versa. See Figure below.

Fig: RS232 connection

RS-232 stands for Recommend Standard number 232 and C is the latest revision of the standard. The serial ports on most computers use a subset of the RS-232C standard. The full RS-232C standard specifies a 25-pin "D" connector of which 22 pins are used. Most of these pins are not needed for normal PC communications, and indeed, most new PCs are equipped with male D type connectors having only 9 pins.

DCE and DTE Devices

Two terms you should be familiar with are DTE and DCE. DTE stands for Data Terminal Equipment, and DCE stands for Data Communications Equipment. These terms are used to indicate the pin-out for the connectors on a device and the direction of the signals on the pins. Your computer is a DTE device, while most other devices are usually DCE devices.

If you have trouble keeping the two straight then replace the term "DTE device" with "your PC" and the term "DCE device" with "remote device" in the following discussion. 

The RS-232 standard states that DTE devices use a 25-pin male connector, and DCE devices use a 25-pin female connector. You can therefore connect a DTE device to a DCE using a straight pin-for-pin connection. However, to connect two like devices, you must instead use a null modem cable. Null modem cables cross the transmit and receive lines in the cable, and are discussed later in this chapter. The listing below shows the connections and signal directions for both 25 and 9-pin connectors.

Interconnection between DTE-DTE and DTE-DCE

Fig (a):-DTE to DCE Fig (b):-DTE to DTE (null modem)

• The minimum interface between a computer and a peripheral requires 3 lines: pins 2, 3 and 7 as shown in figure.
• These lines are defined in relation to DTE; the terminal transmits on pin 2 and receives on pin 3. On the other hand, the DCE transmits on pin 3 and receives on pin 2.
• Now the dilemma is: how does a manufacturer define the role of its equipment? For example, the user may connect its microcomputer to serial printer configured as DTE. Therefore, to remain compatible with the defined signals of RS-232C, the RS-232C cable must be reconfigured as shown in figure (b) above.
• In figure (a), the microcomputer is defined as a DTE, and it can be connected to the modem defined as a DCE, without any modification in RS-232C cable.
• However, when it is connected to the printers, the transmitted and received lines must be crossed as shown in figure (b). This is also known as Null modem connection.