Serial Communication Standards in Microprocessor

Serial Communication Standards in Microprocessor – Various devices like printers and modems use serial communication with the microprocessor or 8051. Hence, there is a requirement for some common standards. A standard is normally defined by professional organisations like IEEE. In serial Communication, data can be transmitted in the form of current or voltage.

RS 232 Communication Standard

When data is transmitted as voltage, this standard is used. It is widely accepted for single-ended data transmission over short distances with low data rates. This standard describes the functions of 25 signal and handshake pins for serial communication. It also describes the voltage level, speed of data transfer, rise and fall time, and maximum capacitance for these signal lines. The 25-pin connector (RS 232C) is shown in Figure 1.

The RS-232C standard is defined in reference to Data Terminal Equipment (DTE) and Data Communication Equipment (DCE). The voltage level methods have the following limitations:

1. The voltage levels are not compatible with TTL levels.
2. The rate of data transmission is limited (up to 20K baud).
3. The maximum distance is only 50ft.

Figure 2 shows the interfacing between TTL and RS-232 signals. The line driver, MC 1488, converts logic ‘1’ into approximately – 9V and logic ‘0’ into approximately +9V. These levels, at the receiving end, are again converted by the line receiver, MC 1489, into TTL-compatible logic. For high-speed transmission, the standards RS – 422A and RS – 423A are used. These standards are differential amplifiers to reject levels and can transmit data at higher speeds over larger cables.

The comparison of these serial communication standards is shown in the following table.

MC 1488 (Line Driver) Standard

The MC-1488 is a quad-line driver. It is used to interface DTE and DCE through the RS-232C bus standard. The pin diagram of MC-1488 is shown in Figure 3. It converts TTL logic ‘1’ (> 3.4V) into approximately -9V and TTL logic ‘0’ (< 0.2V) into +9V. The MC-1488 contains four drivers, and each driver converts a TTL level into an RS-232 level.

MC-1489 (Line Receiver) Standard

This is a quad-line receiver to interface DTE and DCE through the RS-232C bus standard. It converts -9V signal into TTL logic ‘1’ (>3.4V) and +9V signal into TTL logic ‘0’ (<0.2V). The pin diagram of MC-1489 is shown in Figure 4. The MC-1489 contains four receivers, and each receiver converts an RS-232 level signal into a TTL level signal.

Note: MC 1488 and MC 1489 convert the voltage level, hence they are also known as level converters.

Current Loop Standard

When data are transmitted as current, current loops are used. In some current signal systems, like a teletype writer, a current loop of 20mA/60mA is used. Some manufacturer uses 20mA current loop to represent logic ‘1’ or mark, and no current to represent logic ‘0’ or space. Other manufacturer uses 60mA current loop to represent logic ‘1’ and no current to logic ‘0’.

The benefits of the current loop over the other transmission, prior to the RS422, were the long distance and noise-immune transmission of data. The 20mA version is capable of sending data over 2000ft, which is impossible with RS 232. The maximum resistance of the current loop depends on the available voltage, and there are no mechanical or electrical standards. The current loop consists of active and passive devices. This is shown in sections a and b of Figure 5.