lamaPLC Communication: RS-232

In telecommunications, RS-232 or Recommended Standard 232 is a standard originally introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a DTE (data terminal equipment) such as a computer terminal, and a DCE (data circuit-terminating equipment or data communication equipment), such as a modem. The standard defines the electrical characteristics and timing of signals, the meaning of signals, and the physical size and pinout of connectors.

The current version of the standard is TIA232-F Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange, issued in 1997. The RS-232 standard had been commonly used in computer serial ports and is still widely used in industrial communication devices.

A serial port complying with the RS-232 standard was once a standard feature of many types of computers. Personal computers used them for connections not only to modems, but also to printers, computer mice, data storage, uninterruptible power supplies, and other peripheral devices.

Compared with later interfaces such as RS-422, RS-485 and Ethernet, RS-232 has lower transmission speed, shorter maximum cable length, larger voltage swing, larger standard connectors, no multipoint capability and limited multidrop capability. In modern personal computers, USB has displaced RS-232 from most of its peripheral interface roles. Thanks to their simplicity and past ubiquity, however, RS-232 interfaces are still used—particularly in industrial machines, networking equipment, and scientific instruments where a short-range, point-to-point, low-speed wired data connection is fully adequate.

• cheap
• widespread

• multiple body potentials: devices with significantly different potentials often have to be connected; in this case, significant voltage (difference) may occur on the GND wire. In these cases, it is worth adding opto to the communication.
• sensitive to noise: This problem can be significantly reduced by shielding the common GND at both ends of the wire.
• can be used for short distances: Typically (as recommended) it can be used up to 15 m, but with a shielded cable (and in a disturbance-free environment) it can be extended up to 30 m. With a low-capacity cable, you can try to reach up to 100 meters (see baud rate). A classic solution is to convert RS-232 to RS422 (and then convert it back on the other side), so the distance can be increased to 1200 m; its official name: V.35 interface.
• slow

The transmission rate / distance ratio depends significantly on the quality of the used wire and the number of line amplifiers (repeaters). The curves below show typical values only.

The values below can be achieved with UTP CAT-5 cable - 55 pF/m.

• Data signal (TXD) Space, voltage level +3V - +15V Logic 0 signal, control signal (RTS, DTR): ON
• Data signal (TXD) Mark, voltage level -3V - -15V Logic 1 signal, control signal (RTS, DTR): OFF
• Dead band, voltage level -3V - +3V

According to the standard, a dead band is included in RS-232 communication, this is the +3V - -3V range. The task of this is to absorb noise, the actual signals are only produced by the voltage levels that move out of this band.

The voltage moving in the positive direction is a logical “0” signal, called creates a space, and in the negative direction it generates a logical “1”, mark. The voltage level can range up to 15V in both directions, but most devices typically communicate with a voltage of +/- 5V.

The - perhaps the second - most common RS-232 communication device is the null modem. This wire is typically used to connect two stations capable of full communication. If the two stations do not ask for different handshakes, there is no problem, because the “minimum” rs232 functionality (2,3,5) described in the next point can still be provided.

This page has been accessed for: Today: 1, Until now: 94