The Standard of RS232 Communication Protocol

Information transmitted between data processing equipment and peripherals is in the form of digital data, either serially or in parallel. Parallel communication is mainly used for the connection between test instruments or computers and printers, while serial communication is usually used for the connection between computers and other peripheral devices. On behalf of the product has a serial server.

Rs232 protocol transmission involves sending data one at a time over a communication line. Conversely, parallel communication requires at least as many lines as there are bits in the word to be transferred (8 lines minimum for 8-bit words). RS232 serial transmission is useful for long-distance communication, while the parallel transmission is designed for short distances or when very high transmission rates are required.

RS232 standard

One of the advantages of the RS232 protocol is that it can be transmitted over telephone lines. Serial digital data can be converted by a DTU, placed on a standard voice-grade telephone line, and converted back to serial digital data at the receiving end of the line by another modem.

Officially, RS-232 is defined as "the interface between data terminal equipment and data communication equipment using serial binary data exchange". This definition defines data terminal equipment (DTE) as a computer and data communication equipment (DCE) as a modem. Modem cables have pin-to-pin connections designed to connect DTE equipment to DCE equipment.

RS-232 is widely used for the connection between data acquisition equipment and computer systems. RS-232 devices are defined as DTE (usually a computer) or DCE (usually an interface device). A direct connection is required when connecting a DCE device to a computer (DTE). However, not all interface devices or data acquisition systems are DCEs, so a null modem cable is required, which "crosses" the necessary signal lines.

RS232 port

In addition to communicating between computer equipment over telephone lines, the RS-232 protocol is now widely used to connect data acquisition equipment and computer systems. As defined by RS232, a computer is a Data Transmission Equipment (DTE). However, many interface products are not data communication equipment (DCE). Null modem cables are designed for this situation. Null modem cables have the pin connections of modem cables instead of having different internal wiring to allow DTE devices to communicate with each other.

Wiring Options

RS-232 cables usually provide 4-pin, 9-pin or 25-pin wiring. A 25-pin Rs232 cable connects each pin. The 9-pin Rs232 cable doesn't contain many less commonly used connections; the 4-pin Rs232 cable provides minimal connections and has jumpers to provide "handshake" functionality for those devices that need it. These jumpers connect pins 4, 5, and 8, and pins 6 and 20. IBM

The advent of PC AT brought new challenges to RS-232 communications. This computer and many new expansion boards for pc don't have the standard 25-pin connector but instead have a 9-pin serial port. To connect this port to a standard 25-pin port, a 9-to-25-pin adapter cable can be used, or the user can create their own cable specifically for this purpose.

Select RS232 cable

The main consideration in choosing an RS-232 cable depends on the device to be connected. First, are you connecting two DTE devices (null modem cable) or a DTE device to a DCE device (modem cable)? Second, what connectors do you need male or female, and 25 or 9 pins (AT style) on each end ? In general, users are advised to obtain both devices to connect and then determine which cable is required.

Wiring RS232 interface

Most RS-232 devices can only operate with 3 signal lines: transmit (TX), receive (RX), and ground (GND). In order for two RS-232 devices to communicate, the TX of one instrument must be connected to the RX of the other and vice versa.

The ground pins must be connected together. Remember that a 25-pin RS-232 port on a PC transmits on pin 2 and receives on pin 3, while ground is pin 7. The 9-pin RS-232 port on the PC transmits on pin 3 and receives on pin 2. ground is pin 5.

You can't simply connect two devices with a serial cable just because the connectors fit. You must verify the functionality of each pin on each device, as well as verify whether the cable is a straight-through or null modem cable.