The RS-232 protocol
Rs232 protocol transmission involves the sending of data one bit at a time, over a single communications line. In contrast, parallel communications require at least as many lines as there are bits in a word being transmitted (for an 8-bit word, a minimum of 8 lines are needed). RS232 Serial transmission is beneficial for long distance communications, whereas parallel is designed for short distances or when very high transmission rates are required.
One of the advantages of the RS232 protocol is that it lends itself to transmission over telephone lines. The serial digital data can be converted by modem, placed onto 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 communications equipment using serial binary data exchange." This definition defines data terminal equipment (DTE) as the computer, while data communications equipment (DCE) is the modem. A modem cable has pin-to-pin connections, and is designed to connect a DTE device to a DCE device.
RS-232 is widely used for connections between data acquisition devices and computer systems. RS-232 devices are defined as either DTE (usually a computer) or DCE (usually an interface device). When wiring a DCE device to a computer (DTE), a straight-through connection is required. However, not all interface devices or data acquisition systems are DCE, therefore, require a null-modem cable, which 'crosses over' the necessary signal wires.
In addition to communications between computer equipment over telephone lines, RS-232 protocol is now widely used for connections between data acquisition devices and computer systems. As in the definition of RS232, the computer is data transmission equipment (DTE). However, many interface products are not data communications equipment (DCE). Null modem cables are designed for this situation; rather than having the pinto- pin connections of modem cables, null modem cables have different internal wiring to allow DTE devices to communicate with one another.
RS-232 cables are commonly available with either 4, 9 or 25-pin wiring. The 25-pin Rs232 cable connects every pin; the 9-pin Rs232 cables do not include many of the uncommonly used connections; 4-pin Rs232 cables provide the bare minimum connections, and have jumpers to provide "handshaking" for those devices that require it. These jumpers connect pins 4, 5 and 8, and also pins 6 and 20.
The advent of the IBM PC AT has created a new wrinkle in RS-232 communications. Rather than having the standard 25-pin connector, this computer and many new expansion boards for pc's feature a 9-pin serial port. To connect this port to a standard 25- pin port, a 9- to 25-pin adaptor cable may be utilized, or the user may create his own cable specifically for that purpose.
Selecting a Rs232 Cable
The major considerations in choosing an RS-232 cable are based upon the devices 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 are required on each end, male or female, and 25 or 9-pin (AT style)? Usually, it is recommended that the user obtain the two devices to be connected, and then determine which cable is required.
Wiring a Rs232 interface
Most RS-232 devices will operate with only 3 signal wires: Transmit (TX), Receive (RX) and Ground (GND). In order for two RS-232 devices to communicate, you must connect the TX from one instrument to the RX of the second instrument, and vice versa.
The Ground pins must be connected together. Keep in mind that a 25 pin RS-232 port on a PC trasmits on pin 2 and receives on pin 3, and Ground is pin 7. A 9-pin RS-232 port on a PC transmits on pin 3, receives on pin 2 and Ground is pin 5.
You cannot simply connect two devices with a serial cable simply because the connectors fit. You must verify the functions of each pin on each device, as well as verify whether or not the cable is a straight-through or null-modem cable.