Serial cables how many wires

The TD and RD wires carry the data, the Signal Ground wire allows the signals on the other wires to be measured, and the other circuits monitor the state of the connection.

As you can see, the two genders are mirror images of each other so when they they mate, pin 1 connects to pin 1, pin 2 to pin 2, and so on. Click diagram to enlarge Description of Modem Signals The serial port of a computer or terminal is designed and intended to be connected to a modem. Note that the modem receives data on its Transmit Data pin. Received Data RD Carries data from the modem to the computer.

Note that the modem sends data on its Received Data pin. Request to Send RTS Today this signal is generally used to indicate that the computer is prepared to receive data from the modem it can also be used in half-duplex communication to request permission to transmit, hence the name.

Without this, data is likely to be lost or corrupted due to differing speeds or capacities of the connected devices. Carrier Detect CD Indicates that the modem has a telephone connection to another modem and is communicating with it successfully. When the CD signal goes from Off to On, it means the other modem has just "answered the phone".

That's pretty slow when you're used to a network connection, but let me show you how serial data is transferred so you can also understand why it's a lot slower: Figure 1. Transfer of data via serial port The above picture gives you an idea on how serial data is transferred.

Let's have a look at a serial port to see what we are talking about: Figure 2. Let's just have another quick look at the COM ports of a new computer: Figure 3. You use this configuration when you need a cable with a DB-9 connector on each end: Figure 6. You use this configuration when you need a cable with one DB-9 and one DB connector on either end: Figure 7.

You use this configuration when you need a cable with a DB connector on each end: Figure 8. Articles To Read Next:. Important Direct Cable Connection Notes. Direct Cable Connection. It also saves considerable complications by not having to work out which wires to cross over in the serial data cable. Complications do arise when cables are required to connect two items both configured as the same form of equipment, i.

Different data systems utilise different levels of handshaking when using RS Some of the simpler systems just send the data without any hard wired handshaking and as a result the serial data cables required for this are very simple.

These systems require serial data cables that incorporate the transmit and receive data lines as well as an earth line. A suitable cable is shown below. More comprehensive systems may utilise the additional lines provided by RS According serial data cables for these systems require the additional lines to be incorporated into the serial data cables for them to operate correctly.

The RS cable shown above gives the more likely connections seen in an RS cable. Some cables could have more connections for additional functionality, but these are rarely used.

There are several methods that can be used to construct RS cables and the method chosen should be selected according to the requirements, cost timescales, level of reliability, etc.

Serial cables differ in the connectors used on each end, the number and type of wires that are connected end to end, the pinouts which pin on one connector is connected to which pin on the other , and the connections made internally within each local connector, if any. With permutations, there are literally millions of ways you could build a serial cable. Fortunately, only a handful are commonly used. Commonly used serial cables fall into one of the two following general categories:.

A straight-through cable is just what it sounds like? On a DB9-to-DB25 cable, the wires connect different pin numbers, but the same signal. Almost any cable with a DB9 connector connects all nine pins. DB25 cables may have all or only some pins connected, but the existing connections are straight-through. Cross-over cables come in an amazing variety of pinouts, some reasonably standard and others specific to one particular type of connection?

The term null-modem cable is often misused to mean any cross-over cable, but a null-modem cable is really just one variety of cross-over cable. PC serial ports are usually configured as DTE. Modems, mice, trackballs, digitizers, and scanners are usually DCE devices, and so connect to a PC with a straight-through cable. Serial printers and plotters are usually DTE devices, and so connect to a PC using some form of cross-over cable.

Those to be used with hardware flow control require additional connections. Table through Table show the pinouts for some common serial cables. Table shows the simplest possible bidirectional straight-through cables, which may be used to connect a DTE e. These cables are not commonly used because they make no provision for hardware flow control. Table Simple straight-through cables DB9 to DB9.

Table shows the simplest possible bidirectional cross-over cables. These cables, which also make no provision for hardware flow control, are commonly used to attach DTE terminal server ports to DTE terminals in a host environment, but are uncommon in the PC environment. Simple cross-over cables DB9 to DB9. Some programs, notably some bulletin board software, also require that Pins 12 and 23 be connected, which rules out using these programs with a DB9 serial port.

We generally keep only straight-through, wire, DBto-DB25 cables on hand, which can be used with such adapters to connect anything to anything. Pins 1 and 6 are shown bridged because some motherboards cause a "Cable not connected" message to appear unless CD is asserted by being bridged to DSR so that the voltage that appears on DSR anytime the modem is ready also appears on CD to spoof the DTE into believing that a carrier is always present.

You can leave Pin 1 disconnected if this is not the case with your motherboard. However, it doesn't hurt to have Pin 1 connected, even if your motherboard does not require CD be forced high, so the pinouts shown result in a more generally useful cable. Buy the cable if you can. Any cable you need is probably a standard item, and will be cheaper and better than one you build. If you need an unusual pinout, order a custom cable from Black Box, DataComm Warehouse, or another company that produces cables to order.