RS232 serial cable layout

Almost nothing in computer interfacing is more confusing than selecting theright RS232 serial cable. These pages are intended to provide informationabout the most common serial RS232 cables in normal computer use, or in more commonlanguage 'How do I connect devices and computers using RS232?'

RS232 serial connector pin assignment

The RS232 connector was originally developed to use 25 pins. In this DB25 connectorpinout provisions were made for a secondary serial RS232 communication channel.In practice, only one serial communication channel with accompanying handshakingis present. Only very few computers have been manufactured where bothserial RS232 channels are implemented. Examples of this are the Sun SparcStation10 and 20 models and the Dec Alpha Multia. Also on a number of Telebit modem models the secondary channel is present. It can be usedto query the modem status while the modem is on-line and busy communicating.On personal computers, the smaller DB9 version is more commonly usedtoday. The diagrams show the signals common to both connector types inblack. The defined pins only present on the larger connector are shown in red.Note, that the protective ground is assigned to a pin at the large connectorwhere the connector outside is used for that purpose with the DB9 connector version.The pinout is also shown for the DEC modified modular jack. This type of connector has been used on systems built byDigital Equipment Corporation; in the early days one of theleaders in the mainframe world. Althoughthis serial interface is differential (the receive and transmit have their ownfloating ground level which is not the case with regular RS232)it is possible to connect RS232 compatible devices withthis interface because the voltage levels of the bit streams are in the same range.Where the definition of RS232 focussed on the connection of DTE,data terminal equipment (computers, printers, etc.) with DCE,data communication equipment (modems), MMJ was primarily defined for theconnection of two DTE's directly.

RS232 DB25 to DB9 converter

The original pinout for RS232 was developed for a 25 pins sub D connector.Since the introduction of the smaller serial port on the IBM-AT,9 pins RS232 connectors are commonly used. In mixed applications,a 9 to 25 pins converter can be used to connect connectors of differentsizes. As most of the computers are equipped with the DB9serial port version, all wiring examples on this website will use that connectoras a default. If you want to use the example with a DB25, simplyreplace the pin numbers of the connector according to the conversion table below.

DB9 - DB25 conversion

DB9	DB25	Function
1	8	Data carrier detect
2	3	Receive data
3	2	Transmit data
4	20	Data terminal ready
5	7	Signal ground
6	6	Data set ready
7	4	Request to send
8	5	Clear to send
9	22	Ring indicator

RS232 serial loopback test plugs

The following RS232 connectors can be used to test a serial port on your computer.The data and handshake lines have been linked. In this way all data will besent back immediately. The PC controls its own handshaking. The first test plugcan be used to check the function of the RS232 serial port with standard terminalsoftware. The second version can be used to test the full functionality ofthe RS232 serial port with Norton Diagnostics or CheckIt.

DB9	DB25	Function
1 + 4 + 6	6 + 8 + 20	DTR	CD + DSR
2 + 3	2 + 3	Tx	Rx
7 + 8	4 + 5	RTS	CTS

DB9	DB25	Function
1 + 4 + 6 + 9	6 + 8 + 20 + 22	DTR	CD + DSR + RI
2 + 3	2 + 3	Tx	Rx
7 + 8	4 + 5	RTS	CTS

Testing occurs in a few steps. Data is sent on the Tx line and thereceived information on the Rx input is then compared with the original data.The signal level on the DTR and RTS lines is also controlledby the test software and the attached inputs are read back in the software to see if thesesignal levels are properly returned.The second RS232 test plug has the advantage that the ring-indicator RI input linecan also be tested. This input is used by modems to signal an incomingcall to the attached computer.

RS232 null modem cables

The easiest way to connect two PC's is using an RS232 null modem cable. The onlyproblem is the large variety of RS232 null modem cables available. For simpleconnections, a three line RS232 cable connecting the signal ground and receiveand transmit lines is sufficient. Depending of the software used, somesort of handshaking may however be necessary. Use theRS232 null modem selection tableto find the right null modem cable for each purpose. For a Windows 95/98/ME DirectCable Connection, the RS232 null modem cable with loop back handshaking isa good choice.

RS232 null modem cables with handshaking can be defined in numerous ways,with loopback handshaking to each PC, or complete handshaking betweenthe two systems. The most common null modem cable types are shown here.

Connector 1	Connector 2	Function
2	3	Rx	Tx
3	2	Tx	Rx
5	5	Signal ground

Connector 1	Connector 2	Function
2	3	Rx	Tx
3	2	Tx	Rx
5	5	Signal ground
1 + 4 + 6	-	DTR	CD + DSR
-	1 + 4 + 6	DTR	CD + DSR
7 + 8	-	RTS	CTS
-	7 + 8	RTS	CTS

Connector 1	Connector 2	Function
1	7 + 8	RTS2	CTS2 + CD1
2	3	Rx	Tx
3	2	Tx	Rx
4	6	DTR	DSR
5	5	Signal ground
6	4	DSR	DTR
7 + 8	1	RTS1	CTS1 + CD2

Connector 1	Connector 2	Function
2	3	Rx	Tx
3	2	Tx	Rx
4	6	DTR	DSR
5	5	Signal ground
6	4	DSR	DTR
7	8	RTS	CTS
8	7	CTS	RTS

1. Anyone can make a decision given enough facts. 2. A good manager can make a decision without enough facts. 3. A perfect manager can operate in perfect ignorance.

DISCUSSION

A: A standard 'null-modem serial cable' (or a Bluetooth serial adapter), available in good computer shops. The cable needs to be wired for 'full handshaking' (not 'loopback handshaking' or 'partial handshaking'). These cables are architecture-neutral, i.e. they are the same for PC, Amiga and other systems.

The Amiga generally has a 25-pin serial port connector (on the back), whereas PCs use both 9-pin and 25-pin connectors. If the PC does not have a serial port, you can use a USB serial adapter.

On both Amiga and PC systems, be careful not to confuse the serial port (male connector) with the parallel port (female connector). Also, the Amiga has two 9-pin mouse/joystick connectors on the side, which are not serial ports.

Belkin, for example, lists a high-quality, universal (9-pin and 25-pin connectors on both sides, i.e. a total of four connectors) null-modem cable as model number F3X171-10 ('Pro Series File Transfer - PC to PC Cable - Serial'). QVS offers a similar 'Universal Serial File Transfer Cable' as part number CC327-10, which we also tested successfully. Larger computer stores often carry at least one of these two brands. In our experience prices for such 'universal' null-modem cables (i.e. those with four connectors) vary between about $10 and $30, even for the same product sold at different locations. Regular null-modem cables tend to sell in the $5-$7 range.

We did receive a few reports about commercial cables that were not wired properly. Please refer to the section on null-modem cables in the Amiga Explorer documentation for detailed and complete information on this topic, including non-standard Amiga serial ports. The following diagrams show simple schematics for three common null-modem wiring configurations (choose the one that applies based on your combination of PC/Amiga connectors).

DB-25 connectors on both ends

DB-9 connector on one end, DB-25 on the other end

DB-9 connectors on both ends

If you purchased a null-modem cable, and it does not work, please compare it with the schematics in the Amiga Explorer documentation. No parts in this wiring are 'redundant'.

If you need to use a 'gender changer' to attach a null-modem serial cable to the serial port on the back of your computer, it probably means that the cable is not a null-modem serial cable. Null-modem cables should always have female connectors on both sides, and all computers should have male connectors for their serial ports. (A known exception to this are some early Amiga 1000 models. A small number of these had female serial port connectors. If you are in doubt check the indications appearing near the connectors.)

The serial port is normally used for modems, and may have a telephone symbol next to it. The parallel port (which is not used with Amiga Explorer) is normally used for printers, and may have a printer symbol next to it.

Warning: the 25-pin female connector on the back of many computers is a parallel port (not a serial port). While a null-modem serial cable will not attach to it (as it also uses female connectors), certain other combinations of cables and gender changers can in theory be used to try to connect a parallel port on one computer with the serial port of another computer. This should never be attempted, as it may damage one or both computers.

For some applications, including terminal software when no hardware handshaking is used, very simple null-modem cables with three wires are sufficient to exchange data (although not in the most reliable and efficient way). This is accomplished by connecting GND (system ground) on both sides, and cross-connecting TxD (transmit data) with RxD (receive data). Amiga Explorer, like many other programs, additionally uses RTS and CTS for hardware handshaking, and DTR and DSR for each side to inform the other side whether it is ready to process data or not. This ensures, for example, that if there is an overflow in the serial port's input data buffer, the other side stops transmitting, and waits until the buffer is ready to receive more data.

Amiga Explorer requires a complete null-modem cable, with 7 wires. High quality commercial cables follow this specification, but lower quality and 'do-it-yourself' cables sometimes differ. If the wires are not connected properly, Amiga Explorer will not work. In some cases, including proper connections but poor quality or too long cables, Amiga Explorer may transfer short files, but it will fail with longer files (in one or both directions). In other cases, the software will not work at all. The Amiga serial port also uses non-standard signals (see the Amiga Forever documentation), which may damage the equipment on the other side. This cannot happen with a properly wired null-modem cable.

The pins for all DB-9/25 connectors are numbered in such a way that each pin number always corresponds to the same pin number on the other side. For example, pin 1 always connects to pin 1, and is always numbered '1' in all of the above illustrations and tables, regardless of whether it is 'male' or 'female', or viewed from 'inside' or 'outside'. All pins along this electrical path are numbered '1'.

It is very important that the pins which on the different Amiga models supply power (9, 10, 14, 21, 23) be not connected to any active pins on the PC. Also, the two different grounds (shield ground and system ground) should not be connected with each other.

Cables longer than 20 meters (65 feet) should be used with additional care. Special low capacitance cables are better suited for longer distances. Shielded cables and connectors should be used if the cables are close to electrical equipment, which could add noise to the signals. The cable shielding should be connected to the shielding of one of the two sockets. Lower baud rates may help reduce transmission problems over long and/or disturbed lines. The use of additional connectors, adapters and extension wires should be kept to a minimum, as each additional component tends to reduce the quality of the connection.

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