Serial Cabling
Quick Reference to Serial Cabling

I. Serial cable adaptor wiring:

+  DB9     D25    D25 null  DB9 null
b   1       8      20        4
    2       3       2        3
    3       2       3        2
b   4      20       8        1
    5       7       7        5
    6*      6*      6*       6*
a   7       4       5        8
a   8       5       4        7
* Tie DSR to DTR on the same connector end.
For DB9 connectors, that means connect pin 6 to pin 4. On D25 connectors, connect pin 6 to pin 20.
+ For minimum wires, when hardware handshaking is not necessary,
short all pins labeled "a" together at each end, and do the same for those labeled "b". This leaves only three signals required.

II. Serial cable connector pinouts:

 1: Frame ground.        2: Transmit data.     3: Receive data.
 4: Request to send.     5: Clear to send.     6: Data set ready.
 7: Signal ground.       8: Carrier detect.    9: Unused
10: Unused.             11: Printer busy.     12: Secondary tx data.
13: Secondary rx data.  14: Unused.           15: Transmit clock.
16: Unused.             17: Receive clock.    18: Unused.
19: Unused.             20: Data terminal ready.
21: Unused.             22: Ring Indicator.   23: Unused.
24: Unused.             25: External clock.

 1: Carrier detect (CD)    2: Receive data (RX)
 3: Transmit data (TX)     4: Data terminal ready (DTR)
 5: Signal ground (SG)     6: Data set ready (DSR)
 7: Request to send (RTS)  8: Clear to send (CTS)
 9: Ring indicator (RI)

III. Introduction to DTE/DCE concepts.

Serial devices are typically wired internally in one of two ways. Depending on which way the device is wired, the meaning of several RS232 signals is swapped.

DCE stands for Data Communications Equipment, and refers to devices which generate or transfer data. The ports on an IBM type PC, multiple ports device, and modems are typically wired in this way.

DTE stands for Data Terminal Equipment, and refers to devices which primarily output data. Printers and the main port on terminals are typically wired in this way.

The intention of this difference is that a straight through cable (pins with identical numbers are connected to each other from end) can be used to connect a computer to a printer, or a terminal to a modem.

Connecting two identically wired devices requires that some signals be swapped. For example a computer to a modem, or two terminals to each other. This type of cable is referred to as a null modem cable.

In its simplest form, a serial cable needs a minimum of three connections, pins 2, 3 and 7 (on a 25 pin connector). A more complete cable includes pins 2, 3, 4, 5, 6, 7, 8 and 20. The more complete set is required for hardware handshaking.

A null modem cable is made by swapping several pins between the two ends. Typically pins 2 and 3, 4 and 5, 8 and 20 are swapped with one another. One can also connect pin 6 to pin 8 at one or both ends.

Many terminals have two ports, a main and auxiliary. The main is usually wired as DTE, while the auxiliary is wired as DCE, because it is intended for connection to a printer. The 605 and 705 terminals are wired in this way. On a 705 terminal, the D25 connector is considered main and is wired DTE, while the ten pin connection is wired DCE.

IV. RS232 signals:

The following describes the functions of the major signals used in RS232 serial communications. Note that these descriptions are from the point of view of a DCE device. Pins numbers refer to a 25 pin connection.

Pin 1, Frame ground.
Connects to the chassis of one of the communicating devices. Connect this signal AT ONE END ONLY to the metallic outer jacket shield of a serial cable. This improves the cable's resistance to external electrical noise.

Pin 2, Transmit
Sends data out.

Pin 3, Receive
Receives data in.

Pin 7, Signal ground.
Connects signal ground reference between both ends.

Pin 4, Request to send.
Raised when a device is ready to communicate.

Pin 5, Clear to send.
Detects readiness of other device to communicate.

Pin 8, Carrier detect.
Detects that other device is on-line.

Pin 20, Data terminal ready.
Raised when a device is on-line. A device dropping DTR causes Unix to terminate the serial connection.