Serial Communication over Short Distances -- Null Modem (60040)

SUMMARY

If two computers are within a short distance of each other, it is possible to use either a null modem cable or null modem adapter to connect the computers for serial communication.

The null modem cable for IBM computers and compatibles changes the connection of pins so that pin numbers are not connected directly to the corresponding pin numbers at the other end of the cable. In other words, pin number 2 is not connected to pin number 2.

If the lines were straight through, a transmitter pin would be attempting to talk with another transmitter pin. A transmitter pin must be connected to a receiver pin for the transmitter to be "heard."

MORE INFORMATION

The alternative to the null modem cable is the use of a null modem adapter that changes the connection between pins and allows the use of straight through cables. A null modem adapter is also called a modem eliminator. It is a cable connector that is typically 2-3 inches long.

The following are the pin-outs of null modem adapter or null modem cable:

   Pin                 Is connected to
   --------------      ----------------

   Chassis GND  1        1  Chassis GND
   TD           2        3  RD
   RD           3        2  TD
   RTS          4        5  CTS
   CTS          5        4  RTS
   DSR&RSD    6&8       20  DTR
   DTR         20      6&8  DSR&RSD
   Signal GND   7        7  Signal GND

The following table explains the abbreviations in the table above:

   DTR         Data Terminal Ready. This line is brought to active
               when the device is ready to accept data.

   Chassis GND Ground. This is a connection that ultimately is
               connected to earth voltage. It prevents the build-up of
               a voltage difference between a machine and other
               machines or humans.

   Signal GND  Ground. It provides a common zero voltage reference
               connection between points. The communications lines use
               this pin as a reference voltage. There are transient
               differences only between this voltage and Chassis
               ground.

   DSR         Data Set Ready. Indicates the device is ready to
               communicate.

   RTS         Request to Send. This is referred to as the output
               handshake. The standard function of this line is to
               indicate that information is ready to be transmitted.

   CTS         Clear to Send. This is referred to as the input
               handshake. The standard function of this line is to
               enable or inhibit data transmission.

   TD          Transmit Data. This connection sends information. It is
               normally connected to the RD of the other device.

   RD          Receive Data. This connection allows the receipt of
               information. It is normally connected to the TD of the
               other device.

The pins are numbered through a convention that assures that the lines are consistently connected. Male connectors are numbered starting with the upper leftmost pin. The numbering continues from left to right across the connector (viewing it from the end of the cable). The next row starts left to right and repeats for each row of pins. Female connectors are numbered in the opposite direction, from right to left, starting in the upper right corner of the cable (again, viewing it from the end of the cable).

With flat cables, the number 1 pin is connected to the first wire that has a red, black, or other color streak running its full length. Round cables rely on colors to distinguish one line from another.