Note, Nov. 1999: this HTML is from late 1993, early 1994, untouched, except for the head and foot data, and this note. How to wire a Nullmodem cable

I swiped this source from, it was so damn useful I was afraid it might disappear. Finding this document was one of the things that made me consider that the web might actually be useful. It saved me the trouble of walking across the street to my office to pull out a paper copy from the back of some hideous manual. The ambivalence I have towards this event exactly sums up my feelings for the net in general.


                25pin   9pin            9p      25p     
FG              1       -       X       -       1       FG
TD              2       3       -       2       3       RD
RD              3       2       -       3       2       TD
RTS             4       7       -       8       5       CTS
CTS             5       8       -       7       4       RTS
SG              7       5       -       5       7       SG
DSR             6       6       -       4       20      DTR
DTR             20      4       -       6       6       DSR

Frame Ground
Transmit Data
Recieve Data
Request To Send
Clear To Send
Signal Ground
Data Set Ready
Data Terminal Ready

You assemble one connector from each side of the hyphens. The frame ground on the 25 pin plug is optional and should I have been told only be connected on one side of the cable. If you need Data Carrier Detect on your cable (BBS programs, Unix Getty and so on) then you should apply this patch. On a 25 pin plug you can connect pin 6 and 8 On a 9 pin plug you can connect pin 1 and 6 These connections should be made on the same side of the cable.

The standard for RS232 sets these signals on the various pins.

   Common name   RS232C Name          Description

25pin         9 pin        DCE Direction
1                       AA      x       Protective Ground
2       TXD     3       BA      I       Transmitted Data
3       RXD     2       BB      O       Received Data
4       RTS     7       CA      I       Request to Send
5       CTS     8       CB      O       Clear to Send
6       DSR     6       CC      O       Data Set Ready
7       GND     5       AB      x       Signal Ground
8       CD      1       CF      O       Received Line Signal Detector
9                       --      x       Reserved for Data set Testing
10                      --      x       Ditto
11                              x       Unassigned
12      SCF                     O       Secondary Received Line Signal                                          Detector 
13      SCB                     O       Secondary Clear to send
14      SBA                     I       Secondary Transmitted Data
15      DB                      O       Transmission Signal Element                                             Timing  (DTE Source)
16      SBB                     O       Secondary Received Data
17      DD                      O       Receiver Signal Element Timing                                          (DCE Source)
18                              x       Unassigned
19      SCA                     I       Secondary Request to Send
20      DTR     4       CD      I       Data Terminal Ready
21      CG                      O       Signal Quality Detector
22              9       CE      O       Ring Indicator
23      CH/CI                   I/O     Data Signal Rate Selector                                               (DTE/DCE Source)
24      DA                      I       Transmit Signal Element Timing (DTE Source)
25                              x       Unassigned

I will not accept any responsibility for the correctness of this information nor any liability for damages that have been inflicted on you through the use of this information.