To achieve this, the
most obvious connection is that the TD signal of one device must be connected
to the RD input of the other device (and vice versa).
Also, however, many
DTE devices use other RS-232 pins for out-of-band (i.e., "hardware") flow
control. One of the most common schemes is for the DTE (the PC) to assert
the RTS signal if it is ready to receive data (yes, it DOES sound backwards,
but that's how it works), and for the DCE (the modem) to assert CTS when it is
able to accept data. By connecting the RTS pin of one DTE to the CTS pin
of the other DTE, we can simulate this handshake.
Also, it is common
convention for many DTE devices to assert the DTR signal when they are powered
on, and for many DCE devices to assert the DSR signal when they are powered on,
and to assert the CD signal when they are connected. By connecting the
DTR signal of one DTE to both the CD and DSR inputs of the other DTE (and vice
versa), we are able to trick each DTE into thinking that it is connected to a
DCE that is powered up and online. As a general rule, the Ring Indicate
(RI) signal is not passed through a null-modem connection.
Common Null-Modem
Connection
|
Signal
Name |
DB-25
Pin |
DB-9
Pin |
|
DB-9
Pin |
DB-25
Pin |
|
|
FG (Frame Ground) |
1 |
- |
X |
- |
1 |
FG |
|
TD (Transmit Data) |
2 |
3 |
- |
2 |
3 |
RD |
|
RD (Receive Data) |
3 |
2 |
- |
3 |
2 |
TD |
|
RTS (Request To Send) |
4 |
7 |
- |
8 |
5 |
CTS |
|
CTS (Clear To Send) |
5 |
8 |
- |
7 |
4 |
RTS |
|
SG (Signal Ground) |
7 |
5 |
- |
5 |
7 |
SG |
|
DSR (Data Set Ready) |
6 |
6 |
- |
4 |
20 |
DTR |
|
CD (Carrier Detect) |
8 |
1 |
- |
4 |
20 |
DTR |
|
DTR (Data Terminal Ready) |
20 |
4 |
- |
1 |
8 |
CD |
|
DTR (Data Terminal Ready) |
20 |
4 |
- |
6 |
6 |
DSR |
Here's another
null-modem connection that I've seen floating around the net. Some folks
say that it's the cable that's shipped with LapLink 4 Pro.
|
Signal
Name |
DB-25
Pin |
DB-9
Pin |
|
DB-9
Pin |
DB-25
Pin |
|
|
FG (Frame Ground) |
1 |
- |
X |
- |
1 |
FG |
|
TD (Transmit Data) |
2 |
3 |
- |
2 |
3 |
RD |
|
RD (Receive Data) |
3 |
2 |
- |
3 |
2 |
TD |
|
RTS (Request To Send) |
4 |
7 |
- |
8 |
5 |
CTS |
|
CTS (Clear To Send) |
5 |
8 |
- |
7 |
4 |
RTS |
|
SG (Signal Ground) |
7 |
5 |
- |
5 |
7 |
SG |
|
DSR (Data Set Ready) |
6 |
6 |
- |
4 |
20 |
DTR |
|
DTR (Data Terminal Ready) |
20 |
4 |
- |
6 |
6 |
DSR |
We don't think that
a null-modem cable built to the above pinout will work quite as well, but a lot
of folks appear to have success with it. In general, it will work with
some software packages, such as those that only use RTS/CTS hardware flow
control. However, some packages that rely on the proper assertion of the
CD signal will not work with this cable.
Here's a good set of
figures for DB-25 male and female connectors, as viewed from the pin side (not
the solder side).
DB-25 Male
DB-25 Female
Here's a good set of
figures for DB-9 male and female connectors, as viewed from the pin side (not
the solder side).
DB-9 Male
DB-9 Female