1	Protective ground
2	Transmitted data	(TD or TXD)
3	Received data 		(RD or RXD)
4	Request to send		(RTS)
5	Clear to send		(CTS)
6	Data set ready		(DSR)
7	Signal ground		(SG)
8	Carrier detect		(CD or DCD)
20	Data terminal ready	(DTR)

Originally all 25 pins of a DB25 were used but today the above pins are
usually the only ones used.  The most common pinout for DB9 serial
connectors is:

1	Carrier detect		(CD or DCD)
2	Transmitted data	(TD or TXD)
3	Received data 		(RD or RXD)
4	Data terminal ready	(DTR)
5	Signal ground		(SG)
6	Data set ready		(DSR)
7	Request to send		(RTS)
8	Clear to send		(CTS)
	
So a cable for a connection from a DB25 DTE to a DB25 DCE would be wired:

DTE      Cable       DCE
DB25M DB25F  DB25M DB25F
TXD 2>>---------->>2 TXD
RXD 3>>---------->>3 RXD
RTS 4>>---------->>4 RTS
CTS 5>>---------->>5 CTS
DSR 6>>---------->>6 DSR
SG  7>>---------->>7 SG
DCD 8>>---------->>8 DCD
DTR 20>>-------->>20 DTR

In general pin 1, protective ground, is either left open or only
connected to chassis ground on one end of the connection to avoid
ground loops.  Pins 1 and 7 must never be jumpered together on a DB25.

A DB9 DTE to DB25 DCE would be a little more complicated.  It would be
wired:

DTE      Cable       DCE
DB9M  DB9F   DB25M  DB25F
DCD 1>>---------->>8 DCD
TXD 2>>---------->>2 TXD
RXD 3>>---------->>3 RXD
DTR 4>>--------->>20 DTR
SG  5>>---------->>7 SG
DSR 6>>---------->>6 DSR
RTS 7>>---------->>4 RTS
CTS 8>>---------->>5 CTS

If all anyone ever wanted to do was connect DTEs to DCEs all you would
need are straight-through cables.  But what if you want to connect an
MSS1's DTE serial port to a PC's DTE serial port?  That's when you need
a null modem.

The simplest null modem is the 3 wire connection.  On a DB25-DB25
connection it would look like:

DB25            DB25
TXD 2----------3 RXD
RXD 3----------2 TXD
SG  7----------7 SG

Connecting 2 DB9s you'd wire up a null modem this way:

DB9              DB9
TXD 2----------3 RXD
RXD 3----------2 TXD
SG  5----------5 SG

If you're using Xon/Xoff (software) flow control or no flow control
this is all you need.  

A "full handshake" null modem is often wired like this:

DB25            DB25
TXD 2----------3 RXD
RXD 3----------2 TXD
RTS 4----------5 CTS
CTS 5----------4 RTS

DSR 6------|---8 DCD
           |--20 DTR

SG  7----------7 SG

DCD 8---|
DTR 20--|------6 DSR

This assures that all flow control (handshaking) signals are passed
from one side of the connection to the other.  You should be able to
buy a null modem adapter or cable wired this way at any large
electronics store.

To connect a PC's DB9M to a DB25M DTE device like the MSS1 you'd wire
up a cable that looked like this:

 
DB-9F(female)                   DB-25F(female)

TXD 2-----------------------------3  RXD
RXD 3-----------------------------2  TXD
DTR 4-----------------------------6  DSR
GND 5-----------------------------7  GND
DSR 6-----------------------------20 DTR
RTS 7-----------------------------5  CTS
CTS 8-----------------------------4  RTS

If your PC has a DB25M connector for it's serial port the cable would be
wired like this:

 DB-25F(female)                   DB-25F(female)
 
TXD 2-----------------------------3  RXD
RXD 3-----------------------------2  TXD
RTS 4-----------------------------5  CTS
CTS 5-----------------------------4  RTS
DSR 6-----------------------------20 DTR
GND 7-----------------------------7  GND
DTR 20----------------------------6  DSR