***
***      LOOPBACK Documentation
***
***      Version 2.0
***      January 18, 1999
***
***  MarshallSoft Computing, Inc.
***  POB 4543 Huntsville AL 35815
***
***  Phone: 256-881-4630
***  email: ports@marshallsoft.com
***    web: http://www.marshallsoft.com
***    ftp: ftp://ftp.marshallsoft.com/marshallsoft

*** Making a RS232 Loopback Adapter ***

An RS232 loopback adapter can easily be made from a female-female adapter.
Orient the female-female adapter such that the widest part is facing up. In
this orientation, the pins are numbered from right to left, top to bottom.
Connect pins 2 and 3 together with a short piece of wire (or small paper clip)
as shown below.

The loopback adapter can also test RTS/CTS by connecting pin RTS to CTS, as
shown in the diagram below. This is recommended since the RTS/CTS pins are
used for flow control. The loopback adapter can also test DTR/DSR by
connecting pin DTR to pin DSR.

Lastly, connect the female-female adapter to the end of the modem cable, and
you are run to run your test software. 

                    Female DB 25                              Female DB 9                                                  
  +--------------------------------------------------+    +-----------------+
 / o   o   o   o   o   o   o   o   o   o   o===o   o  \  / o   o   o===o   o \
 \                                                    /  \                   /
  \  o   o   o   o   o   o   o   o   o   o   o   o   /    \  o   o   o   o  /
   +------------------------------------------------+      +---------------+
                    Female DB 25                              Female DB 9
  +--------------------------------------------------+    +-----------------+
 /13  12  11  10   9   8   7   6   5   4   3===2   1  \  / 5   4   3===2   1 \
 \                                                    /  \                   /
  \ 25  24  23  22  21  20  19  18  17  16  15  14   /    \  9   8   7   6  /
   +------------------------------------------------+      +---------------+   

          DB 25   DB 9    Pin Definition
          =====   ====    ==============
            7       5     Signal Ground (SG)
            2       3     Transmit Data (TD)
            3       2     Receive Data (RD)
           20       4     Data Terminal Ready (DTR)
            6       6     Data Set Ready (DSR)
            4       7     Request to Send (RTS)
            5       8     Clear to Send (CTS)
            8       1     Carrier Detect (CD)

(Note: The pin positions are reversed for male connectors)

*** Making a RS422/RS485 Loopback Adapter ***

An RS422/RS485 loopback adapter can be made in a similar manner to the RS232
loopback adapter. For a DB25 connector, you want to connect TD- (pin 2) to RD-
(pin3), and TD+ (pin14) to RD+ (pin 16).

          DB 25   Pin Definition
          =====   ==============
            2     - Transmit Data (TD-)
            3     - Receive Data  (RD-)
           14     + Transmit Data (TD+)
           16     + Receive Data  (RD+)