🚀 go-pugleaf

jmorton@cayley.uwaterloo.ca (John Morton) writes:
>I am trying to make a long null modem line.(30+ feet)  I know that there
That's not so long.  That's even shorter than the specification maximum
and no one pays any attention to that.  :-)

>Computer 1         Computer 2
>    Pin                Pin
>     2 ---------------- 3
>     3 ---------------- 2
>    4&5 --------------  8
>     7 ---------------- 7
>     8  -------------- 4&5

Actually an NMC is rather loosley defined but in general it means:

Computer 1         Computer 2
    Pin                Pin
     1 ---------------- 1 (Often omitted)
     2 ---------------- 3
     3 ---------------- 2
     4 ---------------- 5
     5 ---------------- 4
     6 ---------------- 20
     7 ---------------- 7
     8 ---------------- 8
    20 ---------------- 6

From this you have to decide what your system needs.  For example pins 1 and
7 are both ground and generally connect internally and pin 1 is often left
off if there is a shortage of wires.  2 and 3 is for data and are generally
required but in **very** rare cases only one may be required.  Note that
even if data is one way, for example on a printer, you may still need the
back channel for XON/XOFF handshaking.  4 and 5 are used for handshaking.
If one device requires this but the other doesn't support it you can tie
these lines together like this:

     ...
     3 --------------- 2
     4 -----     ----- 5
           |     |
     5 -----     ----- 4
     6 --------------- 20
     ...

If one side requires CD and the other supplies it at appropriate times you
should include pin 8.  If it isn't supplied you can share the input from
pin 20 with pin 6.  Here are 2 "classic" NMCs, a 5 and a 6 wire one:

Computer 1         Computer 2
    Pin                Pin
     2 ---------------- 3
     3 ---------------- 2
     4 -----     ------ 5
           |     |
     5 -----     ------ 4
     7 ---------------- 7
     6 ---------------- 20
         |
     8 ---          --- 8
                    |
    20 ---------------- 6

Computer 1         Computer 2
    Pin                Pin
     2 ---------------- 3
     3 ---------------- 2
     4 -----     ------ 5
           |     |
     5 -----     ------ 4
     7 ---------------- 7
     6 ---------------- 20
     8 ---------------- 8
    20 ---------------- 6

>Is this correct? Do I need more connections?  Is it just my 
>saudering(sp, I know) job?  Should I shield the cable? If so how?

Soldering.  The answers to these questions depend on the particular devices
you are connecting.  Welcome to the world of serial communications.

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