Date   : Wed, 04 Jun 1986 11:47:11 EDT
From   : Robert Bloom AMSTE-TEI 3775 <rbloom@apg-1.ARPA>
Subject: Answer: 2 Computers -> 1 Printer!

I had asked the net how to connect 2 computers to 1 printer 
directly, without use of a AB switch.  Users act as flow control 
so that both don't print at one time.  Michael Neary 
<MNeary.es@Xerox.COM> gave me the answer which worked:

     Yes, all you probably need is a couple of diodes, and maybe 
     a couple of resistors.

     Place a resistor (about 4.7K to 15K) in parallel with each 
     diode.  A diode goes in series with pin 3 (TXD) from each 
     computer.  Join the <cathodes> together and hook this to the 
     printer.

     I am assuming that the 'receive' level translator in the 
     printer will need some bias current to establish a '0' 
     level.  If it doesn't, then the resistors would be 
     unnecessary.

     The nice thing about RS232 is that I have never seen anybody 
     break RS232 unless they apply external voltages to it.  It 
     seems incapable of hurting itself.  :-)

For those mechanical types that can't read words (like me)  and 
need a picture to see what's going on, here's the picture:

                                 /---|<|---\   
                        /-------<           >---- computer
                       /         \--xxx---/    
        printer ------<        
                       \        /---|<|---\
                        \------<           >---- computer 
                                 \--xxx---/    

--xxx-- is the resistor (I used 10k)

--|<|-- is the diode (the marked end is to the left, towards printer

Above fiddling is done on RS232 pin 3.  I made all others 
straight through.  The diodes and resistors were from my spare 
parts drawer, the diodes weren't even matched.  I don't know the 
part numbers either.  But it's working so I won't mess or argue 
with success ...

Thanks Mike,
bob bloom