From pfimdt93@hvdc.hv.se Wed Apr 20 10:28:30 1994
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Date: Wed, 20 Apr 94 14:38:13 +0100
From: PER FINANDER <pfimdt93@hvdc.hv.se>
Subject: Link-interface
To: bed@char.vnet.net
Message-Id: <940420143813.3f5-2.2b8-pfimdt93@hvdc.hv.se>
Status: RO

Hello,

Here is the schematics in windows metafile format. You build this interface
on your OWN risk because I can't promise that this circuit works to 100 
percent. I disclaim all responsibilities. 

Some hints of you decide build one:

* Use a shielded cable between interface <=> TI85 and connect the shield
  to the ground.

* The LM3302 can be replaced by a LM339N (the LM339 is used in the real 
  link85-interface) 

I've included a small program "TESTLINK" to test the interface. To use it
write : "TESTLINK 1" if the interface is connected to com1. Do not connect
the TI85 to the interface when running this program, otherwise the TI85 
thinks it's a transmission and tries to respond. You'll get 4 numbers:

Numbers    Possible errors:
3,1,2,0 <= Correct, the interface works
0,0,0,0 <= Wrong Com-port or no powersupply to the LM3302/LM339
3,2,1,0 <= RTS/DTR or CTS/DSR connections have been exchanged

If you write "TESTLINK 1 X" you must press a key between every number,
but this makes it possible to measure the voltage on the TI85 connection.
 
 IO1 GND
 |   |
<=|=|=|||||| 
   |
   IO2

Number  IO1-GND  IO2-GND
1 (3)   > 4   V  > 4   V
2 (1)   > 4   V  < 0,5 V     | If the voltage on these two lines are reversed
3 (2)   < 0,5 V  > 4   V     | exchange IO1- and IO2-connections on the plug
4 (0)   < 0,5 V  < 0,5 V

I think this will help you.

