LF: NULL MODEMS EXPLAINED!!!!!!!!!!

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LF: NULL MODEMS EXPLAINED!!!!!!!!!!

----------
> From: 		
> To: list-fargo@lists.ticalc.org
> Subject: Re: LF: Modem w/o GraphLink
> Date: Sunday, November 17, 1996 8:00 PM
> 
> Does anyone know what null modem adapters are? If so Reply and I will be
more
> than happy to tell every one how to link their TI-92 to a modem.




The computer's serial port has the transmit and receive lines on certain
pins (I do not know exact numbers).  This means that devices connected to
it have their receive and transmit pins on the opposite ones as the
computer.  That way the computer "speaks" to the devices' "ear"  and the
device "speaks" to the computer's "ear".


The TI-92 is really just a device that plugs into the computer.  If you
want to plug a device into another device or a computer into another
computer (directly) , you must use a null modem adapter.  




Here is a simplfied diagram (there are other things in serial cables but
they are extra indicators, not the actual data transmission wires and not
always used).


RX=Receive
TX=Transmit




Computer<------------------>  Calculator (w/ Official TI Graphlink)


RX <------------------> TX
TX <------------------> RX




Computer <------------------> Modem, Mouse, Any other device


RX <------------------> TX
TX <------------------> RX




Calculator (without null modem adapter)   <------------------> Modem 


RX <------------------> RX         (PROBLEM)
TX <------------------> TX          (PROBLEM)




Calculator (with null modem adapter ) <------------------> Modem 


RX <------------------> TX
TX <------------------> RX








I hope you understand this!


If you are wondering why the TI has 3 connectors on its plug when there is
a TX and RX line,  the third one is ground.  The TX and RX lines are just
positive pulses.  For the parallel port $5 link, it is a clever, deceptive
idea. I do not fully understand it yet but it involves turning on and off
certain pins to produce a pulse at 9600 baud.  This is not normal operation
but it indeed does work.  Serial ports operate around 10 volts and parallel
ports operate at 5 volts, the same as the calc. The chip that converts the
signals is fairly complex and cannot be reproduced cheaply nor easily
because TI calcs don't communicate at EXACTLY 9600 baud as the computer. I
think either the spacing between bits or the length of a bit is a little
different, otherwise you could build your own link with two transistors and
a serial connector!!!!