Re: TI-H: TI LED

To: <ti-hardware@lists.ticalc.org>
Subject: Re: TI-H: TI LED
From: "Thomas Thorsen" <fit@aats.dk>
Date: Thu, 11 Dec 1997 22:50:47 +0100
Reply-To: ti-hardware@lists.ticalc.org



> It's hooked up to pins 2 and 3 .  I dont know the polarity, as the wires
> were both the same length.  Start up your transfer software and just mess
> around with it plugged in and the case off.  Yes, it also blinks.
>                                                               Jeff Rapp

It's very important to use two 10Kohm resistors in seriel with the LEDs. 
the cathode (-) of both LEDs are connected to the shield of the cable, 
which is pin 22 on the parallel-interface.
the two anodes(+) are connected through the resistors to pin 12 and 13
(NOT 2 and 3).

Red 	---------------------------------------------------- 13
			|	|
		             10K	-----D1------- 3
			|
		             LED
			|
G     	---------------------------------------------------- 22
			|
		             LED
			|
		             10K	-----D2------- 2
			|	|
White	---------------------------------------------------- 12


8Kohm resistors may be used but there's a risc, when battery is low, that
they will interfere with com(sends 0 due to grounding through diodes).

The diodes need to be high-bright types, since standard LEDs are barely
visible
with the low power through the resistors.

The current consumed by each LED can be calculated using ohm's law:

Supply voltage/resistance		=>	5V/10.000ohm = .0005A

Such low current won't drag the battery low.

The LEDs will be constantly on, and flashing off when a 1 is transfered.
This is due
to the inverse logic imployed by TI. The blink is very quick, but
noticeable.

To test try starting the duplex-chat program wich constantly tries to
establish connection.

I hope this answers all questions about LEDs in communication. And hey, you
guys working
on the faq, feel free to use this as long as you mention the author "Thomas
Thorsen", me.