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[DPRG] Over dioded?

Subject: [DPRG] Over dioded?
From: Dale Wheat dale at dalewheat.com
Date: Mon Dec 27 18:30:17 CST 2010

I'm designing a new RS-232 to TTL adapter for the NXP LPC Cortex-M0 and 
Cortex-M3 processors.  It uses some of the modem control lines to reset the 
chip and put it into bootloader mode.  I'm using very simple 
resistor-transistor level shifting to constrain the incoming signals to 
typically a 0V-3V range (or whatever is supplied on the Vcc pin).

My original design works well, even with "barely compliant" USB to RS-232 
adapters.  I went and got all fancy by adding some LEDs inline with the 
incoming signals, instead of adding an additional driver transistor for each 
signal.  Using high-brightness LEDs, I can just see the LEDs twinkling when 
bits are flowing to and from the chip.

My question is this:  With the LEDs in series with the incoming signals, do 
I still need the protection diodes (D1, D2, D3) to shunt potential negative 
levels to ground?  Or will the inherent diode-ness of the LEDs prevent this 
from ever happening?

Here is a PDF of the schematic-in-progress:


Any suggestions are appreciated!


Dale Wheat
(972) 486-1317 

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