RE: Opto Isolators & Parallel Ports

From: David Howland dhowland@...
RE: Optoisolators & Parallel Ports

If one wants to opto isolate the parallel port of one's computer, the following facts need to be considered:

The Parallel port pulls the signals up and down, that is, it drives "ones" high (+5 Volts) and "zeros" low (0 Volts or Ground) and the parallel port does this actively with a significant amount of energy.

Opto Isolation devices come in several types. The 4N28, for example, is an open collector transistor style device. Open collector drivers of all types can actively drive the line high OR low and usually a resistor pulls the line back to the other state. Resistors used to passively pull lines high or low can NOT pull the line as quickly as an active driver, and hense there may be a problem in speed. Active drivers are faster than passive pull up or pull down resistors. Active drivers have other advantages, but let's get to the issue.

My favorite way to issolate and drive fast, is to use a high speed TTL output Optoisolator such as the 74OL6000 from QT Optoelectronics connected to a
74LS244 Line Driver and provide power on the business end for the 74LS244 and pin 4 and 6 of the 74OL6000 with a 5 Volt (transformer issolated) Power Supply. You need a seperate power supply (from the computer) to maintain issolation. You must add a few filter caps at the 74OL6000 and the 74LS244 +5 Volt to Gnd connections to keep noise out of your efforts. If you wish to issolate the handshake lines (inputs) on the parallel port as well, you will want to make sure you keep the 5 volt power supply on your business end and don't fall in the trap of crossing parts and the power supply from your business end to your computer end. To avoide this pit fall, seperate your input issolation components and (transformer issolated) power supply and your output issolation components and second power supply by constructing seperate electronics for the inputs and the outputs. Use One power supply to provide 5 volts for the input components, and a seperate (transformer issolated) power supply for the output issolation components. There are short cuts, but if you follow these rules, you won't get into trouble.

The 74OL6000 is $2.72 each from Newark Electronics. (800) 463-9275
The 74LS244 is practically sold at Safeway it is so common. There are stronger drivers if you wish to drive longer lines are heavier loads.




-----Original Message-----
From: Tim Goldstein [SMTP:timg@...]
Sent: Monday, August 02, 1999 11:32 AM
To: CAD_CAM_EDM_DRO@onelist.com
Subject: [CAD_CAM_EDM_DRO] Opto Isolators & Parallel Ports

From: "Tim Goldstein" <timg@...>

I had a person contact me off the list and asked the following:

>> Do you know where I might find information on how to opto isolate the

interface between the drivers and the parallel port? Am now using 4n28's
but these are not working well. <<

Being electronically challenged, I thought I would post tot he list and see
if anyone can help me to understand how to optically isolate the parallel
port.

Thanks,

Tim
[Denver, CO]



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