Re: [CAD_CAM_EDM_DRO] Re: Compters without printer port

andrew.mathison wrote:

> --- In CAD_CAM_EDM_DRO@yahoogroups.com, Andy Wander <awander@...> wrote:
>
>> Hey, Les:
>>
>> I had that same problem with XP(I also had it when I tried Win2K) that the PC would not boot up if the parallel ports were plugged in to the breakout boards.
>>
>> I was beginning to think I was crazy, as I reported it on the Mach forum, and nobody seemed to have ever heard or seen anything like it before.
>> ______________________________________
>> Andy Wander
>>
> Hi Andy,
> with such a problem, the reason may be that the BOB (or equivalent) is drawing far too much current from the individual pins. The boot up check is seeing this and for safety reasons its stopping the boot.
> I know its a fiddle, but you need to measure the current drawn when a pin is activated, not that easy.
> The printer port guarantees only a max of 2.6 ma as a source or 24 ma as a sink (7400 specs). That means that on a BOB with Opto isolation for example that the input resistors need to be around 2K Ohms minimum. More is OK.... I have seen preofessional BOBs with only 370 Ohms...470 Ohms is typical and completely wrong....
> Join DIY-CNC-Interfacing if you wish and read more about this. Also the printer port specs are there to read too....
> If you draw too much current, you could even damage the Printer Port (which is buried in a BIG chip that has many functions other than Printing), or even the mainboard itself. Be carefull.....
> regards
> Andy M.

Based on basic engineering principles, a pull down setup with 240-390
Ohms and one LED drop would be proper from +5 Volts. Why? At 16 mA,
the TTL output should be below 0.4 Volts, but not below 0.2 Volts, see
the TI SN7400 family datasheet
<http://focus.ti.com/lit/ds/symlink/sn74ls04.pdf>. Now, a Fairchild
6N137 <http://www.fairchildsemi.com/ds/6N%2F6N137.pdf> will output
0.35-0.60 Volts at 13 mA with an input current of 5 mA and with an input
of 10 mA dropping 1.4-1.8 Volts across the light emitter.

Even the lowly 4N33
<http://pdf1.alldatasheet.com/datasheet-pdf/view/174531/FAIRCHILD/4N33.html>
family (slow!) will work at 10 mA, but they are 100 uS turn-off time and
30 KHz typical bandwidth, so figure 15 KHz is the safe limit.

Taking 4.75 Volts minus 1.8 Volts minus 0.4 Volts (worst case) yields
2.55 Volts, across 240 Ohms that is 10.625 mA, well within both the
driver capability to pull down and the emitter ability to properly
handle. With high line, the numbers are 5.25 Volts - 1.4 Volts - 0.2
Volts or 3.65 Volts across 240 Ohms is 15.21 mA, also within the safe
operating areas for the optocoupler and parallel ports.

Lower currents and passive optocouplers may not meet these specs, but
390 Ohms should work well AS LONG AS THE OPTOCOUPLER IS USED AS A PULL
UP to the port output. Remember, the chosen device only needs 5.0 mA
for proper switching, 10 mA is overkill.

WARNING -- ASCII ART

+5 Volts -----/\/\/\------>|----- Port pin
390 Ohms optocoupler
emitter
This will work


Port pin -----/\/\/\----->|-----ground

This will not work, not enough drive current available.


Port Pin -------------+
|
+5 Volts ----/\/\/\---+--->|-----ground

This might work, but will fry your port sometimes!


1N4148
Port Pin ------|<-----+
|
+5 Volts ----/\/\/\---+--->|-----ground
390 Ohms

OR
1N4148
Port Pin ------|<-----+
| 1N4148
+5 Volts ----/\/\/\---+--->|----->|----ground
270 Ohms


This will probably work (the 1N4148 plus the port output voltage should
be well below the opto emitter Vf) and should not damage your port,
adding a 1N4148 in series with the emitter will ensure the Vf is low
enough to guarantee the emitter is not on when it shouldn't be. Notice
the series resistor was decreased to account for the increased Vf.



--
David G. LeVine
Nashua, NH 03060