Opto isolators in level converter

I somehow overlooked adding these part numbers:
NEC "High Isolation Voltage SOP Multi Optocoupler"
parts PS2701-1 and PS2701-2. Spec sheets available
at

All optocouplers:
http://www.cel.com/prod/prod_optdevs.asp

2701 specifically:
http://www.cel.com/pdf/datasheets/ps2701.pdf

We don't have to use these (DIP instead of
SOP packages would be more convenient),
but if we're going to slavishly copy the design,
we should check the specs of the optos used.

J. Grable was conservative: he used separate
optos for each direction. You could use a
single quad (and waste one).

His design uses RTS as a constant voltage
source for transistor side of the receiver.
If you are doing RS-232 and don't have access
to power on the computer side, that's a reasonable
design, but if you *do* have power, there are
other options. Doing it this way puts another
burden on the software- it has to keep RTS
on all the time & some drivers don't allow
software control of those lines unless you go
around the driver completely (I think PalmOS is
like that).

At start of communications, RTS has a 2nd duty:
it does the 5 baud startup sequence.

DTR is used as a pulldown for the RXD input to the
computer. I think my note says -12V nominal.

Incidentally, I have a spec sheet for a Sipex +3 to +5V
RS-232 line driver/receiver in my notes. I believe
that's because it's the part used in the Palm and
I was concerned about current draw on DTR and
RTS from this part. The resistors used are pretty
small ('cause a laptop has no trouble supplying
that much current) & could be a lot bigger as long as the
input impedance of RXD, etc. are high.

Hope this helps. I've asked the student to contribute
his schematic & parts list. Let's hope he helps us
out.

-M

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Received on Mon Mar 20 10:29:11 2000