🚀 go-pugleaf

fredm@media.mit.edu (Fred G Martin) writes:

>In article <1992Aug26.192907.10931@aio.jsc.nasa.gov>
>poirot@widget.jsc.nasa.gov (Daniel Poirot) writes: 

[stuff deleted]

>>Are there configuration mods to your programs to disable the
>>requirement for echo?  Do I need to re-compile the programs?  

   Looking at the miniboard schematic, I would say that disabling the 
hardware echo is going to be a semi tough, but not impossible, task.
The reason for the echo is the fact that the miniboard needs a source
of negative voltage to pull the RS-232 line low when it is transmitting
a space.  Since it only has +5V power, the only other source of negative
voltage is the incoming RxD serial line.  So, a resistor between the 
RxD and TxD line holds the TxD line low when the miniboard is
transmitting a space condition.  (A 2N3904 transistor pulls the TxD line
high in a mark condition.)

   So, the answer is to provide a -5V source to drive the line low.
Maybe a small battery pack?  A nine-volt battery, maybe?  Here would be
the mod:  cut the connection between the (10K? don't have the schematic
in front of me right now) resistor on the RxD line (leaving it attached
to TxD) and connect a source of -5V to it.  That should work.

   As an aside, the voltage levels we're using here aren't really up to
the RS-232 spec, but just about every RS-232 receiver (Motorola 1488? 
1489?) can handle this range of input voltages.  As a matter of fact,
we might be able to get away with as little as -3 volts for the
transmit.  Someone with a databook want to verify this?

[more stuff deleted]


      Dave Williams
        dwilliam@jabba.ess.harris.com
           "Huh?  What?  Could you repeat the question?"


-- 
      Dave Williams
        dawill@hubcap.clemson.edu
           "Huh?  What?  Could you repeat the question?"