Tracy Crook wrote:

Looks like MAX243's are RS232 voltage levels in/out inverters with TTL

in/out.

We need RS232 voltage levels both in & out and buffers.
The data sheet is not clear on the MAX 245. Are those RS232 voltage
levels both in & out? I could use two inverter gates in series to make

buffers.

Paul Lamar

Oh, didn't quite understand where you were going.  You could use the

max243

to go to cmos then go thouugh some and gates, enable one at a time then

or

them together and convert back to rs232 again.  Round about way but

would

work.  Don't know of a true tri-state rs232.

Tracy

Yes that already occurred to me except AND gates won't work.
You need tri state buffers. Please take a look at this and see if
there are any bugs before I wire it up.

Paul Lamar

I  don't think this gets you where you want to go.  To 'mix' the two data
streams (GPS & data module), and enable the source one at a time, you would
convert the two RS232 sources to cmos level, buffer through the tristate
buffers, * tie the two outputs together* , then convert back to RS232 with
the max chip then hook that to the computer input channel.  Enable only the
tristate chip you want (as you show).

Or maybe I don't see what you are trying to do yet.

Tracy

Only the tri state buffer outputs can be tied together.
If you tie two AND gates or RS232 outputs together when one is high
and the other is low you have a direct power supply to ground
short.

That is the recent mistake I made with the RS232 outputs.

BTW I was digging though my supply of ICs and I found about 
two dozen MAX238 in a 24 pin skiny DIP package. I will probably use
one of those and save a few caps. Here is a gif of the MAX238


Paul Lamar
 
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