4 channel multiplexer

Started by Vic Fraenckel April 23, 2005
I have a need to distribute a single 3 wire RS232 to one of four seperate
devices depending upon certain conditions in application processor. I may
want to talk to a serial LCD, or a PC or a GPS. I would only need to talk to
one at a time however. I have been looking at a 4052 Dual 4 channel
multiplexer to get the job done. The RS232 RXD line would go to one input
(1Z) and the TXD line to the other input (2Z) some steering logic on the
S0-S1 lindes determine which of the Y outputs would get the output from each
section, i.e. RXD would go to 1Y1, 1Y2, 1Y3 or 1Y4 and TXD to 2Y1, 2Y2,
2Y3, 2Y4 depending on the values on S0-S1.

Is this a viable solution? If not why and what might be doable?

Victor Fraenckel - The Windman
victorf ATSIGN windreader DOTcom
KC2GUI

Home of the WindReader Electronic Theodolite
Read the WIND

"Dost thou not know, my son, with how little wisdom the world is governed?"
-Count Oxenstierna (ca 1620) to the young King Gustavus Adolphus

"People sleep peacefully in their beds at night only because rough
men stand ready to do violence on their behalf."
-George Orwell

"When a true genius appears in the world you may know him by this sign: that
all the dunces are in confederacy against him." -Jonathan Swift



> ... I have a need to distribute a single 3 wire RS232...

Can you redefine COM3 pin assignments to two of eight BX-24 pins? Tom
Tom Becker
--... ...--
GTBecker@GTBe... www.RighTime.com
The RighTime Clock Company, Inc., Cape Coral, Florida USA
+1239 540 5700


Thta's a clever idea. That would allow you to use the single hardware
UART for multiple devices.

Casually looking at the spec sheet... It looks like you will have to
have a VDD and VEE that are beyond the RS232 levels you are switching.

Do you have "REAL" RS232 signals? COM1 of the BasicX is 0-5V only, isn't
it? The serial LCD is probably out output only. But the GPS and PC are
probably +/- volatges. You can measure the RS232 voltages to see what
voltages these devices are using.

If they are lower than a +/- 9, then you may be able to use a MAX232
type chip as your level translator (assuming you are using the software
UART) and as a +/- power supply. (I think the MAX232 chips generate +/-
9V) Otherwise you may need to supply an external +/- 12V or +/- 15V
supply. Then just drive the two address lines lines from BX-24 outputs.

I say this because the spec sheet says:
> Control of analog signals up to 20VP-P can be achieved by digital
> signal amplitudes of 4.5V to 20V (if VDD-VSS = 3V, a
> VDD-VEE of up to 13V can be controlled; for VDD-VEE level
> differences above 13V, a VDD-VSS of at least 4.5V is
> required). For example, if VDD = +4.5V, VSS = 0V, and
> VEE = -13.5V, analog signals from -13.5V to +4.5V can be
> controlled by digital inputs of 0V to 5V.

This is from the Texas Instruments CD4052 data sheet. The 74hc4052 looks
like it only goes to +/- 5V instead of +/- 20V.

So I think if you use a CD4052 with a VDD of +12V and a VEE of -12V and
VSS is ground, then I think it will work with most "Modern" RS232
devices you hook up.

But I recommend you breadboard this and try it before connecting your
BX-24. So you don't blow it up. :-)

I built a 4 channel serial multiplexor using RS232 convertors and TTL
multiplexors. So every channel had RS232 convertors and the multiplexing
was at the TTL levels. I used the DTR and RTS lines (or whichever lines
were outputs) on the mater to select the channel. It was way too many
parts and way too much wiring.

PS: You may need pullup/pulldown resistors for a deselected channel. So
it doesn't receive constant input data. On the RS232 side I think you
need to pull down to your negative supply. But your devices may already
have these built it.

--
Jeff Sampson
http://tcrobots.org/members/jsamp.htm

> Message: 1
> Date: Sat, 23 Apr 2005 13:43:25 -0400
> From: "Vic Fraenckel" <victorf@vict...>
> Subject: 4 channel multiplexer
>
> I have a need to distribute a single 3 wire RS232 to one of four seperate
> devices depending upon certain conditions in application processor. I may
> want to talk to a serial LCD, or a PC or a GPS. I would only need to talk to
> one at a time however. I have been looking at a 4052 Dual 4 channel
> multiplexer to get the job done. The RS232 RXD line would go to one input
> (1Z) and the TXD line to the other input (2Z) some steering logic on the
> S0-S1 lindes determine which of the Y outputs would get the output from each
> section, i.e. RXD would go to 1Y1, 1Y2, 1Y3 or 1Y4 and TXD to 2Y1, 2Y2,
> 2Y3, 2Y4 depending on the values on S0-S1.
>
> Is this a viable solution? If not why and what might be doable?
>
> Victor Fraenckel - The Windman
> victorf ATSIGN windreader DOTcom
> KC2GUI
>
> Home of the WindReader Electronic Theodolite
> Read the WIND