# Multiplexer problem



## geosman (Sep 15, 2007)

Am testing 74HC4051 multiplexer. Sending signals to 378 and software displays byte correctly. Parallel port output is F0-F7 on pins 2-9 (female) which wires to 2-4 to channel selector pins. Put 3 LEDs onto channel selector pins 9-11 and thru single 1k resistor to ground to watch. All are lit regardless. Measurements made without chip in socket. Checked for short found none. 
Why are LEDs not changing as I send to DIGREG0-7 to select channel on the 3 chan select pins? Software uses 5 second delay between changes.


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## octaneman (May 13, 2009)

Are you using assembler code through the multiplexer or switches ?


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## geosman (Sep 15, 2007)

The testing program is written in classic C to drive the LEDs. Linux is the OS. Code sending data thru the MUX ports does not depend on the language used as they represent signal line and ground. 
As I said, the code prints out the values being sent to the DATA Register and those values represent F0-F7, i.e. 240-255.


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## octaneman (May 13, 2009)

From my understanding of the processor, Pin 2 (Y6) is an independent I/O and pin 4 (Y7) is also the same thing, , the input on S0 has to be low ( 1.5 v) and S2 has to be high (3.5v) for the channel to be open (LED on) if using 5 volt power. Any other readings the LED's will be off. 
What is the voltage you are getting on pins ?


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## geosman (Sep 15, 2007)

I need to be specific as to which part the pins are related, i.e. parallel port or the 4051.
Am presuming your Y4 and Y6 are pin positions on 4051.

Since the test arrangement does not employ the 4051 chip, I only use the pin nos. on it to indicate where the LEDs are connected relative to the pins from the MUX's female connector connecting to the parallel port. So, pin 2 of the parallel port goes to pin 11 of the chip, pin 4 on the port connector goes to pin 9 of the chip and port pin 3 to chip pin 10.

The test configuration has an LED anode connected to the chip's pins 9-11 and the cathodes are all connected to a 1k resister to ground.

My 74HC4051 datasheet shows pin 3 (Z) is the universal IO pin on the chip and S0 is on pin 11, S1 on pin 10, and S2 is on pin 9 of the chip, perhaps you dialed in the wrong datasheet inadvertently. I presume that the voltage level from the parallel port would drive the channel selector pins which are for now LEDs, which are just glowing regardless.

When I prove the channel selection via LEDs I will be using 5v @ Vcc with Vee to ground.


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## octaneman (May 13, 2009)

This is the datasheet I'm using as reference the pins are exacally the same:

Can you post the schematic of your layout for analysis ?


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## geosman (Sep 15, 2007)

Attached (if I figure out how) is a text file of the test configuration.


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## octaneman (May 13, 2009)

I can't make heads or tails of the file you sent, can you save your schematic in png extension and compress it to a zip file and re-post it ?


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## geosman (Sep 15, 2007)

Sorry to take so long to respond. As I mentioned the leds were tied to ground via a single 1k resistor. This seems to have been the problem as I took time to build a set of 8 leds and 8 resistors. Tied the anode to pins 2-9, cathodes to a separate resistor and the resistor to ground. It all works and counted 0-255. Guess it was the use of a single resistor, so now I'll go ahead and connect them up to count 0-7 as the 3 selector pins are 2-4, then test the mux ports while watching the selector pin/LEDs. Thanks


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## octaneman (May 13, 2009)

Good job with troubleshooting the problem. Can you post your array set-up with the 74HC4051 so it can benefit others.


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## geosman (Sep 15, 2007)

If you by "array" you mean the test chip with LEDs and resistors I hung off of port that was what I described in last post. If you mean the schematic of what I did w/ 4051, that was to wire its 8 ports to RCA jacks and pins 2-4 to the selector pins of the 4051, just that simple. My software selects port to be activated then I take a reading from the device. There is nothing special or magical.


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