6514 with OC buffer

[cig438]

I have a 6514 output signal that I can toggle between 0 & 5V via
LabView. I send this signal to a SN7407N open-collector buffer.
The buffer is powered by a seperate 5V supply than the supply used to generate the input signal on the SCB100 board.
With the 6514 signal totally disconnected from the 7407 :
7407 input = 1.7V       7407 output = 0V
With the 6514 signal connected to the 7407 input & LOW (0V) :
7407 input  = 1.7V    7407 output = 0V
with the 6514 signal connected to the 7407 input & HI (5V) :
7407 input = 5V     7407 output = 0V

Using a pull-up resistor at the 7407 output just makes the 7407 output = 5V no matter what the input is.
I'm confused by several things :
* why the 1.7V exists in the first place ?
* why the 6514 signal value is not replicated at the 7407 input (even though I wired it straight to it!)

How should I connect the 6514 signal to make it buffered ?

Any help would be greatly appreciated.

ak

[cig438]

I've gotten a better understanding of the issue. All I have now is the
6514 card driving a relay driver whose current draw is less than 0.1mA.
(no buffer).
On the 6514, I use a 5V power supply for Vcc & connect the signal (in my case port 5 line 0) to the driver.
With LabView not even running :
             as soon as turn on my Vcc supply, I get 4V on the signal line !
I'm trying to understand why ?

When the signal is disconnected from the driver, everything works fine :
0V when Vcc = 5V & LabView not running or LabView sets the signal LOW
5V when Vcc = 5V & LabView sets the signal HIGH

ak

[cig438]

Andrew,

   I tied the grounds together & here's what I observe :
              
with LabView off : the 6514 output line when conected to the driver
outputs 4V (should be 0V)
              
with LabView on & output commanded to LOW : output line reads 4V
(should be 0V)
              
with LabView on & output commanded to HI : output line reads 5V
(correct)

              
with the 24V power supply for the driver turned off :
                          
6514 output line commanded to LOW : 0V (correct)
                          
6514 output line commanded to HI : 4V (should be 5V)

Seems like the driver chip is acting a low impedance load & pulls the output line down from 5 to 4V ?
I'll look into the datasheet for the driver.

[cig438]

With the driver powered at 24V & isolated from the 6514 the input reads ~3.4V - this is OK per the spec sheet.
Then I connect the 6514 line & the input still reads 3.4 (should be 0).
I'm pretty sure that driver is good & has no shorts - when I use a
flip switch, bypassing the 6514, I get all the right voltages at all
the pins.

With 24V supply off :
I measured the driver input currents to be < 100uA in the flip
switch circuit & the same when I connect the 6514 - this is well
below the max source current per IO pin possible with the 6514 (~100mA
if I remember right?). The voltages at the input are :
0V when commanded LO & 4V when commanded HI (should be 5).

The driver I'm using is the TI DRV101; it's input currents are <
80uA, so I'm expecting not to need a buffer when connecting the 6514
line to the DRV101. I know the 6514 can only source current, but I
think that's ok.

[cig438]

Andrew,

    I think I'm getting the big picture. True - the
shift from 5 to 4V is a consequence of driving a low-impedance load.
This is why I have to buffer the 6514 outputs (just out of curiousity,
why wasn't the 6514 built with buffered outputs to begin with ?).

    The 3.4V floating issue is something I also see when
using the 7407 buffer. Attached is a picture of my schematic. When I
don't use the 6514, the 7407 works fine. So the problem must be with
introducing the DAQ card. I attach the 6514 signal & note that the
6514 GND is connected to the power supply GND. See the table of values
I get when commanding the 6514 output HI & LO.

     I think the issue is that the 6514 can not
sink current but can only source. When sending the output HI it's
sourcing, when sending it LO it needs to sink but can't. Even if I
connect the 6514 GND to A1, the 7407 still floats.
Do you agree ? Is there any way I can use the 6514 & have the ability to sink as well as source ?

ak


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