SN7407 help

[Scarr]

Hi Guys,

I am trying to figure something out and have spent ages without getting anywhere, just hope you guru's can point out the obviouse to me.

I have a circuit (not mine) that uses a SCN68681 dual rx/tx (DUART), pin 13 (TXDB) goes to pin 3 of a SN7407 hex buffer/driver. when I inject 5v to pin 3 (2A input) of the SN7407 simulating the DUART taking the line high (5v) nothing happens on pin 4 (2Y output) also I have injected TTL comms to pin 3 and again nothing out on pin 4 of SN7407.

When I check the input (pin 3) of the SN7407 whilst injecting it all look fine, so line is not being dragged down or anything. I have even removed the DUART from it's socket and again still nothing on output (PIN 4) of SN7407, I don't think the SN7404 at shot at either.

I'm probably missing something stupid but as I have been looking for ages I think I can see wood for trees now

Thanks in advance

Steve

 

[audioguru]

Hi Steve,
The output of an SN7407 has an open collector so it needs a pullup resistor from its output to a positive voltage of up to +30V to go high.
Its output can sink low with up to 40mA.

 

[Scarr]

Thanks

Thanks AudioGuru your a star, new it was me being stupid!