Re: [CAD_CAM_EDM_DRO] TTL questions

Doug Harrison wrote:

> Hi All;
>
> I am going to drive some H11 optocouplers using 74ACT245 tranceivers for the control lines on CNCpro. Am still somewhat confused with this sinking/sourcing stuff.
>
> Question. Can I drive the optocouplers directly from the data outs of the tranceiver or do I need pull-up resistors?

Yes, sort of. You will still need SERIES resistors to limit current. It works best with LS-type chips
to have the LED part of the optocoupler connected between +5V and the driver, with a series resistor
on either side of the LED. But, with CMOS (like the ACT part) it doesn't matter whether the chip
is sourcing or sinking current, the specs are quite similar. So, which ever way works out for the LED
to be lit, connect it between the chip and either +5 or Gnd, with a resistor in series.

> Second question. Assuming I need pull-ups, the whole thing would go together real nice if I could find resistor networks with .200 pin spacing. Can't find them. Is it considered bad PCB etiquette to clip alternate pins of a 10 pin bussed network? This would sure make it easier to get from the tranceivers to the resistors to the screw terminals.

Why don't you use SIPs with the isolated resistors? There will be an 'input' every .2", and
an 'output' every .2".

> Third question. I only need 4 of the 8 tranceiver lines. Do I tie the unused inputs and outputs to ground or +5V or something else?

Tie unused inputs to Gnd. Don't connect unused outputs.

Jon