[dorkbotsea-blabber] Electronic Logic Design Help: Why Two Inverters In Series?

[Claude Andrew]

I've built a couple of stepper motor controllers by copying other people's designs and, flush with success, I am designing my own... kinda.

There's a circuit snippet for an H-Bridge driver (http://focus.ti.com/lit/ds/symlink/sn754410.pdf) that shows two control inputs hooked up with two inverter (NOT) gates.  I understand that (typically) one would want one of the inputs to be NOT the other... but why do they use TWO inverters?

They have:
----Control----[NOT GATE]--+---------------->Input 1
                           |
                           +--[NOT GATE]---->Input 2
While I would have expected:
----Control----------------+---------------->Input 2

                           |

                           +--[NOT GATE]---->Input 1


I can probably just copy it, but I'd really like to understand why they did it this way in case i am totally misunderstanding something.

Speaking of misunderstanding, I have spent the last two days trying to pick an inverter.  It sounded so simple.  Of the two H-Bridge chips I'm looking at, one claims to be TTL inputs and the other TTL & CMOS.  I'm using PIC Micro 16- and 18-series devices for the controller, but I don't want to use more IO pins that I have to.  After much wikisearch I think that this might do the job for all my glue logic needs:
ST 74ACT04 "designed to interface directly High Speed CMOS systems with TTL, NMOS and CMOS output voltage levels" http://www.st.com/stonline/books/pdf/docs/5079.pdf
Can anyone tell me if this is OK or what I should use instead?

Thanks in advance,


Claude

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