how do bad contacts affect (falling) edges

Hello Jenalee,

As Jack hinted typical breadboards such as the white "plug-and-go" boards or whatever they are called aren't good enough anymore for modern logic. Take a look at VCC, maybe you'll see ringing there as well.

Anyway, a photo and a schematic would help. I really don't think it's bad contacts. Then you should be seeing changes when you tug on the wires.

Breadboards -- the conventional variety -- are death over a few hundred kHz.

Joerg a =E9crit :

Thanks Joerg for your reply. I will try to post pictures and schematics tomorrow.

Thanks, Jenalee K.

Have you placed a decoupling capacitor across each IC (100nF)? It should be right on top of the IC's, not at the end of the vertical tracks.

Meindert

Meindert Sprang a =E9crit :

be

C1 4n7 || ___ |\\ pulse with pulse ------||---o---|___|---o--| >O- weird edges 15Vtt || | R1 | |/ | 4k7 | 74AC14 .-. | | |R2 --- C2 | |10k --- 4n7 '-' | | | | | | | =3D=3D=3D =3D=3D=3D GND GND (created by AACircuit v1.28.6 beta 04/19/05

Original breadboard circuit without C2. The output pulses had weird oscillating rising edges. As Joerg proposed, closer inspection of the power lines reveiled some 100MHz or so ringing on the 5V and GND. After the addition of C2 the problem went away, the ringing became one period of "slow" 20MHz-ish bouncing. I had placed decoupling caps (100n & 10u) as close to the 74AC14 as possible but they didn't seem to have any influence.

There were also clamping diodes just after C1 but they didn't influence the behaviour so I left them out for clarity. The purpose of R1 is protection for the schmitt-trigger and its value doesn't seem to have any influence.

BTW, I made an error, it is on the rising edges only, not on the falling edges.

Thanks, Jenalee K.