Pin 17 must not have been properly connected. It is reading 0V now connected to ground. Measuring the motor wires with everything powered up yields the following.

PIN | Voltage | Function
Motor 1 | 20 | A+
Motor 2 | 22 | A-
Motor 3 | 22 | B+
Motor 4 | 20 | B-

Are you certain that you have a good connection from R5 to SENSE2 and R6 to SENSE1, and that the other sides of the resistors are both connected to ground?
What are you measuring on L298 pins 1 and 15? It should be the same as the top side of R6/R5 and L298 14/13 respectively.

I also measure all of the voltages on the L297 with just the 5V logic on and then again with the 5V and the 24V on. Notice that the low outputs for A and C change from 0V to 3.3V when the 24V supply is turned on. Any idea what might be causing this?

If the chopper drive is working properly, you will see the lines toggling. Measuring them with a meter will give you readings anywhere between 0v and 5v. You need to use an O-scope or a logic probe to see if they are turning on and off. You can make your own logic probe from a few parts.

Could it maybe be leaking through the capacitors? Would it hurt to try running the circuit without the capacitors?

No, don't run it without caps. I notice you don't have the required 0.33uF and 0.1uF caps on the input and output of the 7805 regulator. Didn't you read the datasheet? They aren't optional.

Also I'm using 1N5404, 3A diodes for the output of the H-Bridge. Any idea if those would cause a problem?

Terrible choice. You will get lousy performance from them. You need fast recovery diodes. FR303's would be OK.

Could I possible put voltage dividers on the outputs from the L297 to the L298 to cut that 3.3V to under 1.5V? If I use a 10K and 7.5K I would cut the high voltage from 4.5V to 1.9V. Probably not a good idea, right?

Bad idea.

Build a logic probe. There are more than one ideas for logic probes that have been posted on here.

 

Thank you so much for all your help with this. Turns out that the circuit was working! I didn't hook the chip up to a scope to check if that was indeed the chopping action causing the 3.3V, but I assume it was. I just didn't realize it was working because I was trying to send step pulses with a big old button. I hooked it up to a signal generator and the motor spun right up.

Now that I know this is working I need to build 3 more boards up. In case your wondering this circuit is part of my groups senior design project at the University of Cincinnati. We've been designing and building a CNC router. We are documenting the process and making all of our design and software open to the public at http://www.caffeinesystems.com/

Thanks again for the help.

 

I see you got a 'C' on your CNC.

Good to see you are documenting everything. The ups and downs. Thats the way it should be. That way when other students think they CAN'T do something because of a minor setback, they can still thrive in the end.

http://svn.caffeinesystems.com/SeniorDesign/trunk/Docs/Reviewed Work/Task List - Graded.pdf

 

Any idea where to get heat sinks for the 298s?

 

Glad you got it working.

Try using a copper heat sink with a fan for the old Pentium-type CPU's. Drill and tap a hole in it.

Failing that, go on down to a Big Orange, Big Blue, or Ace Hardware store, and get some aluminum angle stock. You can saw it up and stack it to make a pretty decent and thick heat sink with lots of surface area. If you are not going to use a fan, then have the "fins" oriented vertically to get maximum convection transfer.

Fun facts:
Aluminum is about 59% as efficient as conducting heat as copper.
Water is 1.4 times as efficient conducting heat as copper.
Diamond is more than 5x more efficient than copper as a heat conductor - if you're a high roller.

 

Regarding the fun fact,

carbon nanotubes can be used as heatsinks to ferry heat away from junctions on the nano level. neat.

So, if you got a few hundred miles of nanotubes, you could use them.

But back to the topic at hand. Look at old PCs and video cards, they usually have good sized sinks and fans.