Cortex ports 4 and 5 pulsing

Today I noticed that two motors on my robot pulsed, after an hour of troubleshooting I found that two motor ports, 4 and 5 caused this. They would give power to a motor for a second at most, and then shut off for half a second at most, then turn back on, and so fourth. This would happen to any motor or motor controller plugged into these ports. These motors and controllers would work fine on any other port. I also tried swapping out the battery, rebooting the cortex, re-downloading the software, using other software, and swapping out the back up battery just for the fun of it. I am almost certain it is not a programming problem, I checked it with a program that was previously working. Does anyone know how to solve this problem, or is my cortex shot :frowning: ?


What language/IDE are you programming with?

It could be a problem with the ESD diodes on those ports (they share a package). Usually when a the diode gets damaged the port doesn’t work at all. Plugging in a 2-wire motor into these ports can do this sometimes but it’s usually when they miss the port 1 (or 10) connection and use port 2 (or 9).

What happens when the motor is supposed to be stopped? Does it pulse then? Does it do the same thing going forwards and backwards?

I am programming with easyC v4

No, the motor stops moving like it is suppose to when i give it the command to stop moving. Yes, it pulses in both directions. I also noticed that if i plug a 3 wire extension into port 4 or 5 and the other end straight to the motors 2 wire cable (w/o the motor controller), it spins without pulsing, however it also spins without me giving it the command to. Basically i won’t stop it spinning. Nevertheless, this might not be a problem because it happens with all of the cortex ports (except 1 and 10 because they are 2-wire). Thanks!

By the way, I have had this cortex for about 10 months

Yes, don’t do that, it can damage the cortex.

The connections on a three wire port are as follows.

Black wire - 0V (GND)
Red wire - 7.2V from the battery
White wire - What is called RC pulse width modulation.

The special pwm signal on the white wire consists of a pulse varying from 1mS in duration for full reverse speed to 2mS for full forward speed. The motor is stopped when the pulse is 1.5mS or is missing completely. There’s a scope shot in this post showing the different pulse widths. The pulse is repeated about every 18mS.

When you plugged in the 2 wire motor you either plugged it so that it bridged the black and red wires or the red and white wires. The red/black combination just put full battery voltage across the motor and caused it to run at full speed. The red/white combination is where bad things can happen as you have now effectively shorted the battery to the logic level pwm output. The combination of battery voltage and possible back emf coming from the the motor if it is turned can sometimes fry the ESD diodes that are designed to protect the even more expensive (ie. the cpu) parts inside the cortex.

On the original question, you probably need to have the cortex repaired by VEX unless you can find some other problem, small bits of wire or metal inside the cortex, things like that.

Thank you for the help! When I plugged the 2 wire motor into the extension I made sure I plugged it into the black and red, not the white and red. I know this can still cause problems but i did it just for a second without any load on the motor. I just mentioned that because it was the only way I could get it to run without pulsing. Once again, thanks for all the help!!

We had a similar problem with port 9 on our oldest cortex. I am not sure what originally caused the problem, but for now we are just avoiding it by using different ports and utilizing y-cables to still use ten motors. It does not fix the issue as it seems to be some electrical damage, but it at least works as a work around.

I have already contacted VEX support and they are going to repair it for me, im shipping it out tomorrow :D. VEX customer support is amazing, they contacted me back within 5 minutes after i emailed them :eek: