well so today me and my team started using a power expander


This is great (except the flashing light keeps me awake at night )))

However, the vex Y-Cables are Keyed the wrong way to allow them to be placed between the cortex and power expander


is there a solution to this withous sawing your Y-cables???

thanks you, and may your children live for vex

I’m confused. Why would you want/need to connect a Y-Cable between the CORTEX and the Power Expander?


To drive two motors off seperate ports on the power expander using only one port on the cortex.

k.gen04, you must be congratulated on your knowledge, that is indeed the reason

you need to y cable the motors before they are placed into the power expander (2 motors Y cabled together -> the single end of the y cable in the power expander -> one wire from the opposite matching side of the power expander to the cortex. also make sure that the white/yellow sides of the cables are facing inward on both sides of the expander.)

Alright yes, if that’s what you want then mbrunn’s post should help you out:

“Basic diagram”:

**Motor       Motor**
     **\  /**
 Power Expander


HighSchool Competition has 10 motor limit.
A Cortex has 8 motor ports with three wires, and two ports with two wires: total of 10 motor ports;
Since the Cortex has as many motor ports as allowed motors,
the only reasons to use Y cables on Cortex are:

  • ease of programming paralleled motors, only one SetMotor command needed for multiple motors;
    – There are other equally good ways to do this programmatically.
  • Lack of any 2-wire motors.
    – seems unlikely based on unavailability of new 3-wire motors, and forum enthusiasm over 393 HS motors.

A PIC has 8 motor ports with three wires, so 2 Y cables would be needed just to get from 8 ports to the 10 competition max number of motors. PIC (4A) +PowerExpander (4A) has the same 8A current as Cortex alone.

I had thought (as the Original Poster) that Y cable from CPU (particularly from PIC) to a power expander would be the very useful thing to do.

However, If the power expander has only an overall current limit,
rather than a per-port current limit, (is this true?)
then it wouldn’t matter if the Y cable is before the power expander (as OP)
or after (as Mbrunn and Jordan suggested).

The PWM cables themselves are not a significant current bottleneck.

This thread was started with relation to a college competition team (AURA). Therefore, we have a 12 motor limit

That may be true, but jgraber’s reasoning still holds. There is no electrical advantage to using a Y between the microcontroller and a Power Expander; it should always be just as good to put the Y on the output of the Power Expander.

I suppose you might want to run a pair of Power Expanders off of a Y, so that pairs of drive train motors each had their own battery, separate from the microcontroller battery. I don’t see why that wouldn’t work, though. My Vex Y cable fits fine into a pair of inputs on my Power Expander; the 2 female connectors of the Y are not keyed, so I can plug them in either way.

Perhaps C@meron can provide more details on what he is trying to do, as well as what (specifically) isn’t working.


  • Dean

I’m on the same college team as C@meron, we have had to buy all new parts this year as we are a completely new team.
The new extension cables and y-cables that we were shipped are slightly different to the ones on the products page, they are a different color and the Y-cables are keyed, but keyed the wrong way to the rest of the cables!!!
This is fine when you plug two motors into one but when you want to plug it through the power expander first it makes sense to power expander after the Y-cable so both motors get max power, but then that wont work with the new wrongly keyed Y-cables!

Adding to the annoyance they are also the same color as the rest of the extension cables so can’t easily be identified in a large pile!


That is unfortunate. IFI should take care to be consistent with their keying. In their other products they have put the key near the signal wire for male connectors, and near the ground wire for female connectors. Inconsistent keying is worse than no keying, since it is just as confusing and prevents otherwise valid connections like this.

Rather than cut the key off the connector, I recommend you carefully remove the plastic shell and just re-insert the socket contacts in the correct orientation.

You can use an X-ACTO or similar craft knife to gently lift the little plastic tabs that keep each pin in place, and they should slide right out. Don’t bend the tabs too much, though, or they will distort and won’t be able to hold the contacts when you re-insert them.

If you look at how power flows through the 3-wire system, you’ll see that isn’t really true. There is no advantage to putting a Y-cable between the microcontroller and the power expander.

The 4 ports in the power expander are powered together bus-style, so they will provide a total of 4A. That can be 4A to one port, or 1A to each of the 4 ports. It doesn’t really matter how the Y’s are arranged as long as the total load on the expander is ≤4A.


  • Dean

Actually, after re-reading your post, are you certain the Y-cables are keyed the wrong way around? The Vex keying standard makes it look backwards at first glance. Output ports have the key slot by the white signal wire, but input ports (like on the power expander and LCD) have the key slot by the black ground wire.

That means that extension cables and Y-adapters have the key by the white wire on the male (pins) end and the key by the black wire on the female (socket) end. If that is what you have, then your Y-adapters are not mis-keyed.


  • Dean

Just to clarify here is a picture of an unmodified Y-cable and normal extension cable, both keys are at the bottom side of the plug, also the male ends of both cables are identical.
The Y-cable is at the top and has its white wire running next to the key, wheres the regular extension cable has its black wire running next to the key.
The power expander assumes that the black wire is next to the key and so without any modifications Y-cables fail in power expanders.

i would say that this cable definitely looks defective and probably should not be used. if you have other ones, use them instead

All of my Y and extension cables are unkeyed. The one weird keyed one I do have is keyed on the correct side.

Sounds like a manufacturing defect.

Yep, it looks like the female end of your Y-cables is keyed incorrectly to me, though I don’t know that I would go so far as to call it defective.

The good news is that you really don’t need to use the Y-cables between the microcontroller and the power expander. It will be just as effective to put them between the power expander and the motors.

If you really want to, though, you can fix the keying on your Y-cables by carefully pulling out the contacts and re-inserting them in reverse order.


  • Dean

a problem that my team has been having with y-cables is that when we use them, the joystick wont controll those motors, but without them, we can only have 1 motor in a port. this makes it hard to run two motors in parallel. any thoughts why the y-cables dont always work? becuase sometimes they do…

Are the wires plugged in correctly? (red/red, white/white, black/black)

Since you’re only allowed to have 10 motors anyway, you could plug the motors into individual ports, and then program the motors to move at the same time.


You might be able to Y into 2 non-keyed extensions

yes they are in the correct way, they only go in one way due to the plastic slits on the cortex controller. the thing is, they work sometimes, and not others, and nothing changes in between…

there is nothing wrong with the ports because it works without the y cable, but then theres still only 1 motor. programming is an option, but we havent exactly gotten that far :stuck_out_tongue: