Electronic – Arduino Motor shield external power

The most common reason that Arduino shields "conflict" is because the Chip Select pin for an SPI on the shield is hardwired on the shield. Sometimes this is manifest in an accompanying software library for the shield that assumes the shield's wiring. A forgiving reason things end up this way is a tradeoff between elegance of the software and ease of use for the end user.

If you are optimizing flexibility, you will design your software to have the (potentially conflicting) pin assignments abstracted into a constructor or setter function, and make the default constructor make default assumptions about your hardware. You would also need to provide a jumper on your shield that you can shunt for "default" behavior or wire wherever to any of the I/O pins with a "soft" wire.

If every shield followed that practice, then there would not be such thing as conflicting shields. The drawback of the flexibility is that the more interfaces are exposed to users the higher the barrier to entry for beginners. I don't think anyone would argue that beginners are the majority of Arduino users, and so ease of use wins the day - fewer options and simplicity are better in most cases for the Arduino community.

A good example of a library and a shield that do it the "elegant" way are the LiquidCrystal library that comes standard with Arduino. There are five pins on the LCD interface that are software definable. I made a Basic LCD shield and had to make these kinds of choices in the design. I decided to favor flexibility because (1) the library already supported it, and (2) LCDs themselves are not consistent in their pinout. So I provided a whole bank of jumpers effectively "abstracting" the LCD from the shield in hardware. Maybe that wasn't the right choice in retrospect, but the spirit of my decision was to support whatever LCD you might want to use and whichever Arduino pins you might want to use.

If I had hard wired for certain Arduino pins and a particular LCD pinout, I could have saved users a few minutes of soldering, at the expense of flexibility and interoperability with other shields. In my case though, I saw the LCD shield as something that might be at "the top" of a shield stack, so maybe the flexibility was meritted in this case. There simply isn't a one-size fits all answer I'm afraid.

There are 2 versions of the motor in the datasheet, which is liked in the O.P.: +6V version and +24V version. But the datasheet doesn't say what the markings on the motor are in either of the versions. However, the datasheet tells what the DC resistance of the windings should be in each version (table on p.2). 8Ω for +6V version. 50Ω for +24V version. If you measure the resistances of the windings, you'll be able to tell which version you've got. This stepper is unipolar, so it should have a common connection. Measure resistance between each winding and the common.