If only component manufacturers used straightforward values printed on, rather than
daft codes!   "100n 100V" isn't that hard to print

Yup, the failsafe bias resistors need to be higher than 120 ohms!  Remember they act in parallel with the
termination load resistor, which may have to be increased in value to compensate.  The supply at the failsafe
bias resistors needs to be decoupled at the resistors to provide a low-inductance return path.

The bus cable should match the termination impedance of course, so choose the right cable...

And yes, termination at each end of the cable, not anywhere else.  If intermediate nodes might be unplugged
at some point you can add high-value (10k or so) bias resistors at those nodes so they failsafe when not connected
to the bus.

The specs look a bit odd, claims a 12.5k ohm winding resistance and a current-per-phase of 0.25A,
suspect the winding resistance is 12.5 ohm, not 12.5k (otherwise it would need 3000V supply!).

Any standard bipolar chopper driver that can be set to 0.25A should work fine.

I can't quite read the chip part number from the photo but I'm 90% sure it will be a buck converter,
only able to reduce voltage.   For increasing voltage a boost converter would be needed, or the
fully flexible option is a boost-buck converter (rare).

Don't confuse power and voltage, converter circuits cannot increase power!

Which panels?  A panel good to charge a 24V battery will develop perhaps 40V when open circuit in good sunlight...

Either use a logic level MOSFET or get a MOSFET driver chip such as a MIC4422 to power the gate of the IRFZ44E - you'll need 12V
supply for the MIC4422 and lots of decoupling (0.1uF + 10uF ceramic minimum) on the MIC's supply rails _right next_ to the chip.

logic level is a simpler way to go, but for 10A you'll need a low Rds(on) value (0.01 ohm or less would be great - don't go for a
device with more than twice the voltage you need, Rds(on) increases with device voltage rating.

Can you tell us which exact motor and what supply voltage you are using.  Also tell us the power supply details (link to
info, datasheet, or its max current, something more than "adjustable"....)  With the DRV8825 or other chopper drives
you will get better performance for higher supply voltages.  I think you also need to set it up for the relevant
motor current.  2A is pushing it for the DRV8825 unless there's good heatsinking, IIRC.

What driver are you using?  What voltage power supply?  What mechanical load do you have on the stepper?  Does it
work fine at lower speeds?

[edit:  Also which exact motor - that datasheet covers a range of motors]

If you connect a powered-down CMOS device to your circuit it may clamp the voltage to +/- 0.6V or so
because of the protection resistors (depends on whether the outputs have protection diodes).

But the basic problem seems to be that you are connecting two different outputs together - this will not
work and can even cause damage.  Perhaps you need more pins to talk to these encoders or you need
an input multiplexer chip to handle them?  Maybe if you describe more about what you are trying
to achieve we can work out a better way?

The fan-out of an Arduino pin is large, since it can handle upto 40mA it should be able to drive dozens of CMOS
gates happily if the wiring isn't too long.

Every family of logic chips has a specified "fanout" - the number of inputs a single output can drive without
needing boosting/buffering.  Usually it is at least 10, one seldom has to worry about it.

You could also put a 470 uF cap from Vcc to ground on the SD card... Do you have a separate 3V3 supply for the SD card, Most I've seen work @ 3V3 not 5V and If this is the case the 3V3 supply is limited in current capacity. The filter cap should help keep the supply "stiff".

Doc

SDcards take 3.3V, definitely not 5V.  Many will allow switching to lower than 3.3V after initialization, but this
is only needed for native mode (not SPI).

More decoupling?

Make sure the signal line to the radio module runs with a ground wire parallel to it (ideally a twisted pair), and is
kept away from other wiring.  The SDcard will pull large currents, 100mA+ is likely, it depends entirely on the card itself,
you should have generous decoupling on the 3.3V power to the SDcard (10uF + 0.1uF ceramic for instance).

Whenever you run a digital signal along a wire it should have parallel to it either ground or supply wires - this
reduces the stray inductance and reduces the ability to emit or receive interference from nearby high-speed
switching transients.  When routing several digital signals they can share a single ground return wire.

In particular this is difficult to manage on a breadboarded setup, so occasionally misbehaviour like this will seen,
adding more ground wires can help.

You would never normally take a MOSFET to anywhere like its nominal current rating at continuous
duty cycle - that figure usually represents the maximum power dissipation with an infinite heatsink.

Take the Rds(on) value and work out the dissipation for your load, then decide if thats OK, if not
select a device with lower Rds(on), or parallel them.

The other thing it might be worth checking is the voltage drop from drain to source at your load
current, V = IR.   Again if the voltage drop is more than you'd like, select a better device or parallel them.

220 ohm 0.5W is OK, less than 0.3W will be dissipated, P = IxIxR

It may have a heatsink on the bottom, but the devices are on the top and FR4 PCB material is not a good conductor of heat.
I presume there are lots of vias under them but not 30W's worth!

If it were an aluminium PCB that might be difference, but its clearly FR4 from the edge in the photo, and has through-hole
components to prove it.