Here's a bit of a breakdown on the Diva Mod:
Before I detail the mod, let me explain what it does and why, and a little about PWM.
All the PWM D5 pumps, with the exception of the one from AquaComputer, come exactly the way Lang made them with no reseller / distributor mod.
They are designed to run at 60% when there is no PWM signal.
This is a variation from the Intel standard for PC PWM, whereby every PWM fan you'll find will run at max when no PWM signal is provided.
Based on the Intel standard, each PWM device is supposed to have its PWM line pulled up to 5V thru a resistor internally.
When the PWM line is at 5V, the device runs at max speed because its internal control circuitry applies 12V to the motor..
When you add a controller of some type that connects the PWM line to ground for short intervals at a time, then the internal control circuitry doesn't provide any power to the motor during those intervals.
To put in the real numbers, PC PWM is standardized at 25KHz.
The interval for each possible pulse is 40 microseconds.
The "on time" percentage, or the percentage of the time the motor gets 12V, is called Duty Cycle".
If we have a setup and we have the PWM line at 5V for 8 microseconds, followed by 32 microseconds of 0V, then that would be a 20% duty cycle.
If the PWM line's 5V time was 30 microseconds, followed by 10 microseconds of 0V, that would be 75% duty cycle.
The issue with all the PWM D5s except the one from AC, is that since it's designed to run at 60% with no PWM signal, that precludes having the expected internal resistor circuit that pulls the PWM line up to 5V.
The A6 controller adheres to the Intel standard, and expects a 5V level on the PWM line. . . . Without it, there's no control.
The purpose of the Diva Mod, is to provide that 5V pullup on the PWM line so that the PWM D5 can react electrically the same as standard PWM fans, and you can have full range control via the A6.
There are 3 components in the Diva Mod,
2 resistors and a 5.1V Zener diode.
Zener diodes have a unique electrical characteristic that allows them to act as a voltage regulator with only an additional resistor in series.
The 5.1V Zener diode in series with a 560 ohm to 1K ohm resistor between pins 1 and 2 on a 4 pin fan connector creates a 5V source where they connect together, from the 12V present between pins 1 and 2.
The second resistor, a 2.2K ohm for a dual pump to a 4.7K ohm for a single pump, connects from the 5V source to the PWM line, pulling it up to 5V
With the PWM line pulled up to 5V thru the resistor, the A6 will control it from 0% to 100%.
The D5 typically runs at min speed of about 800rpm below about 15% duty cycle, and maxes out at about 4800rpm at about 70% duty cycle.
It's a significantly different range of speed versus pulse width from what you see with PWM fans.
When you look at pics of the diva Mod, you can see the Zener diode with its anode end connected to pin 1, which is the Gnd pin.
You also see the physically larger resistor, (the 560 ohm to 1K ohm 1/2W one) connected to pin 2, which is the +12V pin and the cathode (banded) end of the Zener diode.
Zener diodes install backwards from what you may already know about diode polarity.
At that junction, you'd measure 5.1V to gnd if you put a multi-meter on it.
The physically smaller, but higher value resistor connects the 5.1V junction to the PWM pin, which is pin 4.
Without getting too technical, if you want to run 2 pumps together, you can do it with a single Diva Mod, just be sure they both have their PWM wires connected together so they both share the 5V pullup.
With a dual pump, a 2.2K ohm resistor is the sweet spot, while if it's a single pump, 3.3K or 4.7K ohms is good.
For a dual pump setup, one of the easiest ways to implement the mod is to do it to a Swiftech PWM splitter cable that has had all the power wires removed, since you don't need them.
That gives you a convenient way to run 2 pumps together on a single channel, with one of them reporting it's rpm to the A6, while the other can report its rpm to the mobo CPU fan header so you don't have a CPU fan error issue.
If you have just a single pump, or two pumps for 2 loops, then you can do the mod right on the 4 pin connector from the pump(s).
Here's some pics of the Diva Mod on a Swiftech PWM splitter: . . . Hope the explanation and pics help it all make more sense now.
Complete and ready to install . . . the mod is on the 4 pin female connector at the upper right that connects to the A6.
A close up of the mod with heatshrink to keep it tidy.
With the heatshrink pulled back . . you can see that it's a 2.2K ohm resistor, (red/red/red) since it's for a dual pump
Here's a close-up that you'll see posted in several threads and posts here on OCN . . this was also for a dual pump setup, but on a scratch made splitter cable. This was one of my first mods when the A6 first came out and turned out not to work with PWM D5's as it came out of the box.