A4988 MS1-MS3 Pull downs (or lack of)?

Guys and Gals,

Looking at the schematic and the data sheet for the A4988, why is only one pin (MS1) tied to ground (and not very hard at that)? Using a 100K to ground will not give you a very good “0” even a 10K is marginal at best. Also, MS2 and MS3 are missing that pull down resistor completely.

I would recommend that the 100K (R4, R7) be replaced by 1K and and pull downs on MS2 and MS3 (also 1K) be added to U3 and U4 (Motor 1 and Motor 2 (pins 10, 11 and 12)).


Bring this up in irc chat room also

Hey, take a look to wiki, about Stepper motor driver. Pololu which design the stepper motor driver (A4988) suggests this value for R4, R7.

The point is all THREE MS stepper pins need to be tied to a good low (1K resistor to ground on PIN side of the PCB) or +5VDC (the Jumper). The 100K is not even close to being a good zero - the leakage current alone coming out of that pin is enough to put it in “no man’s land” and you get very unpredictable operation. The other two are completely floating - that is even worse.

Also posted on IRC chat room. How do I keep that thread highlighted? There is a ton of chaff in with the wheat…

Have you read this, Step (and microstep) size.

Also, i noticed that, this product has been discontinued :confused:.

There is nothing in the link that shows any pull-down resistors on the MS1-MS3 pins. A A4988 is still out there to buy (I just bought 6 of them several weeks ago). They don’t have the heatsink, so I had to buy those from another source as well.

I still stand by the belief that MS1-3 must have a pull-down (1K Ohm) to ground on the chip side of the jumpers to guarantee a solid logic “0”.

From the link:

"Step (and microstep) size

Stepper motors typically have a step size specification (e.g. 1.8° or 200 steps per revolution), which applies to full steps. A microstepping driver such as the A4983 allows higher resolutions by allowing intermediate step locations, which are achieved by energizing the coils with intermediate current levels. For instance, driving a motor in quarter-step mode will give the 200-step-per-revolution motor 800 microsteps per revolution by using four different current levels.

The resolution (step size) selector inputs (MS1, MS2, MS3) enable selection from the five step resolutions according to the table below. MS2 and MS3 have internal 100kΩ pull-down resistors, but MS1 does not, so it must be connected externally. For the microstep modes to function correctly, the current limit must be set low enough (see below) so that current limiting gets engaged. Otherwise, the intermediate current levels will not be correctly maintained, and the motor will effectively operate in a full-step mode."

OK, that sounds good. Thank you very much.

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