Mesa 7i76 vs. M542 vs. Stepper Motor Wiring

Both the Mesa 7i76 and the M542 stepper driver boards use Phoenix-style pluggable screw terminals that simplify the connections: just strip the wires, jam them into the holes, and tighten the screw. That works great in an industrial situation where the equipment gets wired up once and stays that way forever, but I expect that I’ll be doing far too much twiddling… which means the stripped wire ends will fray and shed strands across the boards.

So, while wiring up a stepper motor, I tried soldering the wires to several different terminals I have lying around, just to see how they work.

The M542 stepper driver brick shows the assortment:

M542 Stepper Driver Wiring
M542 Stepper Driver Wiring

On the far left, the four stepper wires end in right-angle PCB pins harvested from surplus connectors. This didn’t work nearly as well as I’d like, simply because the pins are entirely too bulky. I’m not sure quite how the bricks will be arranged, but I think a right-angle connection won’t work well at all.

The field power from the 24 VDC supply arrives on some (cheap) 18 AWG speaker zip cord, terminated in straight-line PCB pins. Those worked better, but they do stick out a goodly amount. Methinks the right thing to do with larger wire is just solder the strands together, clean the end, and not bother with pins. That’s not so good for strain relief (it concentrates at the end of the soldered strands), but, with some tubing added, maybe it’ll be Good Enough.

The 26 AWG input wires from the 7i76 terminate in turret pins originally intended for PCB terminations or test points, back in the day when you (well, I) could actually see such things; I have a bag of 1000 that I’ve been chewing away at for a while. I think these wires are simply too small for the screw terminals, so they really need a pin of some sort and I like the way the turret pins work. The heatstink tubing provides a bit of strain relief, which always comes in handy.

The two stray wires will eventually go to the “Enable” input. It turns out that these bricks defaults to Enabled with no input signal, so you cannot depend on a wiring fault disabling the motor: a broken Enable wire enables the drive output. This seems flat-out dumb, but I suppose there’s some planet on which it makes sense.

I snipped a bunch of 3/8 inch (call it 10 mm) lengths of tubing, but that turns out to be slightly too long for the 7i76 terminal layout:

Mesa 7i76 Wiring
Mesa 7i76 Wiring

So the next iteration must be a bit shorter.

Yes, you can get commercial crimp pin terminations; search eBay for crimp insulated terminal pins, some of which are curving around the planet even as I type. They won’t fit into the tight confines of the 7i76, but they should be better for the M542 bricks. The smallest size fits 22 through 16 AWG wires, so my tiny cable wires may need some steroids to bulk ’em up.

On the stepper motor end of the cable, I picked up a bunch of JST connectors and crimp pins. Unfortunately, the proper crimp tool runs into the hundreds of dollars, even from the usual eBay suppliers, and I really don’t have that much need for those pins. So I just soldered wires from the cable to the pins and mashed them down with needle nose pliers:

Stepper wiring - soldered JST pins
Stepper wiring – soldered JST pins

The alert reader will notice an egregious wire color coding faceplant. I made a corresponding blunder on the other end and nobody will ever know. Next time, maybe I’ll get it right.

That makes for a nice connection at the motor:

Stepper wiring - connector in place
Stepper wiring – connector in place

The thin black cable has nine 26 AWG conductors that I’m doubling up for the motors. In round numbers, 26 AWG stranded has about 120 mΩ/m resistance, so two in parallel work out to 60 mΩ/m. Assuming a meter of cable between the driver and the motor, a 1 A winding current will drop 120 mV along the way and dissipate 1/8 W, which seems defensible. It’s obviously Good Enough for signal wiring.

It is most definitely not good enough for, say, the heaters.

The motivation for using that cable: it’s thin and super-flexy, not the rigid cylinder you get with larger conductors. Plus, I have a huge supply of the stuff… it originally served as RS-232 cable, with molded connectors on each end of a 30 foot length, with four such cables assembled into a super-cable with nylon padding yarn laid inside a protective outer sheath. Must have cost a fortune to the original buyer; decades ago I got three or five of the assemblies and have been harvesting cable ever since.

9 thoughts on “Mesa 7i76 vs. M542 vs. Stepper Motor Wiring

  1. It is supposedly bad form to solder the wires before they go into a Phoenix-style connector. I have seen a few that had big lumps of solder loosen up, and maybe that’s what the warning is about.
    On 26-28ga wire, I have been known to strip it a little long, twist it, fold it double, and then crimp a d-sub pin on the wire.
    That may be overkill.

    1. bad form to solder the wires before they go into a Phoenix-style connector.

      Structural (“the bigger the blob, the better the job”) soldering is definitely its own reward, as I’ve all too often verified to my own dismay.

      The whole point of using stranded wire is to get flexibility, which means converting it into a rigid rod exactly where it needs the most flexibility makes no sense… which is exactly what I’m doing by soldering it onto / into those pins. The best I can hope for is a little flex inside the heatstink tubing. [sigh]

      strip it a little long, twist it, fold it double, and then crimp a d-sub pin

      That’s basically what I do for lug connectors and Anderson Powerpoles, then add some silicone tape and hope for the best.

      I’ve soldered up four cables with motor connectors (with the right color order!) and procrastinated finishing the other ends; if those crimp pins arrive soon and fit well enough, I’ll use your trick.

  2. Nope, you are not suppose to solder stranded wire and put it in a screw terminal. _Especially_ for power connections. The lead in the solder will yield under pressure, causing the connection to loosen a bit, increasing the resistance, increasing the temperature, making the solder softer, etc. I’ve seen this happen more than once.

    1. That certainly matters more for much higher current situations than these piddly steppers, but it’s Yet Another Reason not to do what I’ve both done and seen done. [sigh]

      The min-max stranded wire range for those screw terminals seem to be 24 to 12 AWG and I think they’re happier with larger gauges. It’s certainly true the little bitty wires look lost in there; they certainly don’t get much strain relief.

      Keep nagging me until I get it right…

    1. small enough to fit in the screw terminal?

      A bag of insulated crimp pins specifically intended for screw terminals just arrived; search eBay for “copper crimp pin terminal” and you’ll see an assortment. The insulation sleeve on even the smallest ones won’t fit the Mesa card spacing, but maybe I can trim it down or bend the pin at right angles.

      I should just jam the wires in, crank the screws tight, and be done with it…

  3. Although it takes a little effort, I’ve found the most bullet-proof method is to steal a half inch long piece of wire from some #14 romex and solder this to the stranded wire. With a half inch long piece of heat shrink tubing covering up all but 3/8″ stick out, you get a trouble free connection.

    1. some #14 romex

      Now that I have in abundance!

      The turret pins have a nice hole down the middle that wins on convenience: prime the hole with solder, stick the teeny wire inside up to its insulation, heat pin with iron, and it’s done.

      The Romex definitely wins on raw copper goodness and simplicity; I’ll try some with a right-angle bend on the Mesa card end of the connection; those harvested square pins were entirely too fussy.

      Thanks for the tip!

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