Basic Understanding of PMC-8

Ben K

Hello everyone,

Much thanks to Jerry and his team for all the hard work developing this, and to Chris Moses, for being so vocal and asking great questions.

I'm likely to order a PMC-8 (Exos 2GT) in a day or so, and I wanted to be sure I understood the software topography beforehand. I've got a few basic questions, some of which I believe I know the answer to, some of which I don't.

To be clear, I'm merely trying to understand what PMC-8 will help solve, and what isn't really within its scope, meaning I'm not trying to nitpick, etc.

1) As I understand it, if I have a mount that is ASCOM compliant, I can use any ASCOM software with it, to guide it, plate check things, control cameras (if ASCOM compliant) and so on. Is that right?

2) I also understand that PMC-8 has an ASCOM driver, and the PMC-8 hardware box has a serial connection to run to my computer. Correct, yeah?

3) Jerry has mentioned that because PMC-8 is open source, people can write their own applications for it, to do any number of things. My main question - how is this different than ASCOM? If something is ASCOM compliant, can't I write an application for that (like AstroTortilla, SGP, Stellarium Scope, etc.)?

4) So, why would I write an application specific to PMC-8, instead of simply writing an ASCOM based application? Maybe I'm misunderstanding the role of the PMC-8 software in the topography.

5) I understand that PMC-8 is simply how the hardware is controlled, its, literally, a "precision motor control" system, and ASCOM is much more than that, in that it links to star maps, GPS, whatever you want to connect to it, etc. So, that said, is there really a lot of tinkering to be done on motor control? Isn't it basically "move this gear at this rate for this long, then move this other gear at this other rate for this other length of time"? Please don't be angry if I just simplified things in a way that seems offensive, I know the motor control stuff is complicated, I just thought it was somewhat more concrete/finite than the ASCOM application side of things?

Thank you in advance, looking forward to order my PMC-8 soon!



Hi Ben,  thanks for your questions. I appreciate the kind words about the development of the PMC-Eight.

1) Yes, the PMC-Eight ASCOM driver allows any compliant ASCOM client use the mount. To be clear, the ASCOM Standards API that the mount is controlled with is the TELESCOPE API. There are only certain things the TELESCOPE API allows you to do. The complete ASCOM Standards API contains other objects such as CAMERA, FOCUSER, and also libraries to do plate solves and ephemeris calculations. These are other API's that provide a complete suite of interfaces for all the different types of clients to use.

2) The standard interface for ASCOM is a hardwired serial connection. I have also provided a switch in the PMC-Eight ASCOM driver to connect to it using the computer's WiFi connection. This is transparent to the ASCOM client software and connects to the PMC-Eight's WiFi module.

3) Yes you can create your own programs to use the PMC-Eight controller with its command language. This is mainly for people who either don't want to use the ASCOM TELESCOPE API or if they want to develop solutions on other platforms such as Apple iOS or Android, or anything else for that matter. ASCOM at the current time is a Microsoft Windows only API. You can create ASCOM Client software that utilizes the driver if you like also. I have written an ASCOM Driver test program client that demonstrates this also with the source code for this client included in the SDK.

4) See my answer in 3

5) You are correct in that the PMC-Eight is designed specifically as a precision motion control system. Practically all mount control systems offered by all the other companies are specifically designed as a telescope mount controller. My philosophy is a little different. We have abstracted out the astronomy-centric commands out of the controller and raised them up to the driver and/or the client level. The command language is much simplified compared to other controllers in that it only has around 20 commands versus 40-60 commands. Another aspect of the controller is that it uses raw data (motor count data) versus engineering units (RA/DEC). This helps to keep the performance high and the response time fast when commanding the controller. All engineering unit calculations are done at the ASCOM driver level, or the application level. The ExploreStars application does not use the ASCOM API as it codes directly using the PMC-Eight command language. There is a bit of history there as that was the first application being built even before the PMC-Eight project started over 3 years ago. The PMC-Eight firmware is only concerned about the motors and provides functions for precision tracking and slewing including smooth ramping to provide accurate slews so that motor counts are not skipped. The motor current is controlled to provide the best level of performance of the motors when slewing.

Overall, the philosophy is the opposite of the trend in controllers to push more and more into the hand controller or motor control system. We made the decision to separate out the astronomy centric processing to take advantage of the inexpensive computer systems that are available in the tablets and laptops. Our two main goals were to maximize performance and reliability through our hardware/firmware design, and our separation of functions. Most industrial control systems are designed this way.

The two biggest assets we have based on our micro-controller choice is 1) no interrupts and 2) no operating system. This simplifies and boosts performance. That, plus the fact that we have 8 processors to separate tasks keeps the system very reliable. The other thing we have in our favor is that there is plenty of margin in the design in terms of performance and memory space available. We are also leveraging the hardware and firmware designs and pushing it down into lower end mounts.

Thanks for considering the PMC-Eight for your mount control system.

Ben K


Much obliged for yours answers and your time.

I'll be working with my local scope store to order the Exos 2GT - PMC8, tomorrow morning when they open.

Looking forward to learning more here and over on Cloudy Nights.


W. Christopher Moses

Hey Ben,

Nice to have you aboard.  The threads here and at CN cover most of the questions and issues I've come across, but I'm sure you'll come across something new.  I have a G-11, so I can't help with the mount itself, but let me know if you have any PMC-8 questions and I'll be  happy to share whatever I can.

Chris Moses

Ben K

Chris, thank you as well!

Yeah, I would go for the G-11 if I wasn't new to this, but I'm certain I'll get there in time, and my local scope store has experience with the exos 2 mount, so I imagine I'll be fine there.

Between myself and my wife (data scientist and software developer, respectively), I think we'll be both fairly resourceful but also very inquisitive.

Jerry, ordered my mount just now, I'll let the forum here know once I've got it, and so on. Only thing I know I'll be doing right off the bat is adding tiny glow-in-the-dark stickers to the index arrows and mounting the actual pmc8 assembly more securely. Once I rig up that mount, I'll take photos of it, if others want.

Thank you both!


W. Christopher Moses

Stickers are a good idea. I never thought of that. But, I've found the built-in levels are very, very sensitive. Probably more sensitive than looking at stickers. For resetting zero position, I will stick with them.

I put some painters tape on the bottom of the little metal wedge to keep it from scratching up the mount. And, I never figured out the velcro that came with it.  I just grabbed a long piece I had laying around. 

I just finished mounting a mini-pc on top of the mount.  Now I have the option of mounting the PMC-8 on top! To many options!

W. Christopher Moses

And, yes, please post picts of your rig. I  would love to see it

Ben K

Will do, re pictures.

What mini PC are you using, and how did you mount it?

Much obliged,


Ben K

Also, what do the levels have to do with the index marks? I thought the index marks were for setting it to proper base position before letting the motors take over? I've not played with a proper gem before, so I'm likely missing something.

I think I'm going to create another wedge like the one included with the pmc8, with a 3d printer, then sugru that to the pmc8, and then I'll be able to attach it with two straps. I'll have to see how heavy it is, and might come up with a better solution.

W. Christopher Moses

I was thinking that you were using the marks to indicate zero position. I.e - the mount being "Staight up and down."  The bubble levels perform this function. It isn't like many mounts where getting a good zero position is a chore.  You just release the clutches, line up the bubble levels and retighten the clutches.

At least that is how it is on the G-11. If you are using the Exos, then I'm not sure.

W. Christopher Moses

I'm using an intel brand NUC (their name for a mini-pc)

Not sure if this is the exact model, but it is very close:

I have a dovetail bar on top of the scope and happened to have some extra guide scope rings around, so I took the dovetail clamp off of them and mount the mini-pc to that.  Right now, while I work on spacing and balancing issues, the mini-pc is held to the dovetail clamp by...velcro, lol.

The mini-pc does have a VESA mount with it. So, eventually I'll permanently attach that to the clamp.

I'll post some picture tonight over on CN.

W. Christopher Moses

What is the wedge for?

Ben K

From a video review, it looks like there is a metal wedge / attachment to the back of the PMC unit, that has a Velcro strap threaded through it. In the video, having only one attachment made things hang pretty loosely.

I was going to print a second wedge, attach that to the bottom of the PMC unit, and thread another strap through that, so I could secure it at two points. Will do three if needed, to keep it from banging around.


W. Christopher Moses

Oh, cool. That would be handy. Please send me the url to file.

One thing I did was to put some painters tape on the bottom of the metal wedge do it wouldn't scrape the tripod's finish

W. Christopher Moses

I was thinking about using some dense packing foam also. Cut one side to fir around the leg.

Ben K

Regarding your NUC, are you running all of your ascom programs from that? If so, what are you using for a screen and inputs?

W. Christopher Moses

I'm running everything on it and using RemoteDesktop to connect to it.
I did find that some programs will display dialog boxes very small - to small to read.  I discovered that if I change a setting for the program, in its properties, the dialog boxes will display normally.  Alas, I forget exactly which setting it is, but I'll find it for you if you want.

This does imply that I am either using a wired connection to the pmc-8 or have the pmc-8 set to connect to my network.  Currently, I'm using a wired connection.  Getting the pmc-8 onto my home network is on my to do list

Ben K

Using remote desktop on your phone or tablet or a laptop?

I was thinking of doing something like that, a NUC wired to the PMC, and I connect to the NUC (either adhoc or create a local network or if I'm home, via that), and control it via my Chromebook or Android tablet.

W. Christopher Moses

I'm using a MIcrosoft Surface Pro 4, which can be used as either a traditional laptop or a tablet.

You could easily use an android, mac, or iOS deceive to control it.  I'm not sure RemoteDesktop supports all those platforms but TIghtVNC and similar programs do.

W. Christopher Moses

Realistically, I think a decent sized tablet is probably required to be able to view all the programs required for AP.  For visual, you could get away with a phone - probably.