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ASCOM drivers for Digital DomeWorks - 2018 Reboot

ASCOM Server Status Display

TL;DR

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Overview

In 2006, Tigra Astronomy was commissioned buy Technical Innovations to produce an ASCOM driver for their observatory automation product known as Digital DomeWorks. The project served well for 12 years but here we are in 2018 and it was looking a bit tired. In the intervening years, the ASCOM LocalServer (hub) pattern was created; we created our Reactive Communications for ASCOM library; and we learned a lot about the art of software development based on experience of producing more than 10 commercial ASCOM drivers for telescopes, domes, focusers, rotators, switches and weather safety systems.

But we started to experience problems with the old driver and we were not happy with the robustness in operation nor the code quality. We decided it was time for a new start. So we have produced the 2018 Reboot version, sporting the following features:

  • ASCOM LocalServer/Hub that accepts multiple client connections.
  • ASCOM Dome driver, implementing the IDomeV2 interface.
  • ASCOM Switch driver, implementing the ISwitchV2 interface. Allows applications to control the user output pins and therefore the T.I. Remote Power Module.
  • Low level code is based on a state machine that tracks the state of the hardware. Invalid operations are simply ignored. This improves the robustness of the driver.
  • Reliable shutter state upon connection. If the shutter position is unknown when the driver connects, then it can optionally be forced to close to establish a known starting state.
  • Shutter Position Inference - more reliable shutter state reporting.
  • Hardware simulator built in. To test the driver without hardware, simply check the "use simulator" option in the setup dialog.
  • New graphical user interface shows the current state of the hardware.
  • Open and Close buttons in the status display allow convenient opening and closing.
  • Easy access to the Setup Dialog from the status display.
  • Display of the number of active client connections.
  • Fully asynchronous operation except where the ASCOM Standards indicate otherwise.
  • Event driven code means that the client application never has to wait for an answer, the latest data is alaways available immediately without having to send a command and wait for the response. This means we are never blocking the user interface and we don't slow down applications.
  • Use of the Reactive Communications for ASCOM library means that the driver is thread-safe for use in multi-threaded applications such as SGP.

Hardware Simulator

The driver now incorporates the hardware simulator that we developed to assist with our unit testing. The simulator can work in real time or quick time. In real time mode the simulator tries to be as realistic as possible, simulating the timing characteristics of the Digital DomeWorks hardware. In quick time mode, everything is done as fast as possible.

The simulator can come in very handy if you need to test software without being close to the actual hardware. Previous versions required a serial port but the new version uses an in-memory communications channel built on the technology in our Reactive Communications library. To use the simulator, all that is needed is to check the checkbox in the setup dialog.

The setup dialog, showing the simulator checkbox

Power Automation

Previous versions had some clunky command line utilities and Custom Actions for controlling the user output pins. These pins are typically used with the Remote Power Module. Now, we provide an ASCOM Switch driver making this much easier and more discoverable and usable by applications. And, because the new driver is a LocalServer Hub, both drivers can be in use by multiple clients simultaneously and everything will work.

Shutter Position Inference

We experienced issues with Digital Domeworks reporting the shutter position as 'indeterminate' after closing, even though the shutter was fully closed and everything worked perfectly in terms of the mechanics. We have not been able to determine the source of this issue, which was causing ACP Expert Scheduler to abandon the imaging session and go into "Operator Intervention Required" state. Game over; wasted clear skies.

Digital Domeworks actually has no shutter position sensor. The limit switches activated by the shutter are wired into the Shutter Relay Box, which controls power to the shutter motor, but these signals are not available to the main microprocessor in the control box. This is why the shutter position is always 'Indeterminate' when the unit is powered up even if the shutter is fully open or closed. This lack of direct positional feedback means that Digital Domeworks must infer the shutter position based on the last shutter command and the length of time the shutter moved for. If you tell the shutter to open and it moves for 5 seconds, then Digital Domeworks assumes it is open (and vice versa for closed). Movement is detected by measuring the current drawn by the shutter motor. When the current draw exceeds a threshold, then the motor is assumed to be energised and the shutter is assumed to be moving. When the current draw drops below the threshold for several seconds, it is assumed that the shutter has reached the limit of travel and stopped moving.

It occurred to us that we could use a very similar heuristic within the ASCOM driver and ignore the problematic detection built into the firmware. We have all of the information available to us to do this because Digital Domeworks tells us when a shutter movement has started, which direction it is in and then reports current measurements once per second while the movement proceeds.

So that's exactly what we have done. We introduced a driver option, titled Ignore shutter sensor and infer shutter position. When this option is disabled (the default) then we use the shutter position reported by Digital Domeworks. When the option is enabled, we use our own custom heuristic to infer the shutter position. Our initial testing has indicated that our heuristic works at least as well as the one in the firmware and eliminates some of the phantom shutter errors that we were observing.

This option is experimental and potentially unsafe, because we are overriding the status reported by the firmware. We do not recommend that you use this option unless you are seeing the 'phantom errors' and want to try our workaround and you have carefully considered the implications. If you do enable this option, you will be warned that you are enabling a potentially unsafe configuration. Note that this option will not fix mechanical or electrical issues. If in doubt, leave the option at the default setting of Disabled.

Obtaining and Installing the Driver

The driver may be downloaded from: https://bitbucket.org/tigra-astronomy/ta.digitaldomeworks/downloads/

The source code is available at: https://bitbucket.org/tigra-astronomy/ta.digitaldomeworks

The download is a zip file containing 4 installers. There are Debug and Release builds for each of x86 (32-bit) and x64 (64-bit) architectures. End users should normally use the Release build and install the Release configuration that matches their machine architecture: x86 for 32-bit systems and x64 for 64-bit systems. The installer checks this and will not allow the wrong architecture to be installed.

The Debug configurations contain debugging sybols and produce copious amounts of diagnostic output and therefore run relatively slowly. Debug builds are not recommended for normal use and should only be installed when troubleshooting with a debugger is required.

Open Source

In August 2015, Tigra Astronomy took the decision to open-source the project and we have continued with that policy in the 2018 reboot. The source code is available from a BitBucket Git repository. The code is being made available under the MIT license which is about as permissive as it gets. Basically, anyone can do anything at all with the software with no strings attached, and we are not liable for the consequences, whatever they are.

End users should normally only install the latest release build. Beta builds are likely to contain bugs and are not supported for production use. Integration builds contain up-to-the-minute code changes but might not be production quality so are not supported under any circumstances.

Bug Reports and Feature Requests

Hopefully there will be few problems, but if you do find any, or would like to request a new feature or provide other feedback, then please use our official public/anonymous bug tracker. Unless it's in our bug tracker, we haven't received it and you cannot expect any response.