The XEP198 embedded processor was designed and built in 1998 by Xecon Associates to control and operate certain portions of the 24" Alt-Azimuth Newtonian Reflecting Telescope at the Hobbs Observatory (750) located in Fall Creek, Wisconsin. At approximately the same time Xecon engineers modified the overall control system and wrote a new program (STARTRAC) to operate the telescope and other ancillary equipment. [This is discussed on this web site under the general heading 'Astronomical Computing'.]
Prior to the installation of this device we could only take relatively short time exposures with this instrument because of the inherant problem of field rotation that occurs with all Alt-Azimuth telescopes. After contacting several suppliers of telescope field de-rotators we decided that it would be in our best interest to design and construct the device by ourselves in order to meet our own exacting requirements.
The field de-rotator control electronics unit was designed and integrated into the observatory equipment by Xecon. An image of this device can be seen by clicking on IMGDERCE . The design and construction of the mechanical portion of this device was done by Johnson Tool & Machine Works of Eau Claire, WI. An image of this device can be seen by clicking on IMGDERMA . It should be noticed that the 24" Telescope at the Hobbs Observatory is a joint use project between the University of Wisconsin - Eau Claire, the Beaver Creek Reserve, and the Chippewa Valley Astronomical Society. [A view of this instrument may be seen by visiting the CVAS web site at www.cvastro.org. Click on 'About CVAS' then click on '24" Newtonian/ Nasmyth Reflector' ] Close cooperation by each of the aforementioned organizations resulted in a highly successful project. It has been operating at the observatory since August 1998 through -40F winters and +98F summers with no failures.
The XEP198 controller was designed and developed so that it could easily integrated with the other equipment existing at the observatory. The altitude and azimuth axis of the 24" telescope are individually operated by controllers built by the CompuMotor Corporation. Each axis incorporates a CompuMotor model 2100 Indexer, solid-state driver unit and motor. All three of these devices receive commands and respond with status to a central controlling PC via a single and common serial data channel operating at 9600 baud. The controlling PC runs the STARTRAC program.
At the outset of the project it was jointly decided by the users that the XEP198 controller should at a minimum perform the following functions: (1) Image Field De-Rotation (2 unique modes: simple field de-rotation [Mode 1], or field de-rotation coupled with vertical polar alignment [Mode 2]), (2) Image Focusing control for the JMI motorized focuser, and (3) Filter select control for the Optek MAXFILT 3-position Filter Slider. All of these operations are commanded from the control room via an IBM PC running the STARTRAC program. The main control room is approximately 50 (electrical) feet from the telescope proper.
To accomplish these goals, Xecon chose to construct the XEP198 electronics around an 8051 stored program microprocessor. The 8051 is a device that has been specifically structured to perform REAL-TIME control functions. More accurately it should be referred to as stored program micro-CONTROLLER. Its internal architecture and instruction set is uniquely congruent to applications of this nature. The 8051 contains 128 bytes of RAM, 2 16-bit counter-timers, a full duplex serial communicatins port, 32 I/O lines, a five vector two-level interrupt system, and an on-chip oscillator and associated clock circuitry. In this application it was used in the 40-pin DIP package version. Accompanying the 8051 is an external PROM (programmable read-only memory) and about 16 other MSI/SSI circuits along with a power supply. The motor drive circuits are located on a seperate board and have been placed in an enclosure that is in relative proximity to the devices being driven.
The program code (firmware) for this device is contained in the on-board PROM. The full XEP198 firmware program fits comfortably in a 4K Word 8-bit wide PROM. For optimum efficiency and accurate timing control, the 8051 was programmed in machine (assembly) language and is mechanizd around a Xecon Real-Time Operating System (RTOS).
The RTOS, as implemented for this design, incorporates a 250 microsecond control loop whose timing is developed from one of the 8051's internal counter-timers. Once every 250 microseconds, the 8051 is interrupted and forced to re-enter the real-time in-line code. All time critical functions are performed by this code. Included are the de-rotation motor pulse generator, the filter select pulse generator, the focuser proportional motor control timing, the serial I/O command input processor, and the system 0.1 and 1 second tick counters. This section of code contains a total of about 140 machine instructions and executes (worst-case) in 156 microseconds over two consecutive RT interrupt cycles (500 microseconds). The RT in-line code accordingly consumes about 32% of the microprocessors time resources. The remaining code (about 1200 instructions) is non-time critical.
Non-time-critical tasks are supported by a round-robin task scheduler. Each scheduled task is programmed to execute in as small amount of time as possible. Control is then simply passed on to the next scheduled task. These processes continuously loop until interrupted. Accordinly all tasks are given access to the microprocessor on a equal and shared basis. The task list for this device includes: decode and execute incoming commands, process and generate status replies, service the field de-rotation function, service the focusing function, flash LED test (@ start-up), and run the Xecon internal monitor. Control of the Optek MAXFILT selector is fully handled as part of the in-line code.
We have calculated for this application (in a worst case condition) that less than 45% of the 8051 microprocessor's total timing budget is consumed.
