Chapter 9. Electronics

The LASCO and EIT experiments are both controlled by a single electronics unit, the LASCO Electronics Box (LEB). The main LEB central processing unit (CPU) is a Sandia Lab SA3000, a radiation-hardened, 32-bit processor based on the National 32C016. Three other CPUs in the LEB are Intel 8031 processors integrated onto Application Specific Integrated Circuits (ASICs), which are used to offload some control tasks from the main CPU. The LEB receives commands from the spacecraft, and provides science and housekeeping data to the spacecraft for telemetry. The LEB acts as the main controller for the electronics components of the experiment, including the CCD cameras, the Fabry-Perot, telescope mechanisms such as shutters, filter wheels, and polarizer wheels, and the heaters and thermistors. Two instrument contoller modules provide redundancy. In addition, the LEB contains the power converters used to supply operating voltages to the LEB, the cameras, Fabry-Perot, pointing eyes, boresighter electronics, and M1 mirror controller.

9.1 Primary and Backup Sides

The LEB has a primary and a backup side to increase system reliability. The two sides have identical capability except for the amount of memory. The primary side has 12 Mbytes of random access memory (RAM), while the backup side has 6 Mbytes. The memory is organized into three areas: program space (0.5 Mbytes), an output buffer (1.5 Mbytes), and temporary image buffers (4-10 Mbytes). The redundancy concept is based on a block redundancy approach. The power converter, CPU, and memory units are duplicated for the primary and redundant sides. The primary CPU interfaces to the spacecraft computer primary side, while the backup CPU interfaces to the spacecraft computer redundant side. Similarly, the primary power converter receives power from the primary spacecraft power distribution, while the backup power converter receives power from the redundant side. There are no connections between the primary and backup power busses. The determination of which side of the LEB (primary or backup) will be used is made on the ground. In addition to the block redundant approach for the CPU, memory, and power converter, the instrument controller modules are cross-strapped to allow either CPU subsystem to operate either instrument controller. The redundancy concept allows partial failures in the instrument controllers to be overcome.

9.2 Experiment Control

The LEB controls the experiment operations, manages the interface to the spacecraft computer, and performs image processing of the CCD camera data. In addition, it monitors the operation of the experiments and, via an analog-to-digital converter, provides measured values of voltages, temperatures, and other parameters to the experiment housekeeping data stream.

The CCD cameras contain their own processors, but act as slaves to the LEB. They perform complex operations when commanded by the LEB, while reducing LEB processing requirements, allowing the LEB to act as the overall operational coordinator. The communication link between the LEB and the cameras is by opto-isolated (to mitigate power surges, noise, etc.), Manchester-encoded ( a digital format), serial channels. The CCD cameras provide images with 1024x024 pixel resolution. The four cameras are identical electronically (the EIT camera is unique only in its mechanical interface). Switched power is supplied to the cameras by the LEB, and power returns from the cameras are brought back to the internal LEB single-point ground.

Similarly, the C1 Fabry-Perot contains its own processor acting as a slave to the LEB. Its primary function is to control the Fabry-Perot optical plate position, in order to perform scans of the solar spectrum. It receives commands from the LEB, and provides status reports back to the LEB during operation. The communications link between the LEB and the Fabry-Perot processor is identical to the CCD camera link. The same command and data transfer protocol is used in both the camera and Fabry-Perot links. Switched power is supplied to the Fabry-Perot processor by the LEB, and power returns from the Fabry-Perot are brought back to the internal LEB single-point ground.

9.3 Power Converter

The LEB power converter subsystem provides the secondary voltages to all elements of the experiment except some spacecraft- powered heaters and thermistors. There are over 20 separate mechanisms in LASCO and EIT. The instrument controller allows only one mechanism motor to be driven at a time. This limits the power requirements to a reasonable peak value, and allows for adequate power for individual motor operation. Each mechanism motor can be driven by either or both of the two instrument controller modules in the LEB. Encoders are redundant for each mechanism. The primary encoder set is connected to one instrument controller, and the other set to the other controller. Since either or both of the instrument controllers can be powered-on at any given time, partial failures on either or both controllers can be overcome.

The LEB also provides thermal control of the five heater zones of the Coronagraph Optics Box (COB), and the two zones in EIT. Thermal control of the COB is required to limit the lateral gradients which would tend to distort the telescope images. (In the EIT, the two heater zones are used for focus control). Either instrument controller can drive a given zone heater (the heaters themselves are not redundant), while each instrument controller drives its own set of thermistors (thermistors are redundant)

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