2.5.1 Distributed Processing Overview

Instead of using a single computer to handle all of the data I/O and processing, Ranger NBV data systems were designed to operate using a distributed processing architecture. In this approach, the vehicle utilizes many different computational systems, each dedicated to a specific function. These processors then pass data streams to each other.

Figure 2-18 Distributed Processing Overview

There are many advantages to this approach. Each individual processor may be relatively simple, and specialized for the function that it is to perform. This approach greatly simplifies anomaly resolution because problems may be easily traced to the particular unit that is malfunctioning. Furthermore, the sheer number of subsystems in Ranger NBV make a single processor approach virtually impossible due to the number of sensor and actuator lines that would need to be interfaced with the main processor. The distributed approach allows the number of connections to each processor to be reduced, although in some instances, the wire count is still very high. A very important advantage to the distributed approach is that computers may be located very close to the sensors from which they are acquiring data. This is desirable because long sensor lines are more subject to unwanted noise effects, especially in a very electrically "busy" environment such as the interior of the electronics module. In this environment it is best to digitize the data as soon as possible to reduce unwanted interference.
There are two main classes of computers used in Ranger NBV. There is one main processor which acts as the hub of the entire system. Surrounding this hub are many less powerful specialized computers that interface with the sensors and actuators. These local processor units (LPUs) typically do little processing but mainly perform simple i/o functions, safety procedures, error checking, and basic closed loop control.
Data transfer between the main processor and the LPUs is based on the RS422 serial communications architecture. The serial communications protocol (MiMiCS) was developed at the SSL. Transfer rates vary from 9600 to 38400 baud depending on the system. Communications between the main processor and the control station are carried over a fiber optic ethernet link using the NDDS communications protocol.