PROBLEMS AND SOLUTIONS FOR NONSYNCHRONOUS TELEMETRY AND DATA SYSTEMS

Abstract

The modern pulse code modulated (PCM) telemetry or data transmission system should have its input data rates synchronized by its own, or an external clock. Either arrangement allows all pulsed input data to arrive in a known clocked time sequence. Data transmission is orderly, and all input and output gating occurs at required time intervals. Events in nature do not always follow an ordered sequence, but data transmission requires the pattern and order found in a synchronous system. Today’s missiles and aircraft do not always have the nicety of synchronous systems. In many systems today, the PCM will run at one clock rate, while the navigational computer will be set to a second rate, the autopilot processor at a third and the weapons handling system at a forth. In the slower data use systems such as “1553", data can be stored, deleted, refreshed, and data buffers or memory used. Output data loss is not too important, since output data use rates are well below input data rates. However, research and development aircraft and missile test flight applications require real time or at least time-tagged data. Transient responses cannot be lost because of clock skips or phase differences between clocks. This paper will review and explain data loss through clock skips or phase differences. The paper will show causes and effects with real time flight systems that have flown recently. Solutions such as direct synchronization, phase locked loops between separate clocked systems, and sample and hold first-in, first-out buffers, will be discussed. The applications and limitations of these solutions will be described. A development nonsynchronous system for a flight missile will be reviewed, showing block diagrams, component utilization, circuit schematics, and command application interfaces and software. The intent of this paper is not to give ultimate solutions to the clock synchronization problem, but to alert the telemetry system designer to this data problem. The paper will help define his solutions before he finds his data lost, missing or scrambled on flight test records.