Arbiter Systems 1084A, Operation Manuals

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60 Relay Contacts and Event Inputs
8.2 Event Inputs
8.2.1 Event Timing Input
When configured for event timing, the 1084A/B/C series clocks can provide one input channel with
0.1 microsecond resolution. This channel is primarily used for synchronization via the RS-232 port
with an external computer or other type of device. It may also be used to time an external 5 V
CMOS signal applied to P3; see Figure 5.1 and Section 5.2.2.
Data for individual recorded events can be recalled using either the Event/Deviation key or via
the RS-232C interface (see Section 9.2.2). Data for each event will be retained until it is retrieved
using one of these two methods. Thus, if no event data points are retrieved, recording will be
suspended when the total number of events reaches 500. As soon as data is retrieved for a recorded
event, its address (001 – 500) is made available for data corresponding to a new incoming event.
8.2.2 Event Timing Latency
Event data are recorded using a high-speed capture circuit operating with a 4 MHz time-base.
Latency is limited by the interrupt processing speed of the clock’s microcontroller, which in turn
depends on its workload at the time the event is received. Since the workload varies from time
to time, latency likewise varies. However, response time will, in general, never be less than a few
hundred microseconds nor greater than 10 milliseconds.
8.2.3 Deviation Measurement
The Channel A input can also be configured to display measured event times as 1 pulse-per-second
(1 PPS) deviation measurements. The intended purpose of the deviation measurement function is
to allow comparison of an external 1–PPS signal to the clock’s internal 1–PPS signal. The clock
determines the mean time difference between the two signals, which can be displayed on the front
panel or read via the RS-232 Interface.
8.2.4 Measurement Principle
The measurement technique employed for 1–PPS Deviation uses the same time determination and
recording scheme used for Event Time measurement (refer to paragraph above), but makes the
assumption that the input signal is periodic and continuous. Also, the operation of the circular
memory buffer is modified somewhat, in that recording does not stop after the first 500 events; new
Event Data is given priority over existing data, and will overwrite it. Since the incoming signal
is at 1 Hz and the circular buffer holds 500 events per channel, each Event Time Record will be
overwritten once every 500 seconds.
Once every second, the processor looks at the most recent group of 16 events. When computing
deviation, it uses only the portion of the event data describing fractional seconds (e.g. values
between 0.0000000 and 0.9999999 seconds). By normalizing the 16 fractional-second values around
0.0000000, the range of results from the deviation computations will be centered on zero (-0.4999999
to +0.5000000 seconds). Statistical computations are then performed on the 16 values to determine
their Mean and Sigma (Standard Deviation) values, which are then displayed on the front panel or
output via RS-232.