Sensoray 118 - Manual (Page 36)

Sensoray Model 118 Smart A/D™ Instruction Manual 34 Timing
Because of the unpredictable effect on update rate,
excessively frequent communication traffic between host
and Smart A/D should be avoided.
8.3 Data Age
Data Age is the measure of time that has elapsed since a
channel’s data was acquired by the Smart A/D board.
The age of any channel’s data is directly related to the
update rate.
The minimum possible data age is, of course, zero. This
would be the case if the host retrieves sensor data
immediately after new data is posted by the Smart A/D.
The maximum possible data age would apply if the host
retrieves sensor data immediately before new data is
posted by the Smart A/D. In this case, the data age is the
reciprocal of the minimum update rate.
In practice, data age almost always lies between the
minimum and maximum possible values.
8.3.1 Example
As an example, take the case shown at the end of Section
8.2.1. The minimum update rate is approximately 4.1
Hertz, so the maximum data age would be:
In this example, the age of sensor data from any given
channel may range from zero to 244 milliseconds.
8.4 Communication Latency
Communication Latency is the time elapsed from a host
request for sensor data to the acquisition of that data.
This latency can be viewed as having two components:
host overhead and Smart A/D response time.
Host overhead is the time spent executing those parts of
the Smart A/D interface (API) functions that are in the
computational domain of the host (i.e., not pending on a
Smart A/D response). Since this overhead—which is
purely a function of host clock rate and architecture—
varies from one system to another, it must be
independently determined for each system.
Smart A/D response time is the time spent in an API
function while waiting for a Smart A/D response. For
obvious reasons, short communication latency is a
high-priority design objective in many applications.
Communication functions are event-driven in the Smart
A/D’s local environment and given highes priority in
order to minimize the communication latency. As a
result, Smart A/D response time is predictable with a
good degree of accuracy:
DataAge
max
1
4.1
------- 244ms
≈
=
Table 7: Command response times
Command
Response Time
(microseconds)
GetSensorData 100
GetAllSensors 130