Sensoray 417 - Manual (Page 36)

37 Sensoray Model 417
Chapter 7 Theory of Operation
Channel selection, amplifier gain, and A/D functions are all under direct control of the copro-
cessor’s CPU. Consequently, none of these circuits are accessible to the host.
Timing
Three timing parameters are important from a system integration viewpoint: scan rate, commu-
nication latency, and host processor speed. These timing elements are considered below.
Scan Rate
Channel scan rate is defined as the number of conversions per channel per second. The scan
rate is influenced primarily by the number of active channels in the scan loop. A secondary
influence is the frequency of communication between host and 417 coprocessor.
The basic channel process time—the scan time—is approximately 22 milliseconds at the
default scan rate, or 9 milliseconds when operating in the High Speed Mode. This is the
amount of time required for the 417 to configure its analog section, allow the front end to stabi-
lize and digitize the input signal. Note that the linearization, limit processing and filtering func-
tions don’t require any additional time as they execute concurrently in the digital domain.
Clearly, scan time decreases as the number of active channels decrease. Channels can be
removed from the scan loop by disabling them with the set sensor type command in conjunction
with the disabled channel sensor definition code. The update rate for each channel is approximated
by this function:
Another important parameter is the refresh time—the time elapsed between updates of a chan-
nel’s measured data. This parameter may be critical in control applications that require new
data every time channel data is read. In such applications, it is necessary to know the maximum
refresh time: the minimum time between successive reads from a channel that guarantees fresh
data. The maximum refresh time is given by this function:
Note that time spent communicating with the host typically translates into time stolen from the
scanning function, as denoted by CommTime in the above expression. Communication
between host and 417 will tend to reduce the scan rate and increase the maximum refresh time.
See the next section for details of communication timing.
SamplesPerSecond
1
NumActiveChannels
( )
ChannelScanTime
( ) ×
--------------------------------------------------------------------------------------------------------------
≅
RefreshTime
max
NumActiveChannels 1 +
( )
ChannelScanTime
( ) ×
CommTime + =