Sensoray 7429 - Manual (Page 46)

TIMING
Three timing parameters are important from a system integration viewpoint: channel
scan rate, communication latency, and host processor speed. These timing elements are
considered below.
C HANNEL S CAN R ATE
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 7429 proces-
sors.
The basic channel processing time is approximately 22 milliseconds. This is the
amount of time required for the 7429 to configure its analog section, allow the front end
to settle, and digitize the input signal. Note that linearization and filtering functions
don't require any additional time as they execute concurrently with the digitization.
Clearly, the scan time decreases as the number of active channels decrease. Channels
can be removed from the scan loop by disabling them with the define channel sensor
command, using the disabled channel sensor definition code. The update rate for each
channel is given by this function:
R Ý F(45,N)
R = samples/second
N = number of active channels
All host communication functions are interrupt-driven in the 7429's local environment.
As a result, both communication latency and overhead are kept to a minimum. Even
so, time spent communicating with the host is often time stolen from the scanning func-
tion. Keep in mind that excessive communication traffic between host and 7429 will
tend to reduce the scan rate.
C OMMUNICATION L ATENCY
Communication latency is defined here as communication delays between host and
7429. Perhaps the most important measure of latency is the amount of time elapsed
from a request for sensor data to the acquisition of that data. This is called the acquisi-
tion latency.
Acquisition latency can be viewed as having two parts: time from request to first re-
sponse byte — command response delay — and time between response bytes — data
queue delay.
Two commands return sensor data to the host processor, read all channels and read
channel data. The worst case command response delay is the same for both commands:
140 microseconds. The worst case data queue delay is 40 microseconds.
The read channel data command consumes a maximum of 180 usec. This is computed
as follows: 140 us from command issue to availability of the first response byte, plus
40 us from the time the host reads the first byte to availability of the second response