Sensoray 417 - Manual (Page 37)

Instruction Manual 38
Communication Latency
Communication latency is a measure of communication delays between host and 417. Perhaps
the most important aspect of latency is the time elapsed from a host request for sensor data to
the acquisition of that data. This is called the acquisition latency.
Acquisition latency can be viewed as having two parts. Response delay is the time elapsed
between writing to the command port and availability of the first response byte in the data port.
Data delay is the time elapsed between reading a response byte from the data port and avail-
ability of the next response byte in the data port.
Two commands may be used to return sensor data to the host processor, read channel and read
channel group. For both of these commands, the maximum response delay is 70µs and the
maximum data queue delay is 20µs.
The read channel command consumes 90µs, maximum: 70µs response delay for the first data
byte, plus 20µs delay from the time the host reads the first byte to availability of the second
response byte.
The read channel group command consumes 370µs, maximum: 70µs response delay plus 15
more data bytes with 20µs data delay each.
Note that the above analyses do not consider host processor overhead. In reality, acquisition
latency exceeds the theoretical maximums discussed here. Additional time is consumed by the
host when it polls the 417’s handshake status, writes to the command register and reads from
the data register.
Processor Speed
The coprocessor’s I/O bus interface circuitry is designed to operate at the standard ISAbus
8MHz rate.
There is no bottom limit on host processor speed except for the following: read and write strobes to
the 417 must not exceed 5µs in duration. Handshake integrity cannot be guaranteed if host read/
write strobes exceed 5µs. This restriction is typically not relevent unless you are exercising the
ISAbus with an emulator.