Measurement Computing CIO-DAS800 - Manual (Page 9)

4 OPERATIONAL ASPECTS - CIO-DAS800 VS. CIO-DAS08
4.1 CONVERSION TRIGGER
A trigger is the event that begins an acquisition/transfer cycle. There are three ways to trigger a
CIO-DAS800; software, interrupt service routine or hardware. The hardware trigger is a new feature on
the CIO-DAS800, not found on the other DAS08 family boards. This new, improved trigger allows the
board to run much faster than the other DAS08 boards.
Briefly, there are three methods to trigger a data conversion:
A software trigger starts an A/D conversion when an addressed port is written to. This is how a single
sample may be taken.
An interrupt service routine is a set of instructions executed by the computer when an interrupt is
received. The source of the interrupt is the IR input pin. The signal on the IR input pin is routed to the
PC bus via the interrupt jumper on the CIO-DAS800. The event that causes the interrupt may be the
internal 82C54 counter or an external event.
The hardware trigger is a direct input to the A/D chip's start conversion pin. When the trigger is
received, the A/D starts a conversion. When the conversion is complete, the data is transferred to the
FIFO buffer. The source for the hardware trigger can be the 82C54 counter/pacer or an external event.
A trigger is useful for synchronizing samples to a known time base, such as the on board 82C54
programmable divider. Using an external trigger allows you to synchronize samples to an external event.
4.2 IMPROVED CHANNEL SEQUENCER
When the acquisition strategy you set up requires a scan of data from multiple channels, the channel
sequencer controls the internal multiplexer that routes the signal to the A/D. The DAS08 family employs
a register that must be written to each time the mux is incremented.
The improved hardware channel sequencer on the CIO-DAS800 is set up, then takes over control of the
internal multiplexer, freeing the CPU from that task. This gives faster sampling.
4.3 CONTROL REGISTERS
The CIO-DAS800 is compatible with DAS08 software because the basic I/O register have identical
functions on each board. I/O registers are the locations which the computer writes commands and data to
and reads status and data from. Table 1-1 is a summary of board registers. The register at BASE+2 is a
multi-function register. It and the others are described in the register section of this manual.
Table 1-1. Register Summary
Gain & Range Control / Status BASE + 3
Special Multi-Functions Register BASE + 2
A/D High Byte / Start Conv A/D BASE + 1
A/D Low Byte / Start Conv A/D BASE + 0
FUNCTION Read/ Write I/O ADDRESS
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