Fluke 2680A Service Manual page 88

268XA
Service Manual
Control Signals
Several signals are used by the A3U5 A/D microprocessor to control and receive state
information from the A/D state machine (A3U18). The Trigger line, used to indicate to
the A/D when to begin a reading, was discussed previously.
The A/D state machine has several modes of operation: perform conversion (measure
input); do a reference balance reading with both references on; or do a reference balance
reading with both references off. These modes are selected by the A3U5 A/D
microprocessor through a two-bit parallel port, which consists of two data lines and a
strobe line. The codes for the commands are given in Table 2-15. To send a command to
the A/D state machine, the data lines are set to the values shown, and then latched with a
rising edge on the strobe line.
There are two lines from the A/D state machine (A3U18) that indicate its state. These are
connected as interrupt request signals to the A3U5 A/D microprocessor. The falling edge
of the A/D Interrupt* signal indicates that a reading is complete and the counters are
ready to be read. The A/D Interrupt* signal goes high at the beginning of the Integrate
period, when the counters are cleared, and the signal is read by the A3U5 A/D
microprocessor reads the counters to make sure that they were read in time. The
DE_INT* signal indicates the beginning of the Deintegrate period. See Figure 2-14 A/D
Status Signals.
2-40
Table 2-15. A/D Command Codes
Command
Measure Signal
Reference Balance Reading, Both References
Reference Balance Reading, Neither Reference
AZ
Autozero
Integrate
DE_INT*
A/D Interrupt*
Figure 2-14. A/D Status Signals
0
0
1
I DE
Deintegrate
C1 C0
0
1
0
UAZ
Untimed Autozero
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