Agilent Technologies 6621A, Service Manual

Page: 94 / 156

4-29 Troubleshooting Analog Multiplexer U323
and Readback Using VMUX? Command
Analog multiplexer U323 directs one of eight inputs to the
readback signal comparator U324 (see paragraph 2-31). The
instruction set for the power supply includes the VMUX?
command which measures the voltage at these inputs. This
command can be used to troubleshoot the analog
multiplexer and the readback circuitry on the output board.
The supply may have to be put into the SKIP SELF-TEST
mode in order to program the output (see paragraph 4-126).
The syntax for the VMUX? command is as follows:
VMUX? < ch >, < input no. >
A complete description of the syntax structure is shown as
C4 in Figure 5-2 (sheet 1) in the Operating Manual. The
response to the VMUX? command is SZD.DDD (see Table 5-
2 in the Operating Manual for an explanation of these
abbreviations). The resolution of the returned voltage
reading is approximately 2 mV.
The analog multiplexer circuit is shown on the functional
schematic of Figure 6-3, Sheet 1 and on the block diagram of
Figure 2-4. The eight input signals to the analog multiplexer
U323 are shown in table below.
The VMUX? command reads back approximately -1.01
times the voltage that appears on the selected input. Its
readback range is limited to approximately - 0.1 to + 10
corresponding to +0.1 V to -10 V on the input pins.
However, any positive voltage from approxi 0.1 V
to + 10 V on the input pins will readback about - 0.1 V.
After the VMUX? command is processed, the selected input
will remain connected to the multiplexer output (U323, pin
8) as long as no other A/D conversions (voltage
measurements) take place due to readback either over the
on a particular output channel can be stopped by using the
front panel OUTPUT SELECT key to monitor a different
output channel (or, alternatively, simply pressing the front
panel VSET, ISET, etc. keys will stop the internal A/D
conversions).
For example, if it is desired to measure the multiplexer
output voltage with an external voltmeter when the CV REF
signal (Input No. 6) on output channel 2 is selected, stop the
A/D conversions by pressing the VSET button on the front
panel. Next, use the following program to select input 6 on
channel 2 and to display the value read back to the
computer.
10 OUTPUT 705; "VMUX? 2,6"
20 ENTER 705;A
30 DISP A
40 END
After the program is run, a voltmeter can be used to confirm
that the multiplexer output (U323, pin 8) and input 6 (U323,
pin 11) measure the same voltage and that the VMUX?
result is valid.
4-49
GPIB or internally to the front panel. The A/D conversions