Keithley 194 Instruction Manual page 78

OPERATION
CAUTION
Digital common is connected to chassis ground
and cannot be floated.
NOTE
Shielded cable should be used with the real time
output in order to minimize the possibility of EM1
radiation,
Real time output signal lines include:
Data lines (DO-D15)-These lines have non-inverted A/D
converter data. DO is the least significant bit, and IX5 is
the most significant bit. Note that converter data has 16
bits at sampling rates of 1OOkHz and lower (and if the pro-
grammed number of samples is ~32,768). Above 1OOkHz
(or if the number of samples is >32,768), the converter
operates in the R-bit mode. In this case, relevant data is
located only on DO through D7.
NEW CONVERSION-This signal line will go high when
converter data has been latched into the real time output
data latches and is ready for transfer.
LOW BYTE-This line should be set low to enable the
DO-D7 data lines. Note that the data lines DO-D7 will be
in the high-impedance state when LOWBYTE is high.
HIGH BYTE-This line should be set low to turn on the
De-D15 data lines. De-D15 will be in the higbimpedancc
state when HIGHBYTE is high.
OVERRUN-This flag bit will be set high if data made
available by a previous conversion was not taken. Ont, or
more data words will have been lost under these
conditions.
MEASURING-This line will be high when the instrument
is performing a measurement sequence; in other \vords.
the triggering condition has been met.
3.20.2 Reading Real Time Data
The exact method used to transmit data will depend on
the particular application; however, the pmcedurc belou
gives the basic sequence for transfer of data. The sequence
assumes the instrument is processing samples (MEASUR-
ING high). A flow chart of this stxlucnc~~ is sh<)\vn in
Figure 3-19.
1. Monitor the NEW CONVERSION line and observe
when it goes high. When NEW CONVERSION gws
high, the data byte or word has been latched into the
output data latches.
2. Test the state of the OVERRUN flag. If it has been set
high, you have missed one or more jvords of data. To
correct an overrun condition, either increase the reading
rate of the external equipment, or program a slwwr
sampling rate.
OVERRUN J AS c 0 E F
G H
DATA ,00~0,5~ +t:x
xx
,~,,I,
VALID
VALID
VALID
A. EOC goes true, setting NEW CONVERSION
B. LOWBYTE andbr HIGHBYTE go true. clearing NEW CON"ERSlON
c. same as A
F. EOC goes true with NEW CONVERSION false, string NEW CONVERSION and clearing OVERRUN
G. same as B
H. EOC goes true while LOWBYTE and,or HlGHBYTE are still twe, setting OVERRUN. NEW CONVERSION does not Wt 5~1
I. EOC goes true after LOWBYTE and HlGHBYTE have gone false. clearing OVERRUN and setting NEW CONVERSION
Figure 3-18. Real Time Output Timing
3-39