16
The data will not get lost since the absolute value of the
negative reading will be sent out to the recorder. From the
plot it is fairly easy to determine that the reading is negative,
since the curve comes back up after it hits zero. If in doubt,
it would be wise to rerun the experiment.
4.2.4 Output Impedance
The recorder output found on accessory jack impedance is 409
ohms with a 1 microfarad capacitor across it. This gives much
less than a 0.1% error when looking into a 1 megohm chart
recorder, and gets rid of most radio frequencies. If you want to
get rid of all computer generated spikes as well as 60 cycle
power noise, we recommend putting a 100 microfarad
electrolytic capacitor across the recorder output, preferably
located at the chart recorder.
4.2.5 Character Format
The ten (10) bits that make up each character word are
comprised of one (1) start bit, eight(8) data bits, and at least
one(1) stop bit. Since there is a little delay between words,
you can count on at least two stop bits to be presen t in case
your system needs that extra time. The following diagram
shows the voltage waveform for this serial word.
4.2.6 Word String Format
The ILT1700 sends out a string of serial words
(characters) to transfer the displayed data to other
equipment, such as a printer, computer, or modem. There
are four different modes of readout:
‘auto
-
range’
,
‘fixed
-
range’
,
‘percent’
and
‘facto
r
’
. In the first two modes the
interface sends out ten (10) to twelve (12) characters with
the last being a
‘ca
rriage
return’
. In the percent mode, the
port transmits nine (9) characters, with the last being a
‘carriag
e
return’
. In the first two cases, the extra three (3)
or four (4) characters are sent for the exponent including an
‘e’
to separate the mantissa from the exponent. This makes
it very easy for most receiving systems to input the data,
using an
‘input’
statement. If you find difficulty bringing
the data string into the computer, you can bring it in as
an ASCII text string, then use the letter
‘e’
along with
string manipulatio n commands to separate the mantissa
from the exponent.
Since the serial bus is an open
circuit when not in use, electrical noise could be picked
up on the line before data is transmitted. If this causes a
problem, a 47 kilohm resistor can be tied from pin 11 to
the active line (either pin 2 or pin3) to pull the signal up
to +11 when no data is being transmitted. To guard
against receipt of erroneous information, the string can
be interrogated for the nine (9) characters before the
carriage return, thus rejecting any bogus transmissions
from the open line
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