Reading From Model 776 - Keithley 776 Instruction Manual

4-10. Reading From Model 776
The Reading sequence is used to obtain from Model
776. various information strings such as measurement.
gate time, delay time or trigger level. Each information
string is made up of ASCII alpha and alphanumeric
characters. For more details on the information strings
format refer to paragraph 4-10-I.
The reading sequence is conducted as follows:
1. The controller sets the ATN line true.
2. The Model 776 is addressed to talk.
3. The controller sets ATN false.
4. The instrument sends the information string over the
bus one byte at a time.
5. The controller recognizes that the string is termi-
nated.
6. The controller sets the ATN line true.
7. The UNT (untalk) command is placed on the bus b)
the controller.
NOTE
Most controllers use the CR (Car-
riage Return) orLF (Line Feed) char-
acter to terminate their input se-
quences. but other techniques may be
used as well to recognize the end of
input sequence (for example the EOI
line is low on the bus during the
transfer of the last byte).
4-10-I Data Control commands (R)
The Data Control commands allows access to informa-
tion concerning present operating conditions of the in-
strument. When a data control command other than RO)
is given. the Model 776 transmits the associated data
string instead of its normal data string.
The next
time
it is
addressed to talk,
Model 776 will transmit its
normal measurement data string (RO).
Note that
Model 776 defaults to RO state. It is therefore not nec-
essary to include the RO command in every reading
request.
The Model 776 Data Control commands include:
RO
= Send normal measuring data string
Rl
= Send Gate Time data status string
R2 = Send Delay Time data status string
R3 = Send Trigger Level A data status string
RJ
= Send Trigger Level B data status string
RS
= Send Input conditioning status string
R6 = Send Operating Mode Status string
R7 = Send Error Status string
Table 4-9 shows the general data string format for each
of the seven commands (decimal point floats).
Table 4-10 shows the interpretation for the input condi-
tioning status. operating mode status and error status
strings (RS. R6 and R7).
NOTES
1. Data strings have fixed length of 15 ASCII characters
for the RO. RS and R6 commands without the prefix and
terminator. For all other data strings (RI through R4 and
R7). the length of the data string is 5 ASCII characters
without the prefix and terminator. If the data string is
sent with a prefix. four additional ASCII characters arc
included (refer to paragraph 4-2-3). If the data string is
sent with one or two terminators. the length of the data
string increases by one or two characters respectively.
2. All normal measurement data string information
(RO). besides the status strings (RI through R7). will be
sent only once each time a measurement has been suc-
cessfully completed. This may halt the controller for the
duration of the gate or delay time. It is therefore rccom-
mended that the status byte will bc continuously moni-
tored and normal reading taken only after the READ-
ING DONE bit is set true.
WARNING
Model 776 updates its GPIB
huff-
ers only once at the end of a SUE-
cessful measurement cycle. If a
normal reading is requested before
the gate is closed, the bus could be
held for, at least, the duration of
the gate time. With some control-
lers, this delay may cause control-
ler time-out, or even a complete
bus lockup. It is therefore recom-
mended that the Model 776 is pro-
grammed to assert an RQS on
Reading Done (Q2). and that a nor-
mal reading is requested only if the
Reading Done bit (Bl) in the status
byte is set. The procedure of rcad-
ing the Status Byte, as well as its
interpretation, is given in paw-
graph A-9-21.
3. All status string information. besides the normal data
strings. will be sent only once each time the command
is sent. Once the data string is read. the instrument will
send its normal data string (RO) the next time it is
addressed to talk. CR LF is normal tcrminatoc The
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