Handshake Sequence; Listen Address; Talk Address/Secondary Address; Universal Commands - Wavetek 75 Instruction Manual

3.27.2 Handshake Sequence
The three handshake lines, Data Valid (DAV), Not Ready
for Data (NRFD) and Not Data Accepted (NDAC) are
used in a typical data exchange as described in the next
paragraph.
All devices currentlydesignated as active listeners would
indicate(via the NRFD 1ine)when they are ready for data.
Adevice not readywould pull this line low(ground), while
a device that is ready would let the Iine float high. Since
a lowoverridesapassive high, this linewill stay low until
all active listeners are ready for data. When the talker
senses this, it places the next data byte on the data lines
and then pulls DAV low. This tells the listeners that the
information on the data lines is valid and that they may
read it. Each listener (at its own speed) then takes the
data and lets the NDAC line go high. Again, only when
all listeners have let NDAC go high will the talker sense
that all listeners have read thedata. It can then remove
DAV (let it go high) and start the entire sequence over
again for the next byte of data.
knob until the desired address is displayed. At power on,
the address always matches the internal switches.
3.27.5 Talk Address
Talk addresses are used to command a device to
transmit data over lines D101-D108. There are 31
different available addresses (hexadecimal codes 40
through 5E, ASCI I codes
@
through ). A 32nd address,
called untalk (hexadecimal 5F, ASCll "-") is used to
command all devices to cease talking. The low order five
bits of the Model 75 talk address will be the same as the
loworder five bitsof the listenaddress.Thus, if the Model
75 listen address is hexadecimal 21 (ASCII
"
! "), the talk
address is hexadecimal 41 (ASCII "A").
3.27.6 Secondary Address
Secondary addresses are used following a talk or listen
address to provide the ability to address more than the
31 devices provided for by simple talk or listen
addresses. Secondary addresses are ignored by the
Model 75.
3.27.3 Commands
3.27.7 Universal Commands
Commands are sent over lines Dl01 -Dl08 with ATN true.
universal commands are used to command a device to
They are divided into five classes:
perform designated actions. Universal commands are
1. Listen Addresses recognized at all times. Universal commands performed
2. Talk Addresses
by the Model 75 are:
3. Secondary Addresses
4. Universal Commands
DCL Device Clear
SPE Serial Poll Enable
SPD Serial Poll Disable
LLO Local Lockout
5. Addressed Commands
GTL Go To Local
SDC Selective Device Clear
GET Group Execute Trigger
These commands and command groups are shown with
their binary codes in Appendix A.
3.27.4 Listen Address
Listen addresses are used to command a device to read
any data bytes transmitted over lines Dl01 -D108. There
are 31 different available addresses (hexadecimal codes
20 through 3E, ASCll codes SP through >). A 32nd
address, called unlisten (hexadecimal "3F1', ASCII "?")
is used to command all devices to not read data bytes.
The low order five bits of Model 75 listen address is
selected by internal DIPswitches(see paragraph 2.2.6)
or by the ADRSsoft key. To temporar~lychange theGPIB
address, press the ADRS soft key and turn the Control
1. Device Clear (DCL)
-
Resets the Model 75 to the
default settings. DCL affects all devices on the bus.
Serial Poll Enable (SPE)
-
Causes the instrument
to engage in a serial poll by responding with the serial
poll status byte when addressed as a talker. Data
line D107will beon, if service is being requested on
the SRQ Iine. When the status byte is read, it is reset
to all zeros, and the SRQ line is released (of course,
it may still be held down by other devices). The status
byte is also available by reading the Model 75 talk
message number 1. When this message is read, the
status byte is reset and the SRQ released as for the
serial poll.
3. Serial Poll Disable(SPD)- Discontinuesserial poll.
Returns instrument to normal talk modes.
4. Local Lockout (LLO) -Causes the Model 75 toenter
a state where the front panel LOCAL soft key is
inoperative. In this state the instrument will only
accept parameter changes through the GPIB. To
enable front panel parameter changes, the GPIB
controller can send the GTL(GoTo Local)command.
Scans by ArtekMedia O 2006