Reaction To Bus Commands Dcl, Sdc, And Ifc; Serial And Parallel Polling - HP 7470A Interfacing And Programming Manual

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'-(us Commands
Reaction to Bus Commands DCL, SDC, and IFC
The computer can set all devices on the HP-IB system to a predefined
) or initialized state by sending the device clear command, DCL. The
computer can also set selected devices to a predefined or initialized
' state by sending a selected device clear command, SDC, along with the
\ addresses of the devices. The basic difference is that devices will obey
SDC only if they are addressed to listen, whereas DCL clears all de-
) vices on the bus. The interface clear command, IFC, is used by the
\ computer to override all bus operations and return the bus to a known
quiescent state.
Upon receipt of either a DCL, SDC, or IFC command, the plotter resets
) the I/O to begin accepting a new instruction, and disables any current
output. Any partially parsed HP-GL instruction or parameters will be
1
^ lost.
v I The device clear and interface clear commands do not reset parameters
, in the plotter to their default values. They are not the same as the
v
' HP-GL commands, DF or IN.
."serial and Parallel Polling
!
Polling is the process used by the computer to determine which device
K
) on the HP-IB bus has initiated a require service message. The condi­
tions which will cause the require service message to be sent to the
* ) computer are defined by the input mask instruction, IM, in Chapter 1.
' - he Serial Poll
, ) A serial poll enables the computer to learn the status or condition of
devices on the bus. It is commonly used by the computer to determine
/ who is requiring service.
1 1 The serial poll is so named because the computer polls devices one at a
time rather than all at once. The plotter will respond to a serial poll by
sending the status byte as described under the output status instruction,
' ) OS (Chapter 7). The S-mask parameter of the input mask instruction,
IM, is used to specify which status byte conditions will send the service
) request message and when polled, respond with request service. Unless
' \ the user changes the S-mask value from the default setting of 0, the
plotter will never give a positive response to a serial poll, i.e., request
\ ) service (see The Input Mask Instruction, IM, Chapter 1). Bit position 6
i of the status byte will be set to 1 (if the S-mask value is not 0) when any
of the conditions designated by the S-mask are true. Bit position 6 will
( ) be set to 0 after all conditions which would cause a service request no
,- v longer exist. See IM, Chapter 1, and OS, Chapter 7. Until bit position 6
^ ' has been reset to 0, no additional service request messages, and there-
,' \ fore, no responses to a serial poll are possible.
J-4 HP-IB INTERFACING
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( )
E : 3
A computer must issue special commands to initiate and terminate a
serial poll. During a serial poll, a device must be instructed to talk and
the computer to listen. Therefore, a serial poll cannot be executed when
a plotter is in listen-only mode.
The Parallel Poll
Parallel polling can only be done to plotters with an address 0 through
7. Plotters with address settings from 8 through 30 cannot respond to a
parallel poll. The plotter will respond positively to a parallel poll only if
the conditions specified in the P-mask are satisfied and parallel poll
response is enabled. The P-mask parameter of the input mask instruc­
tion, IM, is used to specify which status byte conditions will result in a
logical 1 response to a parallel poll. The response to a parallel poll is
limited to setting the appropriate data line to a logical 1. The line used
is determined by the plotter's address value as shown in the table below:
Plotter
A d d r e s s
0
1
2
3
4
6
7
Parallel Poll
Bit Position
7
6
5
4
3
1
0
HP-IB D a t a
Line Number
8
7
6
5
_ — , 4
2
1
Plotter Preset Address
To execute a parallel poll, the controller sets the ATN and EOI lines
to 1. The controller reads the eight data lines, and determines from
these lines which instrument on the bus is requesting service. The com­
puter then sends the parallel poll disable command. Not all computers
have parallel poll capability.
It is important to remember t h a t the 7470 will not send a logical 1
unless the P-mask bit value has been changed from the default value of
0 and some condition included in the new P-mask value is true. The
plotter does not respond to a parallel poll in listen-only mode.
Positive responses to parallel polls will continue to occur until all bits of
the status byte included in the P-mask value have been reset to 0. (See
The Output Status Instruction, OS, Chapter 7.)
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HP-IB INTERFACING 9-5
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