Measurement Computing Personal488 rev.3.0 For DOS & Windows 3.Xi - Manual (Page 193)

9H. Data Transfers II. SOFTWARE GUIDES - 9. Driver488/SUB
II-178 Personal488 User’s Manual, Rev. 3.0
Asynchronous Transfers
Driver488 can return to the user’s program while a transfer is in progress. This is useful whenever the
transfer takes a substantial amount of time, and other processing could proceed while waiting. For
example, suppose a certain bus device can transfer only 1000 bytes per second. If there are 10,000
bytes to transfer it takes 10 seconds to complete the transfer. The following statements might be used
to receive this data:
char data [10000];
EnterX (ADC, data, 10000, 1, 0L, 1, 0L);
/*Now do other work while the transfer is processing*/
Wait (ADC);
The “true” async flag tells Driver488 to return to the program after setting up the transfer. The
program is then free to do other processing, as long as it does not need access to the IEEE 488 bus.
Finally, when the program is ready to process the received data, it performs a
Wait to check that the
data has been completely received. In this way, asynchronous transfers overlap IEEE 488 bus data
transfers with program execution.
The use of DMA and interrupts requires proper hardware and software configuration. For more
information, refer to the Sub-Chapter “Installation & Configuration” early in this Chapter.
Introduction
There are four types of IEEE 488 bus devices: Active Controllers, Peripherals, Talk-Only devices, and
Listen-Always devices:
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In simple systems, Talk-Only and Listen-Always devices are usually used together, such as a Talk-
Only digitizer sending results to a Listen-Always plotter. In these systems, no controller is needed
because the talker assumes it is the only talker on the bus, and the listener(s) assume they are all
supposed to receive all data sent over the bus. This is a simple and effective method of
transferring data from one device to another, but is not adequate for more complex systems where,
for example, one computer is controlling many different bus devices.
•
In more complex systems, the Active Controller sends commands to the various bus Peripherals,
telling them what to do. The controller sends bus commands such as:
Unlisten , Listen
Address Group , Untalk , and Talk Address Group to specify which device(s) send data, and
which receive it.
When an IEEE 488 bus system is first turned on, some device must be the Active Controller. This
device is the System Controller and always keeps some control of the bus. In particular, the System
9I. Operating Modes
For Driver488/SUB, W31, W95, & WNT
Topics
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Introduction................................................................................... II-177
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Operating Mode Transitions ...................................................... II-177
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System Controller Mode.............................................................. II-178
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System Controller, Not Active Controller Mode.................... II-179
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Not System Controller Mode ...................................................... II-181
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Active Controller, Not System Controller Mode.................... II-181