Measurement-computing Personal488 rev.3.0 For DOS & Windows 3.Xi Manual de usuario descargar pdf (Pagina 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:

•

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

•

Introduction...................................................................................II-177

•

Operating Mode Transitions......................................................II-177

•

System Controller Mode..............................................................II-178

•

System Controller, Not Active Controller Mode....................II-179

•

Not System Controller Mode......................................................II-181

•

Active Controller, Not System Controller Mode....................II-181

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