Buffer transfers – Measurement Computing Personal488 rev.3.0 For DOS & Windows 3.Xi User Manual

Page 83

8G. QuickBASIC

II. SOFTWARE GUIDES - 8. Driver488/DRV

II-68

Personal488 User’s Manual, Rev. 3.0

and the numeric value of the reading (

+1.23456E-2

). The BASIC

MID$

function can be used to strip

off the range characters and keep only the numeric part (the fifth character and beyond), and the

VAL

function can be used to convert this string to a number:

740 N$=MID$(R$,5)
741 N=VAL(N$)
742 PRINT “The read value is”;N

These may be combined for efficiency:

740 PRINT “The read value is”;VAL(MID$(R$,5))

All the power of BASIC may be used to manipulate, print, store, and analyze the data read from the
IEEE 488 bus. For example, the following statements print the average of ten readings from the 195:

810 SUM=0
820 FOR I=1 TO 10
830 PRINT#1,"ENTER 16"
840 INPUT#2,R$
850 SUM=SUM+VAL(MID$(R$,5))
860 NEXT I
870 PRINT “The average of ten readings is”;SUM/10

Buffer Transfers

Instead of using an

INPUT#2

statement to receive the data from a device, we can direct

Driver488/DRV to place the response directly into a data buffer of our choice. For example, each
reading from the 195 consists of 17 bytes: a four-byte prefix and an eleven-byte reading followed by
the two-byte command terminator. So, we can collect 100 readings in a 1700-byte string.

To do this, we must first allocate the required space in a string variable:

910 R$=SPACE$(1700)

And then we must tell Driver488/DRV where

is located in memory.

In QuickBASIC 4.0, the

VARSEG

function allows us to determine the segment address of a variable.

DS%=VARSEG(R$)

Now that we know the segment address of

, we can get its offset address by using

SADDR

RDESC=0
RDESC=SADDR

Notice that we first create

RDESC

by setting it to zero. This prevents

from being moved as a result

of creating

RDESC

after calling

SADDR

Now we have the segment and offset of

, we can pass it directly to Driver488/DRV with the

ENTER #count BUFFER

command:

PRINT#1,"ENTER16 #1700 BUFFER"; DS%; “:”;RDESC

This command consists of the keyword

ENTER

, followed by the bus device address (

), a number sign

(

), the number of bytes to transfer (

1700

), and the keyword

BUFFER

, followed by the memory address

of the buffer. The buffer address is specified as

segment:offset

where

segment

and

offset

are

each 16-bit numbers and the colon (

) is required to separate them. The

segment

value we need, is the

BASIC data segment value that we have just acquired into

DS%

with

GET.SEGMENT

. The

offset

value is the offset of the string in that data segment, which is

RDESC

Once the data has been received, we can print it out:

980 PRINT R$

The program can continue with other work while the transfer occurs. For example, the program could
process the previous set of data while collecting a new set into a different buffer. To allow the program
to continue, specify

CONTINUE

in the command:

PRINT#1,"ENTER16 #1700 BUFFER"; DS%; “:”;RDESC; “CONTINUE”