Direct i/o & buffered i/o – Measurement Computing Personal488 rev.3.0 For DOS & Windows 3.Xi User Manual

Page 129

8M. Data Transfers

II. SOFTWARE GUIDES - 8. Driver488/DRV

II-114

Personal488 User’s Manual, Rev. 3.0

Direct I/O & Buffered I/O

Direct I/O is communication through the use of the

PRINT

and

INPUT

statements, or their equivalent.

Direct I/O is the simplest method of communicating with Driver488/DRV and, through it, with bus
devices. However, direct I/O has a relatively large overhead and so, for large data transfers, buffered
I/O is preferable. In buffered I/O, the program tells Driver488/DRV where in memory to find or put
the data and Driver488/DRV takes care of the actual transfer.

Direct Bus OUTPUT

The

OUTPUT

command sends data to bus devices. For example, the statement

PRINT#1,"OUTPUT 05;SP1;"

sends the characters

SP1;

to device

. This is an example of direct I/O as the data is communicated

directly to Driver488/DRV through the

PRINT

statement. As discussed above, Driver488/DRV

recognizes the

EOL

output terminator as the end of the data and sends the

TERM

output terminator in its

place after sending the data. Binary direct output is also possible. For example, the following
statements send all

256

ASCII characters:

PRINT#1,"OUTPUT 05 #256;";
FOR I=0 TO 255
PRINT#1,CHR$(I);
NEXT I

The first statement tells Driver488/DRV to expect

256

characters to follow that are to be sent to device

. Notice the semicolon (

;

)just after the

#256

. This marks the end of the actual

OUTPUT

command

and the start of the data. The semicolon at the end of the line tells BASIC not to send anything else,
such as the normal carriage-return and line-feed combination, after sending the quoted characters. The
next three lines send the

256

ASCII characters from

255

in order, to Driver488/DRV for transfer

to device

. The semicolon at the end of the third line has the same function as the semicolon at the

end of the first line: it prevents BASIC from sending any extra characters. In this example, we are
performing a binary transfer. Driver488/DRV knows how many characters are to be transferred and
neither requires

EOL

output terminators to end the command, nor sends

TERM

output terminators to the

bus device. The data is transferred to the bus device exactly as sent from the program.

Direct Bus ENTER

The

ENTER

command is used to read data from bus devices. For example, the statements:

PRINT#1,"ENTER 16"
INPUT#2,A$

read data from device

and store the returned data in the

variable. This is an example of direct

ENTER

input since the data received from the bus is read into the program via the

INPUT

statement that

reads the result directly from Driver488/DRV. As discussed above, Driver488/DRV accepts data from
device

until it detects the

TERM

input terminator. It replaces the

TERM

input terminator with the

EOL

output terminator and returns the result to the program. BASIC accepts the data just as it accepts
character data from any file. This allows us to use the varieties of BASIC input statements to control
how the data is received. For example, if the data read form the device is in the form of a valid number
then we can read it as a number:

PRINT#1,"ENTER 16"
INPUT#2,N

Or, if the data consists of several values separated by commas, we can read it as several values:

PRINT#1,"ENTER 16"
INPUT#2,A$,N,B$,I

Or, if we want to read the entire input, even if it includes commas or other special characters, we can
use

LINE INPUT

PRINT#1,"ENTER 16"
LINE INPUT#2,L$