National Instruments GPIB-MAC User Manual page 70

November 1985
Because you may not know for certain the number of
bytes actually read from the GPIB, the GPIB-MAC
returns the received GPIB data to you as follows. First,
the GPIB-MAC returns to you all bytes it read from the
GPIB. Next, it sends null bytes until the total number of
bytes returned to you matches your requested count.
Finally, it returns a numeric string representing the
number of bytes that it actually read from the GPIB.
For example, if you send the GPIB-MAC the pro-
gramming message "rd lO"<CR>, it reads data from the
GPIB until it receives 10 bytes of data, the END
message, or an eos byte. Let's say the GPIB-MAC
receives END with the fourth data byte. The GPIB-
MAC then returns to you the four data bytes, followed
by 6 null bytes, followed by an ASCII 4 and
always read back count bytes of data from the serial
port, then look at the remaining bytes to determine how
many of the count bytes were read from the GPIB.
Refer to the example at the end of this description.
The GPIB-MAC aborts the GPIB read and records the
EABO error if, at any time during the GPIB read, the
time limit set for I/O functions expires. This limit is 10
seconds unless you use tmo to change it.
If the GPIB-MAC is CIC, rd will cause the GPIB-MAC
to address itself to Listen if it is not already addressed.
If you specify the address of the Talker, the GPIB-MAC
will also address that device to Talk. If you do not
specify the address of the Talker, the GPIB-MAC will
assume that the Talker has already been addressed.
The GPIB-MAC then places itself in Standby Controller
state with ATN off and remains there after the read
operation is completed
If you specify an address, the GPIB-MAC must be CIC
to perform the addressing.
59 SECTION FOUR - FUNCTIONS