Table of Contents
Advertisement
IEEE-488 NAMED DEViCE CLOSURE
For IEEE-4888 devices, which were opened with file names, a special listener command sequence,
with the special secondary address of thehexadecimaf EO DR'ed with the secondary address from the
OPEN is sent. This allows devices such as disc files to be closed by the peripheral controller.
ERROR DETECTION: GENERAL
The basic concept of the PET operating system is that the user will be allowed to operate in a free-form
format; reading and writing on tapes, discs, and printers, in the manner that Is most comfortable for him.
Because 110 is totally free-form, it is most important that the operating system should have means of
informing the user when transmission errors or end of data conditions occur.
STATUS WORDS
In order to facilitate IN PUT/OUTPUT operation error detection, the PET uses the "status word" concept in
which a byte in memory is manipulated by each of the 110 operations for the PET, and can be sampled by
the programmer at any time by calling the function ST. Each bit in the staus word has a general meaning
for all operations and a specific meaning for the individual I/O device.
Table 7.17 shows the errors as a function of the ST word value for the tape cassette units. IEEE read/write
operations, tape verify and load operations.
ST
ST
Cassette
Tape
Bit
Numeric
R....
IEEE AM
Verify
Position
Value
+
Load
0
1
Time out
on write
1
2
Time out
on read
2
4
Short block
Short block
3
8
long block
long block
1------
- -
4
16
Unrecoverable
Aoy
read error
mismatch
5
32
Checksum
Checksum
error
error
6
64
End of file
EOr line
7
-128
End of tape
Device not
End of
present
tape
Table 7.17. Status Word (5T) values correlated with
tape cassette, unit and IEEE bus readlwrite errors.
IEEE DEVICE ERRORS
There are basically three errors that can occur during an IEEE-488 transfer. First, the entire bus does not
respond to an attention sequence. If this occurs, the IEEE-488 subroutine sets the DEVICE NOT PRESENT
bit (7 or -128). The PET also terminates the current program with ?DEVICE NOT PRESENT ERROR. If the
bus responds correctly to the attention, but when the PET goes to write the first character to the bus and
the physical device is not present as indicated by having NRFD or NDAC low, the PET, again, gives a
device not present indication.
The second error occurs during the process of transferring data to -the device. The bus does not respond
80
Advertisement
Table of Contents
