Tektronix 7D20 Operator's Manual page 201

GPIB-7D20
4-26
Title
(Device Dependent :)
Fatal Error
TABLE 4-4
7D20 Status Bytes
Binary
Decimal
(not busy (busy
(System :)
Power On 010X 0001 65 81
Command Error OR1X 0001 97 113
User Request 010X 0011 67
83
Execution Error OR1X 0010 98 114
Internal Error OR1X 0011 99 115
Execution Warning OR1X 0101 101
117
Internal Warning OR1X 0110 102 118
Operation Complete OROX 0010 66
82
No Status To Report OOOX 0000 0
16
Priority
1R1X 0011
I
227
I
243
I
1
1
2
2
2
2
2
2
2
3
R is asserted with the GPIB command : ROS :ON . If the 7D20 is in the
ROS :OFF mode and polled, the status byte is sent without DI07 asserted . X is
the busy bit, and is asserted only when the 7D20 is in AVE, AVEN, ENV,
ENVN, or HOLDNEXT at the time of the SRO . The first column of decimal
values is without the busy bit set and the last column is with the busy bit set .
Both columns assume ROS :ON .
It should be noted that status bytes are not stacked; that is, only the current status
byte (in the interface) and the status byte of highest priority (status byte pending) are
saved . All status bytes of lower priority than the one pending are discarded. Also,
the status byte in the pending register is discarded and replaced if one of higher
priority appears . Thus, with the exception of the status byte stored in the interface
(status byte current), the highest priority status byte is stored in the pending
register.
One exception to the above is the Fatal Error status byte, which acts as a system
safeguard . If a Fatal Error occurs (which only happens if the 7D20 fails), this status
byte goes directly to the interface, and the 7D20 will refuse to respond further to
any bus commands, except serial poll . Thus, the fatal error status byte never has a
corresponding event code, for none could be read . The reason for the shut down is
to prevent transmission of erroneous bus commands that might adversely affect
other instruments on the GPIB .
Unlike status bytes, as many as 40 event codes are saved for later examination .
Referring again to Figure 4-14, note that except for the "LIFO" (last-in, first-out)
buffer, the structure of event-code storage is similar to that of the status bytes . New
event codes that occur are sorted to determine the priority of each ; if one is of
higher priority than that stored in the Event Pending buffer, the code in the buffer is