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HYDRA
Service Manual
7-8
7-12. Troubleshooting
7-13. Power-Up Problems
The following discussion identifies probable fault areas if the installation of an IEEE-
488 Option causes power-up failure for the instrument. The problem is probably a short
on A5J1; the Microprocessor on the Main Assembly is prevented from accessing ROM
and RAM correctly.
• First check if VCC is shorted to GND on the IEEE Assembly.
• The short may also be caused by an interface signal to either VCC, GND, or another
interface signal. The logical signals to check are D7 .. D0, A2 .. A0, RD*, IEEE*,
WR*, E, RESET*, and IRQ2*.
• The short may be due to a CMOS input that has been damaged due to static
discharge; the short is then detectable only when the circuit is powered up. Use an
oscilloscope to check activity on each of the interface signals. Verify that signals are
able to transition normally between 0 and 5V dc.
7-14. Communication Problems
7-15. Failure to Select IEEE-488 Option
IEEE-488 Interface selection procedures are described in Section 3 of the Hydra User
Manual.
If the IEEE-488 option is not detected by instrument software, there may be a problem
with the OPS* signal. The IEEE option grounds the OPS* signal (A5J1-22), which is
normally pulled up to VCC on the instrument Main PCA. The Microprocessor
determines that the IEEE-488 option is not installed if OPS* (A1U4-29) is high during
the power-up option detection.
7-16. Failure to Handshake on IEEE-488 Bus
After power-up or when the active computer interface is changed from RS-232 to IEEE-
488, the Microprocessor sends six write cycles to initialize A5U1. The IRQ2* interrupt
is then enabled, and the serial poll status byte is initialized. At this point, the IEEE-488
option is ready to respond to transactions on the IEEE-488 bus.
7-17. Failure to Enter Remote
If the IEEE-488 option does not enter remote, check that the remote/local control circuit
is operating properly. When the IEEE-488 option is the active instrument interface, the
remote/local control state is polled by the Microprocessor approximately every 400 ms.
Normally, A5U1-4 goes low for approximately 800 ns during the read cycle that checks
the state of A5U1. If D(0) (A5U1-11) is low during the read cycle, A5U1 is in the local
state. If A5U1-11 is high during the read cycle, A5U1 is in the remote state. When A5U1
indicates that it is in remote, the REM indicator on the display is turned on.