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Chapter 2
VMEbus Timeout Value
The VME-MXI contains the VMEbus Bus Timeout Unit (BTO) circuitry for the VMEbus. The
BTO monitors the current bus cycle and asserts the bus error (BERR*) signal if either data strobe
(DS1* or DS0*) remains active for a given amount of time.
The VMEbus system should have one, and only one, device that functions as the BTO Monitor
on the VMEbus. The VME-MXI is factory configured to be the VMEbus System Controller
(installed in Slot 1) and to provide the VMEbus BTO function. It is recommended that you use
the VME-MXI as the VMEbus System Controller. If, however, you use another device as the
VMEbus System Controller, it is recommended that you still retain the VME-MXI as the BTO
Monitor, if possible. In this case, disable the BTO function on the System Controller and leave
it enabled on the VME-MXI. If it is not possible to disable the BTO function on the System
Controller, you can disable the BTO function on the VME-MXI instead. However, be sure that
the BTO timeout value on your System Controller is sufficient to allow transfers to complete
across the MXIbus. This is especially important for multiple-chassis systems or for systems with
more than one VME-MXI.
You can either disable the VMEbus BTO value or set it to 100, 200, or 400 µsec by rearranging
the jumper selection at location W8, as shown in Figure 2-6.
© National Instruments Corporation
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a. 100 µsec BTO
(Default Setting)
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c. 400 µsec BTO
Figure 2-6. VMEbus Timeout Value Selection
Hardware Configuration and Installation
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b. 200 µsec BTO
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d. Disable BTO
2-9
VME-SB2020/NI-VXI/Solaris Getting Started
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