Appendix B
Programmable Configurations
© National Instruments Corporation
B-5
VME-MXI-2 User Manual
VMEbus Timer Limit
The VMEbus Bus Timeout (BTO) is a watchdog timer for transfers on
the VMEbus Data Transfer bus. After the specified amount of time has
elapsed, the BTO circuitry terminates a VMEbus cycle if no slave has
responded. The VME-MXI-2 must provide the VMEbus BTO for
proper operation because when a MXIbus cycle is involved, the
VMEbus timeout must be disabled and the MXIbus BTO enabled.
You should disable the BTO of any other BTO module residing in the
mainframe. If this is not possible, set it to its maximum setting to give
the MXIbus cycles as much time as possible to complete.
The lowest value in the allowable range is 15
µ
s and the highest is
256 ms. The default value is 125
µ
s.
To change the VMEbus timeout limit of the VME-MXI-2, write the
EEPROM byte at offset 206F hex from the VME-MXI-2 base address.
The following table gives the value that should be written for the
corresponding time limit.
Time Limit
Value (Hex)
Timer Disabled
C0
15
µ
s
C1
30
µ
s
C2
60
µ
s
C3
125
µ
s
C4 (default)
250
µ
s
C5
500
µ
s
C6
1 ms
C7
2 ms
C8
4 ms
C9
8 ms
CA
16 ms
CB
32 ms
CC
64 ms
CD
128 ms
CE
256 ms
CF
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