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Configuration and Installation Chapter 3
VXI-MXI User Manual 3-30 © National Instruments Corporation
In a properly configured MXIbus system, the first and last devices in the daisy-chain each have
only one cable connected to their device connector. MXIbus devices that are neither the first nor
the last device in the daisy-chain have two (and only two) MXIbus cables attached to their device
connector.
System Power Cycling Requirements
A distributed architecture such as MXIbus does not have a common power bus or reset signal to
ensure that all devices within the extended system are initialized at the same time. Therefore,
powering-on a device after other devices in the system have already received power and become
functional may cause unintended bus activity due to the power-up state of that device. This bus
activity could result in a powered, functional MXIbus device making an attempt to respond. If
this response initiates bus arbitration, for example, the arbitration mechanism could become hung
while waiting for a nonexistent master to assume control of the bus.
To guard against this type of bus activity, you should keep in mind that two types of systems
exist with respect to power cycling. The first of these is one in which power is supplied to all
devices at roughly the same time as is the case for a system with a master power switch. In such
a system, proper operation is guaranteed if the last device to reach 5V does so within roughly
half of a second of the first device to reach 5V. The second type of system is one in which power
is applied to each MXIbus device separately. In this type of system, you must power-on devices
starting with the device at the end of the MXIbus link opposite the MXIbus System Controller
and progress towards the MXIbus System Controller. The MXIbus System Controller should be
the last device on the MXIbus link to receive power. Conversely, when removing system power,
you should power-down the device farthest from the MXIbus System Controller last so that the
termination resistors on the end device ensure that the MXIbus lines remain unasserted
throughout the power-down sequence.
When shutting down a system, you must ensure that all devices that could be adversely affected
by unintended bus cycles or interrupts have their windows and interrupt mapping disabled. The
VXI-MXI itself will not be affected during power-down; however, there may be devices in the
same frame as the VXI-MXI that could be affected.
Table 3-1 summarizes MXIbus system power cycling requirements.
Table 3-1. MXIbus System Power Cycling Requirements
System Type Power-On Requirements Power-Off Requirements
Master Power
Switch
All devices must receive power
within 0.5 seconds of each other.
Disable all A24 and A32 inward
mapping to devices that could be
adversely affected by an
unintended access. Disable all
mapping of interrupts.
Distributed Power-on devices along the
MXIbus link beginning with the
non-MXIbus System Controller
end of the MXIbus link. Make
sure the MXIbus System
Controller is the last to receive
power.
Power-off devices in opposite
order of power-on sequence
(power off MXIbus System
Controller first). Disable all A24
and A32 inward mapping to
devices that could be adversely
affected by an unintended access.
Disable all mapping of interrupts.