Radstone PME 68-1B, Manual

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VMEbus Interrupt Handling
All on-board interrupts have a higher priority in the internal interrupt acknowledge
daisy chain than VMEbus interrupts at the same interrupt level.
VMEbus interrupt signals, IRQ1 to IRQ7, can be enabled/disabled separately using
jumpers. Refer to Chapter 4 for further details. Default condition is all interrupts en-
abled.
VMEbus Interrupt Acknowledge Daisy Chain
The VMEbus specification defines 7 interrupt request signals IRQ1* to IRQ7*, plus 3
special control signals which allow an unlimited number of interrupt sources in the
system. Each of the seven VMEbus IRQ signals may be shared by two or more
interrupter modules. The Interrupt Acknowledge (IACK*) Daisy Chain is used to
ensure that only one of these modules places its interrupt vector on data bits D0 to D7 at
the required time.
An active Interrupt Acknowledge (IACK*) signal informs all cards within the system
that the current read cycle is an interrupt vector acquisition. This signal is connected at
slot 1 of the motherboard to the Interrupt
Acknowledge In (IACKIN*) signal (Figure 10). The IACKIN* signal passes through
each board on the bus.
If a board receives an active IACKIN* signal and has not produced an interrupt on that
level, its control logic must pass the signal through its own IACKOUT* signal to inform
the next board (if it is the interrupt requester), that it can place its interrupt vector onto
the data bus.
A functional diagram of the Interrupt Acknowledge Daisy Chain is shown below.
Figure 10 The Interrupt Acknowledge Daisy Chain with Slot 1 Interrupt Handler.
The low driven IACK* signal is wired to slot 1 of the VME motherboard and runs down
the IACK daisy-chain.
Slot 1 Slot x
Slot x+ 1
Interrupt
Control
Logic Interrupt
Control
Logic
Interrupt
Control
Logic
IACK*
IACKIN*
Backplane Link
IACKOUT* IACKOUT*
IACKOUT*
PME68-1B Manual
Page 35 Issue 5