A one (single) level Arbiter only honours requests on a single, predefined, Bus Request
(BR) line and relies on the daisy-chain structure for priority determination.
The VMEbus specification defines four BR levels, BR0* to BR3* and specifies that
BR3* should be used by one-level Arbiters. This is the default setting of the
PME 68-1B Arbiter but the user may select any other BR level if required; see
Chapter 4.
Using PME 68-1B as the System Bus Arbiter
A bus arbitration scheme with a minimum of system overhead can be built using the
on-board bus Arbiter. This uses four types of VME signal: "Bus Request", "Bus Grant In
and Out", "Bus Clear" and "Bus Busy".
The BR* signal is driven low by a VME module to inform the Arbiter that it requires
control of the DTB (Data Transfer Bus).
When the current bus master releases control of the bus and a bus request is pending, a
new arbitration cycle is initiated.
The Arbiter then starts the bus grant daisy-chain by driving a Bus Grant Out
(BGxOUT*) signal low. This BGxOUT* signal is the BGxIN* signal to the next slot in
the motherboard. It informs the board in this slot that it can become Bus Master if it has
requested DTB control. If the board has not requested DTB control, it must pass on the
BGxIN* signal to the next slot, and so on.
When the DTB requester receives the BGxIN* signal, it acquires DTB control by
driving Bus Busy (BBSY*) low and de-activating its BR* signal. This informs the Arbiter
that DTB acquisition is complete. Figure 12 shows the bus grant daisy-chain.
PME68-1B Manual
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Содержание PME 68-1B
Страница 8: ...Figure 1 The PME 68 1B Board Photograph not available in PDF PME 68 1B Manual Page 2 Issue 5...
Страница 12: ...Figure 4 Component Layout Diagram P5 P4 P3 BAT PME 68 1B Manual Page 6 Issue 5...
Страница 56: ...Figure 14 Link Areas BAT PME68 1B Manual Page 50 Issue 5...
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Страница 59: ...Figure 15 Location of System EPROM Area 1 2 3 4 1 2 3 4 WC1 WC2 PME68 1B Manual Page 53 Issue 5...
Страница 80: ...Figure 41 Location of BERR Jumpers 1 2 3 4 WK1 WK2 BR26 2 3 1 BAT PME68 1B Manual Page 74 Issue 5...
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