Functional Blocks

VMEbus to Local Bus Interface

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The VMEbus to local bus interface allows an off-board VMEbus master access to onboard resources. The VMEbus to local bus interface includes the VMEbus slave, write post buffer, and local bus master.

Adhering to the IEEE 1014-87 VMEbus Standard, the slave can withstand address-only cycles, as well as address pipelining, and respond to unaligned transfers. Using programmable map decoders, it can be configured to provide the following VMEbus capabilities:

Addressing capabilities:

A24, A32

Data transfer capabilities: D08(EO), D16, D32, D8/BLT, D16/BLT, D32/BLT, D64/BLT (BLT = block transfer)

The slave can be programmed to perform write posting operations. When in this mode, the chip latches incoming data and addressing information into a staging FIFO and then acknowledges the VMEbus write transfer by asserting DTACK. The chip then requests control of the local bus and independently accesses the local resource after it has been granted the local bus. The write-posting pipeline is two deep in the non-block transfer mode and 16 deep in the block transfer mode.

To significantly improve the access time of the slave when it responds to a VMEbus block read cycle, the VMEchip2 contains a 16 four-byte deep read-ahead pipeline. When responding to a block read cycle, the chip performs block read cycles on the local bus to keep the FIFO buffer full. Data for subsequent transfers is then retrieved from the onchip buffer, significantly improving the response time of the slave in the block transfer mode.

The VMEchip2 includes an onchip map decoder that allows software to configure the global addressing range of onboard resources. The decoder allows the local address range to be partitioned into two separate banks, each with its own start and end address (in increments of 64KB), as well as set each bank’s address modifier codes and write post enable and snoop enable.

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Motorola MVME172 manual VMEbus to Local Bus Interface, Functional Blocks