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EPC-6A Hardware Reference
26
VMEbus Mapped Registers
EPC-6A follows the lead of the VXIbus specification in defining a standard set of
configuration registers that are mapped into the VMEbus A16 space and thus accessible
by other VMEbus modules. These registers are 16-bit registers occupying 64 bytes of A16
space at a base address defined by the EPC-6A’s logical address. The base address is:
1111 111a aa00 0000
where
aaa
is the value of the ULA field in the response register at I/O port 814A.
The VME-mapped registers are a subset of those defined previously as I/O ports in the
EPC-6A. The registers are dual-ported in that they are accessible both from VME and
from within the EPC-6A as ports in its I/O space. The VME mapped registers are defined
in the next table.
Table 4-2. VME mapped registers
The registers occupy the first 16 bytes of the 64-byte space; the remainder of the space is
undefined. (Actually, the registers are mapped into each 16-byte chunk of the 64-byte
space.)
Reads and writes of the registers from VME and as I/O ports have identical results and
effects except for the following:
1.
Changing the RELM, ARBPRI, and ARBM fields of the status/control register from
VME will appear to have changed the fields (for example, if the register is then read),
but the new values will not effect the EPC-6A’s bus-control logic. To use these fields
for their intended purpose, they must be set by I/O port accesses.
2.
A read of the response register from VME clears the LOCK bit (immediately after the
current value of the response register is returned).
Offset
Upper byte
Lower byte
0
ID (8141)
ID (8140)
2
Device type (8143)
Device type (8142)
4
Status/control (8145)
Status/control (8144)
6
Slave offset (8147)
Slave offset (8146)
8
Protocol (8149)
Protocol (8148)
A
Response (814B)
Response (814A)
C
Message high (814D)
Message high (814C)
E
Message low (814F)
Message low (814E)
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