
VXIbus Interface
Where a bit position has been described by a 0 or 1, the bit is a ROM bit, and writing
to it has no effect. Bit positions labeled "RAM" are register storage bits that have no
special hardware interpretation. Unless otherwise noted below, all registers and bit
values are readable and writeable.
EXM Configuration Reg
reserved
Slot Number
96
This register is used to assert a -EXMID signal to a specific EXM slot. For instance,
writing the value 02 causes the -EXMID signal to be asserted for slot 2. Writing FF
causes all -EXMID signals to be deasserted. Slot number can be assigned a
value 0-5.
Battery Backed Register
ARBPRI
RELM
ARBM
RAM
SDMA
EVME
0
8102
This register is used by the BIOS to control certain system configuration options.
The BIOS transfers information into the register at initialization time from the battery
backed CMOS RAM.
ARBPRI Arbitration priority. This defines the level at which the EPC-7 will ar-
bitrate for the VMEbus. 11 means 3, 10 means 2, 01 means 1, 00 means
0.
RELM
Bus release mode. If set, the bus release mode is ROR (release on
request); otherwise it is the VXI RONR "fair requester" mode (request on
no request).
ARBM
Arbitration mode. This bit is pertinent only if the EPC-7 is jumpered to be
the VXIbus slot 0 controller. If set, the EPC-7 is a priority arbiter; other-
wise it is a round-robin arbiter.
7
7
SDMA
SCSI DMA channel. If set (1), the SCSI controller uses 16-bit DMA
channel 5. Otherwise it uses 8-bit DMA channel 0. This bit can be used
to avoid conflicts with EXM modules that might use one of these DMA
channels.
EVME
Enable VME master access. If set (1), master accesses to the VXI data
transfer bus are enabled. Otherwise these accesses are mapped to the
EXM expansion interface (using the lower 24 bits of address) and
typically time out, meaning that writes appear to complete and reads return
F's. EVME can be used to inhibit VXI accesses by certain operating
systems (e.g., OS/2) which probe through the address space during
initialization.
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