Specifications

Chapter 6: The VMEbus Interface
Page 77
VMEbus Mapped Registers
The EPC-5A 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-5A’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-5A. The registers are dual-ported in that they are accessible both from VME and
from within the EPC-5A as ports in its I/O space. The VME mapped registers are de-
fined below.
Offset from
ULA
Upper byte Lower byte
0 ID (8141h) ID (8140h)
2 Device type (8143h) Device type (8142h)
4 Status/control (8145h) Status/control (8144h)
6 Slave offset (8147h) Slave offset (8146h)
8 Protocol (8149h) Protocol (8148h)
A Response (814Bh) Response (814Ah)
C Message high (814Dh) Message high (814Ch)
E Message low (814Fh) Message low (814Eh)
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 (i.e., if the
register is then read), but the new values will not effect the EPC-5A’s bus-
control logic. To use these fields for their intended purpose, they must be
set by I/O port accesses.