Specifications

EPC-6A Hardware Reference

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)