RadiSys EPC-5A, Hardware & Software Reference Manual

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Chapter 6: The VMEbus Interface
Page 79
Low-Level Programming the VMEbus Interface
It is recommended that rather than performing accesses in this low-level hardware
dependent form, the Bus Manager component of the EPConnect software package
be used instead.
VMEbus Accesses
Two examples are given here including both a verbal description and the Microsoft C
source code for performing VMEbus accesses through the memory window.
Example #1 performs a 16-bit read from the VMEbus A16 space.
1. Set the VME access bit in Register 8104h.
2. Determine the correct address modifier for A16 supervisory access (2Dh).
3. The unused address lines A31-A16 may float when not being used. Registers
8150h and 8130h must be set so that each line is a 1.
Set register 8130h to FCh and register 8150h to FFh.
4. Set the access mode in the VME Modifier Register (8151h) as follows:
VME WA23-22 BORD IACK AM5 AM4 AM2 AM1
(Note that register bits are not defined for the VMEbus address modifier lines AM3
and AM0 since, for all defined address modifier values in the VMEbus specification,
AM3 is 1 and AM0 is the inverse of AM1. Therefore these two bit values are
generated by hardware.)
Bits 7 & 6 Since the A16 space does not use VMEbus address lines A23 &
A22, set these values to 1.
VME WA 23-22 = 11
Bit 5 Set the byte order to “little endian”.
BORD = 0
Bit 4 Clear the IACK bit so this is not an interrupt acknowledge cycle.
IACK = 0