Specifications

Chapter 4: Theory of Operation
39
This ID byte is the same identification byte discussed earlier in Chapter 3, BIOS
Configuration in the section on the EXM Menu.
The EXM expansion interface is provided on rows A, C, and D of the EPC-8A’s 4-row
DIN P2 connector. The subplane carries the EXM interface to other modules, such as to
EXM modules and the EXP-MX Mass Storage module. These EXM interface signals are
not passed through to the VMEbus.
Further information on the EXM expansion interface, its connectors, and standards for
building EXMs is available upon request.
VME Interface
Connectivity
The EPC-8A module connects to the VMEbus J1 connector directly and uses all of the
defined VMEbus lines except SERCLK, and SERDAT. Connection to the J2 connector is
through the subplane's 4-row DIN connector B row. Connections to the VME J2
backplane are on the B row and are power and ground, address lines A31–A24, data lines
D16–D31.
VMEbus System (Slot-1) Controller Functions
Every VMEbus system must have a System (Slot-1) Controller. The Slot-1 controller
provides the following functionality:
• Serves as the bus arbiter (priority or round-robin)
• Drives the 16 MHz SYSCLK signal
• Starts the IACK bus grant daisy chain.
• Provides Bus time-out function.
When configured as the Slot-1 controller, the EPC-8A detects and terminates data transfer
bus timeout. Once it sees either the DS0 or DS1 lines asserted, a counter is started. If the
counter expires before both DS0 and DS1 are deasserted, the EPC-8A asserts the VMEbus
BERR signal until both data strobes are deasserted. The duration of the VMEbus timeout
counter is approximately 100–120 µsecs.
When the EPC-8A is configured as the slot-1 controller, this timeout cannot be disabled
and the duration cannot be changed.
VMEbus Access
VMEbus accesses are available either by mapping a 64 Kbytes segment of the VMEbus
through the 0E0000–0EFFFF “E page” window or by direct mapping above 256 Mbytes.