Motorola MCP750 Specifications download pdf (Page 56)

evice Driver Programming

4-8

Bus Time-Out 4

The (H)VME bus measures with a bus timer the duration of data transfers accessing slave

devices. If a data transfer malfunctions, the bus timer detects the malfunction and gener-

ates a bus time-out, preventing a dead VME slave from hanging the I/O channel.

After a device applies an address to the bus and asserts the address strobe (AS*) and data

strobe (DS*) signals, the VME device addressed must assert the data transfer acknowl-

edge (DTACK*) signal within 51.2 microseconds. If it does not assert the DTACK* signal

in a timely manner, the HVME bus controller asserts bus error (BERR*) and generates a

system fault.

Data transfer malfunctions on the bus occur for the following reasons:

• Invalid address

• Invalid address modiﬁer

• Invalid transfer

• Nonexistent device addressed

• Device correctly addressed but malfunctioning

The kernel tries to recognize VME bus errors and determines their cause. The most com-

mon response by the kernel is to panic the system. A panic halts the system so that the

administrator can fix a malfunctioning board or device, or take some other corrective

action.

An alternative system service, iobus_err(2), can handle some types of VME bus

errors without panicking the system. This service supports environments in which panick-

ing the system is an undesirable response to bus errors, such as real-time or production

mode. See Chapter 16, “Special Considerations”, “Device Drivers and VME Bus Errors”

and the iobus_err(2) man page for details on this service.

VME Device Address Assignment and Conﬁguration 4

The HN6200 or HN6800 Architecture Manual documents the range of addresses reserved

within the system memory map for I/O purposes. Another document, the (H)VME Address

Speciﬁcation, further documents this I/O address space and divides it into ranges occupied

by those (H)VME vendor boards Concurrent Computer Corporation supports. Prototype

devices use selected areas within this mapping, which includes instructions for selecting

appropriate device addresses. The system probes the address range occupied by the device

to detect and identify it.

Jumpers, switches, or programmable assemblies (Programmable Read Only Memory

(PROM) or Programmable Array Logic (PAL)) normally set VME device addresses:

• If set by jumpers or switches, refer to the device installation manual for

selecting the proper valid address and address modiﬁer.

• If set by programmable assembly, and if either the address falls outside

valid VME address space or generates the wrong VME address modifier,

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Motorola MCP750 Specifications Page 56