Interfaces; Registers - HP 9835A Programming Manual

134
I/O Handling
The backplane serves as an intermediary between the processor and the peripheral interfaces.
The internal addressing of the backplane is transparent, both to the interfaces and to the
programmer.
Interfaces
The processor does all its talking, through the backplane, to peripheral interfaces, never di-
rectly to a peripheral itself. An interface is a complex electronic circuit which provides mechani-
cal, electrical, data format, and timing compatibility between the 9835A/B and the peripheral
device to which it is connected. From a programmer's point of view, the primary task of an
interface is to provide a means of exchanging data between the 9835A/ B and the peripheral. A
well-designed interface isolates the programmer from the details of electronics and timing,
appearing as a simple "black box" through which information is exchanged.
The processor can talk to as many as 14 peripheral interfaces through the backplane. Each can
be talked to individually, and there may be a mix of peripherals using programmed, interrupt,
or DMA types of transfers.
Individual I/O operations (Le., exchanges of single words) occur between the processor and
one interface at a time, although interrupt and DMA modes of operation can be programmed to
allow automatic interleaving of individual operations.
A peripheral is addressed through a select code and a transfer occurs through four special
registers reserved for the purpose. These will each be discussed shortly.
Discussion of the techniques and methods presented in this chapter uses the common HP
interfaces as examples. A full discussion of the operation of these interfaces can be found in the
Interfacing Concepts manual (HP part number 09825-90060) and also from your Sales and
Service office (list in Appendix K).
Example programs utilizing various I/O techniques with a number of the standard interfaces
can be found in Appendix H.
Registers
All I/O operations go through a set of four registers maintained by the 9835A/B. The
four - R4, R5, R6, and R7 - are the sole means of communicating data between the proces-
sor and peripheral interfaces. While the registers are actually on the interface cards, they may
be thought of as being in the computer memory. This makes the cards themselves accessible by
simple memory referencing instructions.