The computer and external device repeat these
sequences for each successive word of data until they
have transferred the block of data words specified by
the input buffer control words.
Intercomputer I/O Operations
Parallel channels are often used to communicate
between two stand-alone computers. In this mode, the
computers will appear as external devices to each other.
One computer will be designated the transmitting
(outputting) computer; the other computer will be
designated the receiving (input) computer. A similarity
exists between intercomputer channels and normal
channels. The two cables are identical; in this mode all
the signals remain the same except ODA and ODR,
which become ready and resume respectively. Figure
7-29 illustrates the interface between two computers.
The two types of information transferred over the
intercomputer channels data lines are command words
and data words.
Command words are used to
exchange external function data, which includes
external functions, forced external functions and
external function buffer words, between the
transmitting computer and the receiving computer.
Data words are sent as part of output data buffers from
the transmitting computer and accepted as part of the
receiving computers input data buffer. Command
words use additional interface signals to identify their
function and to coordinate their transfer. When the
transmitting computer generates an external function
acknowledge signal with the ready signal, the data word
transmitted is identified as a forced external function or
an external function command word. The external
interrupt enable signal is set to identify the command
word as an external function command word. If the
external interrupt enable is not set, the command word
is a forced external function.
The sequence of events for intercomputer
command word and data transfers is as described in the
INTERCOMPUTER COMMAND WORD
TRANSFER (BUFFERED). Whenever the
transmitting computer has an EFR line and the receiving
computer has an EIE line, transfer of buffered
command words is possible. As you read, refer to figure
7-29; we designate computer A as the sending computer
and computer B as the receiving computer. Whenever
an EF buffer has been established in the transmitting
computer for a channel, the transmitting computer and
the receiving computer do the following to transfer a
Computer B, under program control, sets the
EIE line when it is ready to accept an EF
command word from computer A.
In accordance with internal priority, computer
A recognizes the EIE as an EFR and places the
EF code on the data lines. The EF command
word will be held on the data lines until
computer B sets the resume line or until
computer As program intervenes to resolve the
no resume condition.
Computer A sets the EFA line to indicate that
the EF command word is on the OD lines.
Figure 7-29.Intercomputer interface.