U UNIVAC 1218 - Bitsavers

All steps of this sequence except the first are repeated for every
external function message until the number of words specified in the
external function buffer have been transferred.
The computer also has the option of forcing any word of an external
function buffer; that is, it can, under program control, send an external
function code regardless of the state of the external function request
line for that channel. This option is necessary so that the computer
can override whatever function the peripheral equipment is performing in
order to re-establish positive control.
2.1.2 INTERCOMPUTER OPERATION
Any I/O channel can be selected as an intercomputer channel by a channelassociated
switch on the control panel. The selection of a given channel as
an intercomputer channel affects only the logic concerned with the output and
external function buffers. A channel which is sending data or external functions
to a given peripheral device holds the data in the output registers
for a fixed minimum time period, after which any output or external function
request on any other channel which is part of the same 4-channel group can cause
the data to be changed. However, a channel sending data or external functions
to another computer must hold the information in the output register(s) until
the receiving computer acknowledges receipt of those words. This acknowledge
signal is received on what is known as the output data request line (when not in
intercomputer mode). This line, in the intercomputer mode, is known as the
resume line.
This resume line is connected to the input acknowledge line of the receIvIng
computer (see .Figure I-C-2). Activation of the resume signal on the transmitting
computer channel causes the setting of the resume flip-flop for the
even or odd group of four channels. It is this flip-flop whichv when set, .
allows the transmitting computer to proceed to the next highest priority output
function (the next output data word or external function message). If an
output channel is holding data for another computer and no resume is received
from that computer, the output registers are tied up indefinitely and no output
buffers or external function buffers to other equipment can proceed. To limit
the possibility of this hang-up occurring, two instructions are provided by
which the computer program can monitor the status of the resume flip-flop.
These instructions are: skip on no resume (50 57k) and set resume (50 20k).
The former allows examination of the resume flip-flop, and the latter allows
the program to correct the situation in which the hang-up exists.
2.1.3 FORCED TRANSFERS (OVERRIDE)
The computer has the ability, under program control, to force the transfer of
a word from an external function buffer or output buffer regardless of the
state of the request line on that channel. Peripheral devices should have the
ability to accept such forced transmissions, realizing that loss of data or
even loss of a previous external function word is unimportant under conditions
when this option is used. Instructions 50 26 and 50 27 are the override
instructions used to accomplish forced transfers.
I-C-7