Control Data Corporation INFORMATION SYSTEMS TERMINAL II Hardware Maintenance Manual page 83

• These eight lines are driven by the terminal during
module outputs (external write active) and by the
device during terminal inputs (external read active).
NOTE
Unless a device is requested to place
data on the data lines, its data line
drivers must be in the high impedance
state.
• Address Line 0 through Address Line 5 - These six
output lines, driven by the terminal, determine the
device to receive or send data and the kind of func-
tion to be performed on the data.
• External Write - When active, this control signal,
driven by the terminal, indicates that the informa-
tion on the data bus goes from the terminal to the
device defined by the address bus. The device
should get ready to receive data.
• External Output - This control signal, driven by the
terminal, complements the External Write signal.
External Write defines the direction of the flow of
data; External Output active indicates that the data
is on the data lines and stable; and the device
defined by the address lines is to act on it.
• External Read - When active, this control signal,
driven by the terminal, indicates that the device
defined by the address lines is to place the
requested information on the data lines. Only at
this time can the device turn on its data line
drivers.
• External Ready - When active, this signal, driven by
the addressed device, indicates that the I/O cycle
initiated by the terminal has been completed by the
device (data was taken by the device during terminal
outputs or placed on the data lines by the device
during terminal inputs).
• Interrupt Line - This line is common to all devices
on the parallel channel and any of them can activate
it at any time. Its purpose is to tell the proces-
sor to process data over the parallel channel, since
all the activities on this channel are initiated by
the processor. When the processor has acknowledged
and serviced the interrupting device, it must deac-
tivate this line.
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