Control Data Corporation 6000 Series Reference Manual page 11

FUNCTIONAL DESCRIPTION
Two stations, the local and the remote, constitute a communication network (Figure
1-1). The local station includes the 6000 Series computer and the 6675 Data Set
Controller; the remote station contains the other computer and peripheral equipment.
With the exception of the computer and associated equipment, each station contains
compatible DSC' s and identical data sets. A leased transmission line connects the
stations through the terminating data sets.
Each DSC operates in half-duplex; that is, data is exchanged in only one direction at
a time. When one DSC is in the Transmit mode, the other must be in the Receive
mode; to exchange data in the opposite direction, both DSC' s must reverse modes.
Although data is transferred in half-duplex, control signals and response indications
utilize the full-duplex capability of the data set. Both the Response and the Control
signals are automatic functions.
During transmission from the local station to the remote station, the local station is
in the Transmit mode and the remote station is in the Receive mode. With both
stations active, data at the local station is exchanged in the following manner
(Figure 2-2):
1) The local computer selects a local DSC (0-3) for the Transmit mode.
2) The DSC transmits a sync word.
3) The computer selects the DSC and sends it a data word.
4) The remote station acknowledges the sync word with a Response.
5) The DSC disassembles the first parallel data word received from the
computer and transmits it serially, highest order bit first.
6) By sampling the status-all bits, the computer detects when a specific DSC
(0-3) requires another data word for transmission (or contains a complete
data word received from the remote station).
7) When the complete data block is transmitted, the DSC transmits the cyclic
code word.
Rev. A
2-3