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2-24
System Overview
A Data II, III or UDC Channel Module, when set for a particular application, has program plugs
PP1 and PP2 set to support that application. The settings of these two plugs select up to four control
signals from the device connected to the channel at that node. Both data channel modules can trans-
mit the On or Off state of the selected signals across the TDM link to the data channel module at
the opposite node. These signals are then passed from the data channel module to the device con-
nected to the channel at that node. The program plugs on the data channel module at that node pass
the received control signals to the correct pins of the channel interface, so that the ~ receiving device
can properly interpret the state of the control signals. In this way the devices at each end of a TDM
channel link exchange control signals to begin and end data transmission.
An interface type is selected for each data channel in the system. The display for an interface type
indicates the signals selected by the program plugs at each node for transmission across the TDM
link. Each of the control signals from the device are labeled as SEND controls (because they are
being sent across the TDM link). The display also shows the control signals being received at each
node from the opposite-end channel (these control signals are labeled RCV). The position, left-to-
right, of the SEND and RCV control signals for each node reflect their selection as SEND control
signals 1-4, and RCV control signals 1-4. Examples of each standard interface type display appear
on the following pages.
Table 2-2 lists and defines all control signals multiplexed by a MEGAMUX PLUS data channel.
The table indicates those signals which are outputs from data terminal equipment (DTE) and inputs
to data communications equipment (DCE); conversely, those control signals which are inputs to
data communications equipment are outputs from data terminal equipment.
Control signals are often "crossed over" to complete a link between two devices on opposite ends
of a data channel. If a CPU port communicates with a terminal, for example, both devices are data
terminal equipment (DTE). Each device sends Data Terminal Ready (DTR) as an input into the
channel interface, and receives Data Set Ready (DSR) as an output from the channel interface. The
setting of the program plugs for this arrangement would select DTR as a SEND control at each
node, and the interface display would indicate that selection. The control signal would be transmit-
ted across the link; the RCV control signal would be passed out through the channel interface DSR,
with the program plug settings placing the signal on the proper interface connector pin. The inter-
face type display would label the RCV control as DSR. Thus the CPU port and terminal would each
indicate readiness for operation through the state of control signal DTR, and recognize the readiness
of the opposite-end device through the state of RCV control signal DSR.
Control signals may also be "forced On" or "forced Off' by the TDM system. The On or Off state
of a signal is set in the TDM system. The interface type may define a signal to be forced On; this
allows data transmission to occur whether or not the device connected to a channel turns control
signals on. For example, Interface Type CTB1 forces SEND controls RTS and DTR On at both the
Node M and Node S channel interfaces. The RCV control signals DCD (Data Carrier Detect) and
the DSR (Data Set Ready) at both interfaces remain On, regardless of the state of control signals
from the devices connected to channel interfaces at either node. The MEGAIVIUX PLUS operator
is also given the ability to set signals On or Off through a Diagnostic Functions routine.
SEND Control 1 has special significance. The input signal selected as SEND Control 1 is treated as
a "fast control." Normally, the control signals are transmitted as separate bits of control data, that
are distinct from the channel transmit data multiplexed across the aggregate link. Because channel
data have priority over channel control data, there is a slight delay (on the order of milliseconds)
between a control signal transition at the channel interface, and the transition appearing as an output
at the opposite-end channel interface.
036R660-000, Issue 10
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