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2-28
S E N D
RCV
CPU to Terminal - Polling (CTP2)
This interface type supports a polling operation. A front-end processor port is connected to the
channel interface at Node M; a terminal is connected to the channel interface at Node S. Polling is
performed by the front-end processor; the terminals at Node S responds to a poll by raising RTS
(Request to Send), which results in output signal DCD (Data Carrier Detect) going On at the front-
end processor interface.
As shown in Figure 2-7 B, the state of Request to Send is transmitted in each direction across the
link; at the opposite-end channel interface, the signal is "wrapped" back to the channel interface as
a control input signal. That input is transmitted back across the link, and passed through the channel
interface as output signal CTS (Clear to Send). The result of this arrangement (called a remote RTS-
CTS wrap) is that a delay is created between the Off-to-On transition of Request to Send, and the
Off-to-On transition of Clear-to-Send. The delay is greater than any potential delay in transmitting
Request-to-Send across the link once; this ensures that the front-end processor will detect the Off-
to-On transition of DCD before the remote terminal begins to transmit data.
S E N D
RCV
CPU to Modem - Polling (CPM2)
This interface type supports a polling operation. A front-end processor port is connected to the
channel interface at Node M; a master modem on a multipoint line is connected to the Node S chan-
nel interface. Polling is performed by the front-end processor; a remote terminal connected to a re-
mote modem responds by raising Request to Send, which results in output signal DCD (Data
Carrier Detect) going On at the front-end processor site.
036R660-000, Issue 10
CPU/TERM - POLLING
NODE M
: RTS CTS
DTR
: DCD CTS
DSR
CPU/MODEM - POLLING
NODE M
: RTS CTS
DTR
: DCD CTS
DSR
System Overview
NODE S
RTS CTS
DTR
DCD CTS
DSR
NODE S
DCD CTS
DSR
RTS CTS
DTR
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