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2.2.1.4 Request To· Send (RTS) - The direction of RTS is from the LA120. The on condition of RTS
means that the LA120 intends to transmit data. After turning this circuit on, the LA120 waits for a
clear to send (transmit enable) condition before starting transmission.
2.2.1.5
Clear To Send (CTS) - The direction of CTS is to the LA120. Although the LA120 physically
receives this signal, it is not used for any purpose. Depending on the modem control protocol in use,
either RLSD, SRLSD, or a timeout after asserting RTS is used to provide a clear to send (transmit
enable) condition.
2.2.1.6 Data Set Ready (DSR) - The direction of DSR is to the LA120. The on condition of DSR
indicates that the user's equipment is capable of transmitting and receiving data signals. The off condi-
tion of DSR causes the LA120 to ignore all other interface inputs except ring indicator (RI). In full
duplex without EIA control, this circuit is assumed to always be in the on condition.
2.2.1.7 Signal Ground - This circuit establishes the common ground reference potential for all inter-
face circuits except protective ground. The circuit is permanently connected to the protective ground
circuit.
2.2.1.8 Carrier Detect (RLSD) - The direction of RLSD is to the LA120. The on condition of RLSO
indicates that data transmission from the user's equipment to the LA 120 is enabled. In full duplex
without EIA control, this circuit is assumed to always be in the on condition.
2.2.1.9 Secondary Request To Send (SRTS) - The direction of SRTS is from the LA120. In certain
half-duplex modes, the on condition of SRTS indicates that the LA120 is capable of successfully
processing the received data from the user's equipment. In restraint mode, the off condition of SRTS
indicates that the user's equipment should temporarily suspend the transmission of data. When SRTS
goes on, transmission may be resumed.
2.2.1.10 Speed Indicator (SPDI) (Full Duplex Only) - The direction of SPDI is to the LA 120. The on
condition of SPDI indicates that the baud rate is 1200, regardless of the rate selected by the operator.
The off condition indicates that the operator-selected baud rate is being used.
2.2.1.11
Secondary Carrier Detect (SRLSD) (Half Duplex Only) - The direction of SRLSO is to the
LA 120. The on condition of SRLSD indicates that the user's equipment is capable of successfully
processing the transmitted data from the LA 120.
2.2.1.12 Data Terminal Ready (DTR) - The direction ofDTR is from the LA120. The on condition of
DTR indicates that the LA 120 is capable of transmitting and receiving data signals. The off condition
of DTR may cause the user's equipment to set the DSR (data set ready) to the off condition. When
DTR is off, the LA120 ignores all interface inputs except ring indicator (RI).
2.2.1.13 Ring Indicator (RI) - The direction of RI is to the LA120. If data terminal ready (DTR) is
off, then the on condition of RI causes DTR to turn on. DTR remains on until data set ready (OSR)
turns on or 30 seconds elapses, whichever occurs first. Then DTR turns off. If DTR is on, the on
condition of RI causes a 30-second timeout. If no data is received in 30 seconds, DTR is pulsed low for
233 ms
±
10 percent.
2.2.1.14 Speed Select (SPDS) (Full Duplex Only) - The direction of SPDS is from the LA120. If the
operator-selected bau.d rate is 600 or higher, the LA120 asserts an on condition of SPDS; otherwise,
the LA120 holds this circuit in the off condition.
2.2.2
EIA Interface Cables
BC22A and BC22B interface cables are described in the following paragraphs.
2-9
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