EIA Connection Types (X.25)

Introduction
A device connected to a port can establish and maintain a connection only after a proper handshake using control signals has occurred. This is called the EIA connection establishment and should not be confused with the physical connection to the port. A port’s physical level is in an idle state when there is no EIA connection and when it is disconnected.
Connection Types
Different types of EIA connections can be used depending on the setting of the Connection Type parameter (in the Port Record):
• SIMP: Simple connection with no control signal handshake.
• SIMPv: The modem switches from leased to dial-only mode when leased line goes down. • DTR: Connection with DTR control signal handshake.
• DTRD: Same as DTR but control signals drop.
• DTRP: When DTR needs to be passed end-to-end.
• DIMO: Dial modem attached to the port and does dial-in/out handshake.
• DIMOa: Same as DIMO except DSR not raised.
• DIMOb: Same as DIMO except DSR follows DTR.
• DIMOv: The port handshakes with attached V.25 bis dial modem.
• EMRI: Port emulates a modem and does dial-in/out handshake with RI.
• EMDC: Port emulates a modem and does dial-in/out handshake with DCD.
Disable/Enable Ports
When a port is disabled, its EIA connection type is changed to NULL and all input control signals are ignored. All output control signals are dropped. If the parameter Port Control is set to MB (Make Busy), RI (pin 22) is raised. When a disabled port is enabled, its EIA connection type changes back to the configured EIA connection type. If the parameter Port Control is set to MB (Make Busy), RI (pin 22) is lowered.
SIMP (Simple) Connections
Introduction
This connection type is used when terminals are connected to a port with a cable that has minimal conductors. Most control signals are absent because of the lack of conductors. This kind of cabling provides only ground, transmit and receive data, transmit and receive clock.
• Note
For DCE ports, DCD, DSR, and CTS control signals remain high. For DTE ports, RTS, DTR, and DRO control signals remain high.
DCE EIA Status for SIMP
Connection - Outbound control signals DCD, DSR, and CTS (pins 8, 6, and 5) are held high at all times. On asynchronous PAD ports, if EIA data restraint is enabled, CTS and RTS (pins 5 and 4) may change according to the requirements of data restraint. Inbound control signals DTR and MB (pins 20 and 25) are ignored. DTE EIA Status for SIMP
Connection - Outbound control signals RTS, DTR, and DRO (pins 4, 20, and 14) are held high at all times. On asynchronous PAD ports, if EIA data restraint is enabled, DCD and DRO (pins 8 and 14) may change according to the requirements of data restraint. Inbound control signals are ignored: DCD, DSR, and CTS (pins 8, 6, and 5).
SIMPv
This is a combination of SIMP and DIMOv Connection Types. It starts as SIMP and after the SIMP connection goes down (leased line), the Connection Type switches to DIMOv (dial line). This is used with dial restoral modems.
DTR Connections
Use this connection type when the device connected to the port provides basic control signals to maintain the EIA connection. The remote user calling the device through a PAD port will know if the device is disconnected or powered down because the call will not be completed. Users connecting to a PAD port will access the terminal handler. They can manually call or be automatically connected if the port is configured for autocalling.
DCE Port States:
Idle - DCD, DSR, and CTS (pins 8, 6, and 5) are held high at all times.
Connection - The port monitors DTR (pin 20). If it is detected high, the EIA connection is established. RTS is ignored. A device on the asynchronous PAD port connects to the terminal handler. A call from the network is accepted if DTR is active. On asynchronous PAD ports, if EIA data restraint is enabled, CTS (pin 5) may go low during the connection.
Disconnection - The port monitors DTR (pin 20). If it goes low for more than 1.5 seconds, disconnection occurs. A call clear is sent to the network if disconnection occurs. DTRD Connections Introduction Use this connection type only on asynchronous PAD ports. Some devices require APAD ports to lower the control signals for a short period after the call is terminated. DCE Port States : (This table describes the conditions during various states for
DTR connections on DCE ports.
Idle - DCD, DSR, and CTS (pins 8, 6, and 5) are held high at all times.
Connection - The port monitors DTR (pin 20). If it is detected high, the EIA connection is established. RTS is ignored. A device on the asynchronous PAD port connects to the terminal handler. A call from the network is accepted if DTR is active. On asynchronous PAD ports, if EIA data restraint is enabled, CTS (pin 5) may go low during the connection.
Disconnection - The port monitors DTR (pin 20). If it goes low for more than 1.5 seconds, the port drops DCD, DSR, and CTS (pins 8, 6, and 5) for one second. A call clear is sent to the network, and the port returns to the idle state. During the control signal drop, the port cannot receive calls from the network. If the user clears the call by entering [CLR] the signals do not drop. If the call is cleared by an X.29 invitation to clear, the signals remain high when the parameter Invitation to clear = CLRWO: the signals are dropped when the parameter Invitation to clear = CLRWD.
DTE Port States:
(This table describes the conditions during various states for DTR connections on DTE ports)
Idle - RTS, DTR, and DRO (pins 4, 20, and 14) are held high at all times.
Connection - The port monitors DSR (pin 6). If it is detected high, the EIA connection is established. DCD is ignored. A device on the asynchronous PAD port connects to the terminal handler. A call from the network is accepted if DSR is active. On asynchronous PAD ports, if EIA data restraint is enabled, DRO (pin 14) may go low during the connection.
Disconnection - The port monitors DSR (pin 6). If it goes low for more than 1.5 seconds, the port drops RTS, DTR, and DRO (pins 4, 20, and 14) for one second. A call clear is sent to the network, and the port returns to the idle state. During the control signal drop, the port cannot receive calls from the network. If the user clears the call by entering [CLR], the signals do not drop. If the call is cleared by an X.29 invitation to clear, the signals remain high when the parameter Invitation to clear = CLRWO: the signals are dropped when the parameter Invitation to clear = CLRWD.
DTRP Connections Port States forDTRP
(Originate End: Autocall Configured)
Idle - DCD, DSR, and CTS (pins 8, 6, and 5) are maintained low.
Connection - The port monitors DTR (pin 20). If it is high or goes high, the port makes a network call according to the autocall mnemonic and waits for the call to be accepted by the remote PAD. If the call is accepted, the port raises DCD, DSR, and CTS (pins 8, 6,and 5) and the connection is established. If the call is not accepted, the port continues to autocall until it reaches the autocall limit. DCD, DSR, and CTS (pins 8, 6,and 5) will remain low.
Disconnection - The port monitors DTR (pin 20). If it goes low for at least 50 milliseconds, the port drops control signals DCD, DSR, and CTS (pins 8, 6, and 5), clears the call, and returns to the idle state. If the call is cleared from the network or by the user entering [CLR] at the port, the port, immediately drops the controls signals.
DTRP connections on DTE ports:
Idle - RTS, DTR, and DRO (pins 4, 20, and 14) are maintained low.
Connection - The port monitors DSR (pin 6). If it is high or goes high, the port makes a network call according to the autocall mnemonic and waits for the call to be accepted by the remote PAD. If the call is accepted, the port raises RTS and DTR (pins 4 and 20) and the connection is established. If the call is not accepted, the port continues to autocall until it reaches the autocall limit. RTS,DTR and DRO (pins 4, 20, and 14) remain low. Disconnection - The port monitors DSR (pin 6). If it goes low for at least 50 milliseconds, the port drops RTS, DTR, and DRO (pins 4, 20,and 14), clears the call, and returns to the idle state. If the call is cleared from the network or by the user entering [CLR] at the port, the port immediately drops the controls signals.
Port States forDTRP (Answer End: No AutoCalling) connections on DCE and DTE ports.
Idle - DCD, DSR, and CTS (pins 8, 6, and 5) are maintained low.
Connection - When a call arrives from the network, the port raises DCD, DSR,and CTS (pins 8, 6,and 5) and monitors DTR (pin 20). If DTR is high or goes high, the PAD accepts the call.
Disconnection - The port continues to monitor DTR (pin 20). If it goes low for at least 50 milliseconds, the port drops DCD, DSR, and CTS (pins 8, 6,and 5), clears the calls and returns to the idle state. If the call is cleared from the network or by the user entering [CLR] at the port, the port immediately drops the controls signals. If DTR is not raised within three seconds after the call arrives from the network, the port drops the control signals and clears the call.
DTRP connections on DTE ports.
Idle -RTS, DTR, and DRO (pins 4, 20, and 14) are maintained low.
Connection -When a call arrives from the network, the port raises RTS, DTR,and DRO (pins 4, 20, and 14) and then monitors DSR (pin 6). If DSR is high or goes high, the PAD accepts the call.
Disconnection - The port continues to monitor DSR (pin 6). If it goes low for at least 50 milliseconds, the port drops RTS, DTR, and DRO (pins 4, 20, and 14), clears the call, and returns to the idle state. If the call is cleared from the network or by the user entering [CLR] at the port, the port immediately drops the controls signals. If DSR is not raised within three seconds after the call arrives from the network, the port drops the control signals and clears the call.
DIMO Connections
Introduction
Use this connection type with a crossover cable to connect a dial modem to the DCE port. When calls are made, the port handshake uses the modem control signals. There are several types of operation that can occur with this connection type including:
• Dial In
• Dial Out
• Dial In/Dial Out Collision
Dial In
When a user dials into a PAD port through a telephone network, the connection depends on whether the port is configured for manual calling or autocalling. When the port is configured for manual calling, the user is connected to the terminal handler when the EIA connection is completed. When the port is configured for autocalling, the call request must be accepted before the EIA connection is completed. This prevents users from being charged for the telephone call if the call cannot be completed.
States for DIMO (Dial In, No Autoconnect).
DCE ports
Idle - DCD, DSR, and CTS (pins 8, 6, and 5) are maintained low.
Connection - The port monitors MB (pin 25) [modem RI]. If it goes high, the port raises DSR, DCD, and CTS (pins 6, 8, and 5) [modem DTR, RTS, and DRO (pins 20, 4, and 14)], then waits up to 240 seconds for DTR and RTS (pins 4 and 20) [modem DSR and DCD] to go high. If the timer expires, DCD, DSR, and CTS (pins 8, 6, and 5) are dropped, the network call is cleared, and the port returns to the idle state. The connection is established when DTR and RTS go high. After the port receives the MB signal, it cannot receive calls from the network, so the dial procedure can be completed.
Disconnection - The port monitors DTR and RTS (pins 20 and 4) [modem DSR, DCD]. If either goes low for at least 50 milliseconds, the port immediately drops DCD, DSR, and CTS (pins 8, 6 and, 5) [modem RTS and DTR] and a call clear is sent to network. A PAD port also drops the control signals and returns to the idle state if the user fails to establish a call within the time configured by the Port Record parameter Call Accept Timeout or makes three unsuccessful call attempts. If the call is cleared by an X.25 clear from the network, the port immediately drops DCD, DSR,and CTS [modem RTS, DTR, and DRO]. The port waits for DTR and RTS [modem DSR, DCD] to go low, at which time the port returns to idle state, ready for another dial-in sequence. If the call is cleared from the port by the user entering [CR] at the port, control signals are not dropped until Call Accept Timeout expires. The port is unavailable to take network calls while waiting for the control signals from the modem to drop. If a call is cleared by an X.29 invitation to clear, the signals remain high when the parameter Invitation to clear = CLRWO: the signals are dropped when the parameter Invitation to clear = CLRWD.
DTE ports
Idle - RTS and DTR (pins 4 and 20) are maintained low.
Connection - The port monitors RI (pin 22). If it goes high, the port raises RTS, DTR, and DRO (pins 4, 20, and 14) then waits up to 240 seconds for DSR and DCD (pins 6 and 8) to go high. If the timer expires, RTS, DTR, and DRO (pins 4, 20, and 14) are dropped, the network call is cleared, and the port returns to the idle state. The connection is established when DSR and DCD go high. After the port receives the RI signal, it cannot receive calls from the network so the dial procedure can be completed. D
isconnection - The port monitors DSR and DCD (pins 6 and 8). If either goes low for at least 50 milliseconds, the port immediately drops RTS, DTR, and DRO (pins 4, 20, and 14) and a call clear is sent to network. A PAD port also drops the control signals and returns to the idle state if the user fails to establish a call within the time configured by the Port Record parameter Call Accept Timeout or makes three unsuccessful call attempts. If the call is cleared by an X.25 clear from the network, the port immediately drops DTR and RTS. The port waits for DSR and DCD to go low, at which time the port returns to idle state, ready for another dial-in sequence. If the call is cleared from the user entering [CLR] at the port, control signals are not dropped until the Call Accept Timeout expires. The port is unavailable to take network calls while waiting for the control signals from the modem to drop. If a call is cleared by an X.29 invitation to clear, the signals remain high when the parameter Invitation to clear = CLRWO: the signals are dropped when the parameter Invitation to clear =CLRWD
DIMO (Dial In, With Autoconnect).
DCE ports
Idle - DCD, DSR, and CTS (pins 8, 6, and 5) are maintained low.
Connection - The port monitors MB (pin 25) [modem RI]. If it goes high, the port makes a network call according to the autocall mnemonic. When the call is accepted, the port raises DCD, DSR, and CTS (pins 8, 6, and 5) [modem RTS, DTR, and DRO (pins 4, 20, and 14)], then waits up to 240 seconds for DTR and RTS (pins 4 and 20) [modem DSR and DCD] to go high. If the timer expires, the DCD, DSR, and CTS (pins 8, 6, and 5) are dropped, the network call is cleared, and the port returns to the idle state. If DTR and RTS go high before the timer expires, the connection is established.
Disconnection - The port monitors DTR, and RTS (pins 20 and 4) [modem DSR and DCD]. If either goes low for at least 50 milliseconds, the port immediately drops DCD, DSR, and CTS (pins 8, 6, and 5) [modem RTS, DTR and DRO] and a call clear is sent to network. A PAD port also drops the control signals and return to the idle state if the user fails to establish a call within the time configured by the Port Record parameter Call Accept Timeout or makes three unsuccessful call attempts. If the call is cleared by an X.25 clear from the network, the port immediately drops DCD, DSR,and CTS [modem RTS, DTR, and DRO]. The port waits for DTR and RTS [modem DSR and DCD] to go low, at which time the port returns to idle state, ready for another dial-in sequence. If the call is cleared from the port, control signals are not dropped until the Call Accept Timeout expires. The port is unavailable to take network calls while waiting for the control signals from the modem to drop. If the call is cleared by an X.29 invitation to clear, the signals remain high when the parameter Invitation to clear = CLRWO: the signals are dropped when the parameter Invitation to clear = CLRWD.
DTE Ports
Idle - RTS and DTR (pins 4 and 20) are maintained low.
Connection - The port monitors RI (pin 25). If it goes high, the port makes a network call according to the autocall mnemonic. When the call is accepted, the port raises RTS, DTR, and DRO (pins 4, 20, and 14) then waits up to 240 seconds for DSR and DCD (pins 6 and 8) to go high. If the timer expires, the RTS, DTR and DRO (pins 4, 20, and 14) are dropped, the network call is cleared, and the port returns to the idle state. If the DSR and DCD go high before the timer expires, the connection is established.
Disconnection - The port monitors DSR and DCD (pins 6 and 8). If either goes low for at least 50 milliseconds, the port immediately drops RTS, DTR, and DRO (pins 4, 20, and 14) [modem DCD, DSR and CTS] and a call clear is sent to network. A PAD port also drops the control signals and returns to the idle state if the user fails to establish a call within the time configured by the Port Record parameter Call Accept Timeout or makes three unsuccessful call attempts. If the call is cleared by an X.25 clear from the network, the port immediately drops RTS, DTR, and DRO. The port waits for DSR and DCD to go low, at which time the port returns to idle state, ready for another dial-in sequence. If the call is cleared from the port, control signals are not dropped until the Call Accept Timeout expires. The port is unavailable to take network calls while waiting for the control signals from the modem to drop. If the call is cleared by an X.29 invitation to clear, the signals remain high when the parameter Invitation to clear = CLRWO: the signals are dropped when the parameter Invitation to clear = CLRWD. If the call is cleared from the port, control signals are not dropped until Call Accept Timeout expires.

Dial Out
In this case a modem is connected to a PAD port. Calls from the network connect to the PAD port and use the modem to call through the telephone network.
States for the connection type DIMO (Dial Out).
DTE Ports
Idle - DCD, DSR, and CTS (pins 8, 6, and 5) are maintained low. Connection - This is for modems with the autodial feature (the modem can dial the number when the DTR input goes from inactive to active). When a call arrives at a port that is idle and available, the call is accepted. The port raises DSR, [modem DTR]. The
modem autodials the destination and, when a connection is made, raises its DCD output. The port monitors RTS (pin 4). If it goes high and if DTR remains high, the port raises DCD, DSR,and CTS. If RTS and DTR are not raised within three minutes after the call is accepted (and DSR being raised), the call is cleared.
Disconnection - The port monitors DTR and RTS (pins 20 and 4) [modem DSR, DCD]. If either goes low for at least 50 milliseconds, the port immediately drops DCD, DSR, and CTS (pins 8, 6, and 5) [modem RTS, DTR, and DRO] and a call clear is sent to network. A PAD port will also drop the control signals and return to the idle state if the user fails to establish a call within the time configured by the Port Record parameter Call Accept
Timeout or makes three unsuccessful call attempts. If the call is cleared by an X.25 clear from the network, the port immediately drops DCD, DSR, and CTS [modem RTS, DTR, and DRO]. The port waits for DTR and RTS [modem DSR and DCD] to go low, at which time the port returns to idle state, ready for another dial-in sequence. If the call is cleared from the port, control signals are not dropped until the Call Accept Timeout expires. The port is unavailable to take network calls while waiting for the control signals from the modem to drop. If the call is cleared by an X.29 invitation to clear, the signals remain high when the parameter Invitation to clear = CLRWO: the signals are dropped when the parameter Invitation to clear = CLRWD.

DTE Ports
Idle - RTS, DTR, and DRO (pins 4, 20, and 14) are maintained low.
Connection - This is for modems with the autodial feature (the modem can dial the number when the DTR input goes from inactive to active). When a call arrives at a port that is idle and available, the call is accepted and the port raises DTR. The modem autodials the destination and, when a connection is made, raises its DCD output. The port monitors DCD (pin 8). If it goes high and if DSR remains high, the port raises RTS, DTR, and DRO (pins 4, 20, and 14). If DCD and DSR are not raised within threeminutes after the call is accepted (and DTR being raised), the call is cleared.
Disconnection - The port monitors DSR and DCD (pins 6 and 8). If either goes low for at least 50 milliseconds, the port immediately drops DCD, DSR, and CTS (pins 8, 6, and 5) [modem RTS, DTR, and DRO] and a call clear is sent to network. A PAD port will also drop the control signals and return to the idle state if the user fails to establish a call within the time configured by the Port Record parameter Call Accept Timeout or makes three
unsuccessful call attempts. If the call is cleared by an X.25 clear from the network, the port immediately drops RTS, DTR, and DRO. The port waits for DSR and DCD to go low, at which time the port returns to idle state, ready for another dial-in sequence. If the call is cleared from the port, control signals are not dropped until the Call Accept Timeout expires. The port is unavailable to take network calls while waiting for the control signals from the modem to drop. If the call is cleared by an X.29 invitation to clear, the signals remain high when the parameter Invitation to clear = CLRWO: the signals are dropped when the parameter Invitation to clear = CLRWD.

If the attached modem does not store telephone numbers, or the caller uses standard AT commands, the modem must be configured so DCD output is always high so the port can send dial information to the modem. The modem’s DSR must be strapped to follow DTR inputs so that when the network disconnects by dropping all EIA control signals, the modem will drop DSR to complete the disconnection. (DTR Control on the modem must be configured as 108.2. This drops the connection when DTR goes from on to off.)

Dial In/Dial Out Collision
This is the case of a telephone call causing the MB [modem RI] signal to arrive at the port at the same time a network call arrives at the port, thus causing the port to raise DCD, DSR, and CTS [modem RTS, DTR, and DRO]. The port can detect this circumstance because the MB signal is not the expected response. The port resolves the collision by clearing the call to the network while the DCD, DSR, and CTS stay raised at the modem. If DTR and RTS are not raised within one minute, the port drops DCD, DSR, and CTS [modem RTS, DTR, and DRO]. Call collision is resolved in favor of the telephone network caller, that is, the call is completed, not cleared. After the collision is resolved, the call is handled like any other incoming call from the telephone network.

Variations of DIMO Connections
DIMOa - This is the same as DIMO, except that the DSR signal is treated differently. Use DIMOa when modems do not have DSR raised on incoming calls.
DIMOb - This is the same as DIMO, except that the DSR signal is treated differently. Use DIMOb when modems have DSR following DTR on incoming calls.
DIMOv - This connection type provides the capability for interfacing to V.25 bis type modems and is the same as DIMO as far as EIA handshaking is concerned.

EMRI/EMDC Connections

Introduction
This case is for a situation where a PAD port connects to a host computer and replaces a modem.
Note
Do not use EMRI with hunt groups or autocalls or when using EIA-232-D DIMs
in the DTE position.

DCE Port States for EMRI
Conditions during various states for EMRI connections on DCE ports:

Idle - The front panel switch RI/TM is set to RI and DCD, DSR, and CTS (pins 8, 6, and 5) are maintained low.
Connection - When a call arrives from the network, the RI (pin 22) is pulsed (two seconds on, four seconds off) for up to five cycles (30 seconds). During the ringing, DTR (pin 20) is monitored. If it is high or goes high, the PAD clears RI (pin 22) and raises DSR and DCD (pins 6 and 8) and waits for RTS (pin 4) to go high. When RTS goes high, the PAD raises CTS (pin 5). The PAD accepts the incoming call from the network only after DTR and
RTS are detected high.
Disconnection - After DTR is detected high, the PAD monitors DTR (pin 20) and if it is low for at least 50 milliseconds, the call is cleared. DSR and DCD (pins 6 and 8) are dropped and the PAD returns to the idle state. If the call is cleared by the network while waiting for RTS to be raised, DSR and DCD are dropped and the PAD waits for DTR to drop before completing the disconnect. The PAD will not accept another dial-out attempt until DTR is lowered. If RTS is not raised within 30 seconds of RI first being raised, then DCD and DSR (pins 8 and 6) are dropped and the call is cleared. If the call is cleared by the network while waiting for DSR to be raised, RI is immediately dropped. Once the call is connected, if the call is cleared from the network DCD, DSR, and CTS are dropped.

EMDC
This is similar to EMRI, but DCD is used to signal the host about arrival of the call.
Note
Do not use this setting with hunt groups or with autocalls.
Note
A change in a EIA control signal may not be detected for up to 50 milliseconds (average 25 ms). As a result, the Vanguard ignores data sent to port before the connection was recognized as valid. To prevent this, before passing data wait at least 50 milliseconds after the EIA handshake or until the Vanguard sends a connection prompt.

DCE Port States for EMDC
Idle - The front panel switch RI/TM is set to TM and DCD, DSR, and CTS (pins 8, 6, and 5) are maintained low. DTR (pin 20) may be high.
Connection - When a call arrives from the network, the DCD (pin 8) is raised. DTR (pin 20) is monitored. If it is high or goes high, the PAD raises DSR (pin 6) and waits for RTS (pin 4) to go high. When RTS goes high, the PAD raises CTS (pin 5). The PAD accepts the call from the network only after DTR and RTS are detected high.
Disconnection - The PAD monitors DTR (pin 20) and if it is low for at least 50 milliseconds, the call is cleared. The control signals DSR and CTS (pins 6 and 8) are dropped, and the PAD returns to the idle state. If the call is cleared by the network, while waiting for RTS to be raised, and then DSR and DCD (pins 6 and 8) are dropped
and the PAD returns to the idle state for the period after DTR is lowered. The PAD will not accept another dial-out attempt until DTR is lowered. If RTS is not raised within 30 seconds of RI being raised, DCD and DSR (pins 8 and 6) are dropped and the call is cleared. If the call is cleared by the network while waiting for DTR to be raised DCD is immediately dropped. Once the call is connected, if the call is cleared from the network DCD, DSR, and CTS are dropped.

DTE Port States for EMDC
Idle - The front panel switch RI/TM is set to TM and RTS, DTR, and DRO (pins 4, 20, and 14) are maintained low. DSR (pin 6) may be high.
Connection - When a call arrives from the network, the RTS (pin 4) is raised. DSR (pin 6) is monitored. If it is high or goes high, the PAD raises DTR (pin 20) and waits for DCD (pin 8) to go high. The PAD accepts the call from the network after DSR and DCD are detected high.
Disconnection - The PAD monitors DSR (pin 6) and if it is low for at least 50 milliseconds, the call is cleared. The control signals RTS, DTR,and DRO (pins 4, 20, and 14) are dropped, and the PAD returns to the idle state. If the call is cleared by the network, while waiting for DCD to be raised. RTS and DTR (pins 4 and 20) are
dropped and the PAD returns to the idle state after DSR is lowered. The PAD will not accept another dial-out attempt until DSR is lowered. If DCD is not raised within 30 seconds of MB being raised, RTS and DTR (pins 4 and 20) are dropped and the call is cleared. If the call is cleared by the network while waiting for DSR to be raised, MB is immediately dropped. Once the call is connected, if the call is cleared from the network RTS, DTR, and DRO are dropped.