3COM CORPORATION

US ROBOTICS COURIER INTERNAL I-MODEM

Card Type

Modem (synchronous/asynchronous)

Chip Set

Unidentified

Maximum Modem Rate

56Kbps

ISDN Transfer rate

64Kbps

Maximum Fax Rate

14.4Kbps

Data Modulation Protocol

Bell 103/212A

ITU-T x2, V.34, V.FC, V.32, V.32terbo, V.32bis

ISDN Protocol

V.120, V.110

Fax Modulation Protocol

Unidentified

Error Correction/Compression

MNP5, V.42, V.42bis, HST

Fax Class

Class I & II

Data Bus

8-bit ISA

Card Size

Full-length, full-height card

CONNECTIONS

Function

Label

Function

Label

RJ-45 connector

CN1

RJ-45 connector

CN2

USER CONFIGURABLE SETTINGS

Function

Label

Position

í Load &F0 setting upon reset

SW1/1

Off

Load NVRAM settings upon reset

SW1/1

On

Modem acts on AT commands

SW1/2

On

Modem ignores AT commands

SW1/2

Off

í Factory configured - do not alter

SW1/3

Unidentified

í Factory configured - do not alter

SW1/4

Unidentified

SERIAL PORT ADDRESS

Setting

JP1

JP2

JP3

COM1 (3F8h)

Open

Open

Open

COM2 (2F8h)

Closed

Open

Open

COM3 (3E8h)

Open

Closed

Open

COM4 (2E8h)

Closed

Closed

Open

Plug & Play

Open

Open

Closed

 

 

INTERRUPT

IRQ

JP4

JP5

JP6

JP7

JP8

2

Open

Open

Open

Open

Closed

3

Open

Open

Open

Closed

Open

4

Open

Open

Closed

Open

Open

5

Open

Closed

Open

Open

Open

7

Closed

Open

Open

Open

Open

SUPPORTED COMMAND SET

Basic AT Commands

AT, ‘+++’, A/

A, B, C, E, F, O, V

&C, &G, &Z

S Registers

S0, S1, S2, S3, S4, S5, S6, S7, S8, S9, S10, S11, S12, S18, S26

Note: See MHI Help File for full command documentation.

Proprietary AT Command Set

ADDITIONAL RESULT CODES

Type:

Configuration

Format:

AT [cmds] &An [cmds]

Description:

Controls the display of the additional result codes

Command

Function

í &A0

Do not display ARQ result codes

&A1

Display ARQ result codes

&A2

Display ARQ result codes, HST, V.32, V.FC, V.34 or Digital modulation indicator

&A3

Display ARQ result codes and error control indicator LAPM, HST, MNP, SYNC, V.120, or NONE

BREAK HANDLING

Type:

Configuration

Format:

AT [cmds] &Yn [cmds]

Description:

Controls the break handling

Command

Function

í &Y0

Destructive, don’t send break

&Y1

Destructive, expedited break sent

&Y2

Nondestructive, expedited break sent

&Y3

Non destructive un-expedited break sent

 

 

 

 

 

 

 

 

CONNECTION RATE

Type:

Configuration

Format:

AT [cmds] &Nn [cmds]

Description:

Controls the connection rate

Command

Function

í &N0

Variable rate

&N1

300bps

&N2

1200bps

&N3

2400bps

&N4

4800bps

&N5

7200bps

&N6

9600bps

&N7

12Kbps

&N8

14.4Kbps

&N9

16.8Kbps

&N10

19.2Kbps

&N11

21.6Kbps

&N12

24Kbps

&N13

26.4Kbps

&N14

28.8Kbps

&N15

31.2Kbps

&N16

33.6Kbps

DATA COMPRESSION

Type:

Configuration

Format:

AT [cmds] &Kn [cmds]

Description:

Controls the data compression

Command

Function

&K0

Data compression disabled

í &K1

Use auto enable/disable data compression

&K2

Data compression always enabled

&K3

Selective data compression

DATA SET READY (DSR)

Type:

Configuration

Format:

AT [cmds] &Sn [cmds]

Description:

Selects DSR options

Command

Function

&S0

DSR forced high

í &S1

DSR high only while modem is handshaking or connected

&S2

Pulsed DSR sent with CTS following CD when the carrier is lost

&S3

Pulsed DSR sent following CD when the carrier is lost

&S4

Send the computer a DSR signal at the same time as the CD

&S5

Send DSR normally, and follow CTS with CD

 

 

 

DATA TERMINAL READY (DTR)

Type:

Configuration

Format:

AT [cmds] &Dn [cmds]

Description:

Selects modem response to DTR

Note: The action each variant of &D causes depends on the setting of &Q

Command

Function

í &D0

Modem does not respond to DTR

&D1

Modem goes to command mode after DTR goes is off

&D2

Modem goes to command mode and disconnects (hangs up) after DTR goes off; Auto-Answer is disabled.

DIAL

Type:

Immediate

Format:

AT [cmds] D<#> [cmds]

Description:

Dials telephone number according to any modifiers included in the string

Note:

Any combination of modifiers can be used to produce the desired dial functions in sequence.

Command

Function

DL

Re-dial last number

DP

Pulse dialing enabled

DR

Answer mode enabled; originate mode disabled following handshake initiation.

DS=n

Dial stored telephone number n

DT

Tone dialing enabled/Pulse dialing disabled

DW

Dialing resumed following dial tone detection

D,

Dialing paused for amount of time specified in S8 register

D"WXY"

Optional method of denoting telephone numbers. Letters enclosed in quotes are interpreted as numbers according to the system found on a telephone keypad.

D@

Wait for Quite Answer function enabled. Modem waits until a "quiet answer," a ring-back signal followed by silence up to the time specified in S7, is received prior to executing the rest of the dial string.

D$

Wait for prompt tone detection function enabled. Waits for prompt tone for amount of time specified by the S7 command.

D;

Modem returned to idle state after dialing. The semicolon can only be placed at the end of the dial command.

D/

Pause for 125 milliseconds

 

 

 

 

 

 

 

 

 

 

 

 

 

ERROR CONTROL

Type:

Configuration

Format:

AT [cmds] &Mn [cmds]

Description:

Selects the error control protocol

Command

Mode

í &M0

Normal mode, no error control

&M1

On-line synchronous mode, no error control

&M4

Normal/ARQ mode,

&M5

ARQ asynchronous mode

&M6

V.25bis synchronous mode.

&M7

V.25bis synchronous mode using HDLC link protocol

FACTORY DEFAULT PROFILE

Type:

Configuration

Format:

AT [cmds] &F [cmds]

Description:

Sets values in active profile to values found in the default profile

Command

Function

í &F0

Load No Flow Control template settings

&F1

Load Hardware Flow Control template settings

&F2

Load Software Flow Control template settings

HARDWARE RESET

Type:

Immediate

Format:

AT [cmds] Z! [cmds]

Description:

Performs a hardware reset

HELP

Type:

Configuration

Format:

AT [cmds] $ [cmds]

Description:

Display the help commands for the selected command sets

Command

Function

#$

Display the help pannels for the octothorpe (#) command set

*$

Displays the help pannels for the * command set

HOOK CONTROL

Type:

Immediate

Format:

AT [cmds] Hn [cmds]

Description:

Selects whether the modem is on-hook or off-hook

Command

Function

í H0

Modem commanded to go on-hook (hang-up)

LINE SELECTION

Type:

Configuration

Format:

AT [cmds] &Ln [cmds]

Description:

Selects the phone line type

Command

Function

&L0

Switched line (PSTN/Dial-up)

MODEM CLOCK

Type:

Configuration

Format:

AT [cmds] Kn [cmds]

Description:

Controls the modem clock

Command

Function

í K0

If on-line, display current call duration; if off-line, display the last call’s duration

K1

Display the actual time (set clock using ATI3=HH:MM:SS K1)

RECEIVE DATA FLOW CONTROL

Type:

Configuration

Format:

AT [cmds] &In [cmds]

Description:

Controls the receive data flow control

Command

Jack Type

í &I0

Disables XON/XOFF flow control

&I1

Modem acts on typed XON/XOFF commands, Ctrl -S, or Ctrl -Q and passes them to remote device

&I2

Modem acts on typed XON/XOFF commands but removes them from data stream

&I3

Hewlett Packard host mode

&I4

Hewlett Packard terminal mode

&I5

Enable flow control on the phone link when the connection is not under error control

REPORT INFORMATION

Type:

Immediate

Format:

AT [cmds] In [cmds]

Description:

Displays information requested

Command

Function

I0

Reports 4 digit product code

I1

Reports results of ROM checksum

I2

Reports results of RAM test

I3

Reports the product name

I4

Reports the current modem settings

I5

Reports the settings stored in NVRAM

I6

Reports statistics for the last call

I7

Reports product configuration

I10

Reports dial security account status information

I11

Displays high speed connection report

I12

Display the ISDN settings

RESULT CODES

Type:

Configuration

Format:

AT [cmds] Qn [cmds]

Description:

Enables modem to send result codes to the DTE

Command

Function

í Q0

Result code sending enabled

Q1

Result code sending disabled

Q2

Suppress result codes when answering

RTS/CTS

Type:

Configuration

Format:

AT [cmds] &Rn [cmds]

Description:

Selects RTS/CTS options

Command

Function

í &R0

CTS follows RTS in data mode; RTS is ignored in command mode.

&R1

CTS forced high, RTS is ignored.

&R2

Enable hardware flow control of received data

SELECT CALL PROGRESS RESULT CODES

Type:

Configuration

Format:

AT [cmds] Xn [cmds]

Description:

Enables selection of tone detection and associated result code format options

Command

Function

X0

Busy and dial tone detection disabled; result codes 0 - 4 enabled.

X1

Busy and dial tone detection disabled; result codes 0 - 5 & 10 enabled.

X2

Busy tone detection disabled, dial tone detection enabled; result codes 0 - 6 & 10 enabled.

X3

Busy tone detection enabled, dial tone detection disabled; result codes 0 - 5, 7 & 10 enabled.

í X4

Busy and dial tone detection enabled; result codes 0 - 7 & 10 enabled.

X5

Busy tone detection enabled, dial tone detection disabled, all result codes enabled

X6

All result codes enabled

X7

Voice detection disabled, all result codes enabled

SERIAL PORT RATE

Type:

Configuration

Format:

AT [cmds] &Bn [cmds]

Description:

Controls the serial port rate

Command

Function

í &B0

Variable serial port rate

&B1

Fixed serial port rate

&B2

In answer mode use fixed rate for ARQ calls and variable rate for non ARQ calls.

TEST MODES

Type:

Immediate

Format:

AT [cmds] &Tn

Description:

Selects test options

Command

Function

&T0

End current test

&T3

Begin local digital loopback

&T4

Grant remote digital loopback request

&T5

Deny remote digital loopback request

&T6

Request remote digital loopback

&T7

Request remote digital loopback and self-test

 

 

TRANSMIT DATA FLOW CONTROL

Type:

Configuration

Format:

AT [cmds] &Hn [cmds]

Description:

Controls the transmit data flow

Command

Function

í &H0

Flow control disabled

&H1

Hardware flow control enabled

&H2

Software flow control enabled

&H3

Both hardware and software flow control enabled

WRITE CURRENT SETTINGS

Type:

Immediate

Format:

AT [cmds] &W [cmds]

Description:

Writes the current settings to NVRAM

Extended AT Commands

ASSIGN PASSWORD

Type:

Immediate

Format:

AT [cmds] %V=PWn [cmds]

Description:

Assign the password in account n in security account

CALL TYPE

Type:

Configuration

Format:

AT [cmds] *Vn [cmds]

Description:

Sets the call type

Bit

Unit

Function

*V1

0

1

3.1 kHz audio (modem/fax)

Speech

*V2

0

1

2

3

4

5

Autodetect

V.120 rate adaption only

V.110 rate adaption only

Modem or fax only

Clear-channel synchronous

Asychronous to synchronous PPP conversion

CENTRAL OFFICE PROTOCOL

Type:

Configuration

Format:

AT [cmds] *Wn [cmds]

Description:

Sets the protocol used by the telephone company’s central office

Command

Function

*W0

AT&T 5ESS custom

*W1

Northern Telecom DMS-100

*W2

National ISDN-1

*W3

National ISDN-2

CLOCK SPEED

Type:

Configuration

Format:

AT [cmds] %Nn [cmds]

Description:

Sets clock speed for synchronous mode

Command

Function

%N2

1200bps

%N3

2400bps

%N4

4800bps

%N5

7200bps

%N6

9600bps

%N7

12000bps

%N8

14400bps

%N9

16800bps

%N10

19200bps

DIALING MODE

Type:

Immediate

Format:

AT [cmds] *On [cmds]

Description:

Sets the dialing mode

Command

Function

*O0

En-bloc dialing

*O1

Overlap dialing

DIRECTORY NUMBER

Type:

Configuration

Format:

AT [cmds] *P [cmds]

Description:

Sets the Directory number assigned by the phone company

Command

Function

*P1

The DN for Voice B-channel

*P2

The DN for Data B-channel

ERASE SECURITY SETTINGS

Type:

Configuration

Format:

AT [cmds] %En [cmds]

Description:

Erases selected security settings

Command

Function

%E1

Erase local access password

%E2

Erase autopass password

%E3

Erase passwords in accounts 0-9

%E4

Erase phone numbers in accounts 0-9

%E5

Disable account, dialback, and new number fields in accouns 0-9

LOCAL ACCESS PASSWORD

Type:

Configuration

Format:

AT [cmds] %Ln [cmds]

Description:

Set local access password

 

PASSWORD SECURITY

Type:

Immediate

Format:

AT [cmds] %P [cmds]

Description:

Disable password security

REMOTE CONFIGURATION

Type:

Configuration

Format:

AT [cmds] %Cn [cmds]

Description:

Controls the remote configuration

Command

Function

%C0

Defer configuration changes until the call is ended

%C1

Cancel configuration changes and restore the original configuration

%C2

Force configuration changes to take effect immediately

REMOTE CONFIGURATION

Type:

Configuration

Format:

AT [cmds] %Fn [cmds]

Description:

Configure another device’s data remotely

Command

Function

%F0

No parity, 8 data bits

%F1

Mark parity, 7 data bits

%F2

Odd parity, 7 data bits

%F3

Even parity, 7 data bits

RINGING SIGNAL VOLUME

Type:

Configuration

Default:

Unidentified

Range:

0-9

Format:

AT [cmds] *Rn [cmds]

Description:

Sets the ringing signal volume

SECURITY ACCOUNTS

Type:

Configuration

Format:

AT [cmds] %An [cmds]

Description:

Create and configure security accounts

SECURITY ACCOUNTS

Type:

Configuration

Format:

AT [cmds] %Sn [cmds]

Description:

Access security accounts

 

 

 

 

 

 

 

SERIAL PORT RATE

Type:

Configuration

Format:

AT [cmds] %Bn [cmds]

Description:

Remotely configure a modem’s serial port rate

Command

Function

%B0

110bps

%B1

300bps

%B2

600bps

%B3

1200bps

%B4

2400bps

%B5

4800bps

%B6

9600bps

%B7

19200bps

%B8

38400bps

%B9

57600bps

%B10

115200bps

SERVICE PROFILE ID

Type:

Configuration

Format:

AT [cmds] Sn=? [cmds]

Description:

Sets the service profile ID

Command

Function

*S1

The SPID for the Voice B-channel

*S2

The SPID for the Data B-channel

TELEPHONE COMPANY PROTOCOL

Type:

Configuration

Format:

AT [cmds] *Mn [cmds]

Description:

Set switch protocol

Command

Function

*M0

Point-to-point

*M1

Multipoint

TERMINAL ENDPOINT ID

Type:

Configuration

Format:

AT [cmds] *T [cmds]

Description:

Sets the Terminal endpoint ID

Command

Function

*T1

0-63 for the Voice B-channel

*T2

0-63 for the Data B-channel

TONE FREQUENCIES

Type:

Configuration

Format:

AT [cmds] %Tn [cmds]

Description:

Enables recognition of the tone frequencies

 

 

VOLUME

Type:

Configuration

Format:

AT [cmds] *Cn [cmds]

Description:

Sets the volume of the sound that comes out of the analog device jack

Command

Function

*C0

Off

*C1

Volume level 1

*C2

Volume level 2

*C3

Volume level 3

*C4

Volume level 4

*C5

Volume level 5

*C6

Volume level 6

*C7

Volume level 7

*C8

Volume level 8

*C9

Volume level 9

S(status) -REGISTERS

BIT MAPPED REGISTER

Type:

Register

Format:

AT [cmds] S15=n [cmds]

Unit:

Bit-mapped

Description:

Controls fallback

Bit

Value

Function

0

1

Disable the modem’s extra high frequency equalization if it causes problems on shorter -link calls- for HST modulation only

1

2

Disable online fallback

2

4

Disable 450bps back channel ---HST only

3

8

Reset non-ARQ mode transmit buffer from 1.5Kbytes to 128

4

16

Disable MNP level 4

5

32

Set backspace key to delete

6

64

Set for some incompatable 2400bps MNP-I modems

7

128

Custom applications only

BIT MAPPED REGISTER S16

Type:

Register

Format:

AT [cmds] S16=n [cmds]

Unit:

Bit-mapped

Description:

Controls loopback tests and test pattern

Bit

Value

Function

2

4

Test pattern

3

8

Remote digital loopback

 

 

 

 

 

BIT MAPPED REGISTER S53

Type:

Register

Format:

AT [cmds] S53=n [cmds]

Default:

126

Description:

Bit mapped register

Bit

Value

Function

0

1

Dial security enabled

1

2

Prompting enabled

2

4

Local access password protection enabled

BIT MAPPED REGISTER S54

Type:

Register

Format:

AT [cmds] S54=n [cmds]

Unit:

Bit mapped

Bit

Value

Function

0

1

Disable 2400 symbol rate

1

2

Disable 2743 symbol rate

2

4

Disable 2800 symbol rate

3

8

Disable 3000 symbol rate

4

16

Disable 3200 symbol rate

5

32

Disable 3429 symbol rate

6

64

Disable Call Indicate

7

128

Disable V.8

BIT-MAPPED REGISTER S13

Format:

AT [cmds] S13=n [cmds]

Default:

170

Range:

0-174

Unit:

Bit-mapped

Description:

Controls echo, result codes and display, dial mode, and answer/originate mode.

Bit

Value

Function

0

1

Reset when DTR drops

1

2

Reverse normal auto answer mode

2

4

Disable 250 msec pause before result code display

3

8

On DTR signal, Autodial the number stored in NVRAM at position 0

4

16

At power on/reset, auto dial number stored in NVRAM at position 0

5

32

Disable HST

6

64

Disable MNP level 3

7

128

Hardware reset

 

 

 

 

 

 

 

 

BIT-MAPPED REGISTER S14

Format:

AT [cmds] S14=n [cmds]

Default:

170

Range:

0-174

Unit:

Bit-mapped

Description:

Controls echo, result codes and display, dial mode, and answer/originate mode.

Bit

Value

Function

0

í 1

Disconnect on escape code

1

2

Send result codes only when originating a call

BIT-MAPPED REGISTER S21

Format

AT [cmds] S21=n [cmds]

Default:

Unidentified

Range:

Unidentified

Unit:

.01 second

Description:

Sets the duration of breaks sent from the modem to the computer or terminal

BIT-MAPPED REGISTER S28

Format

AT [cmds] S28=n [cmds]

Default:

Unidentified

Range:

8

Unit:

.1 second

Description:

Sets the duration of the extra 3000/600 Hz answer tones sent during handshaking.

BITMAPPED REGISTER S27

Format

AT [cmds] S27=n [cmds]

Default:

Unidentified

Range:

0

Unit:

Bit-mapped

Description:

Selects modulation

Bit

Value

Function

0

1

Enable ITU-T V.21

1

2

Enable un-encoded modulation in V.32 mode

2

4

Disable V.32 modulation

3

8

Disable 2100Hz answer tone

4

0

16

0

16

V.42 detection, LAPM error control, MNP

Disable MNP

Disable V.42 detection and LAPM

Disable detection phase

5

0

32

0

32

V.42 detection, LAPM error control, MNP

Disable MNP

Disable V.42 detection and LAPM

Disable detection phase

7

128

Unusual software incompatibility

 

 

BIT-MAPPED REGISTER S34

Type:

Register

Format:

AT [cmds] S34n [cmds]

Default:

0

Range:

Unidentified

Unit:

Unidentified

Description:

Controls V.32bis

Bit

Value

Function

0

1

Disable V.32bis

1

2

Disable modem’s enhanced proprietary V.32bis modulation

2

4

Disable the faster retrains that occure during proprietary V.32terbo modulation

3

8

Enable V.23

4

16

Force the MR LED to show DSR

5

32

Enable MI/MIC

6

64

Disable remote access busy message

7

128

Disable V.32terbo

BIT-MAPPED REGISTER S51

Format

AT [cmds] S51=n [cmds]

Default:

0

Range:

Unidentified

Unit:

Bit-mapped

Description:

MNP/V.42 control

Bit

Value

Function

0

1

Disable MNP/V.42 for V.22

1

2

Disable MNP/V.42 for V.22bis

2

4

Disable MNP/V.42 for V.32/V.32bis/V.32terbo

6

64

Disable selective reject

7

128

Enable handset exclusion delay

BIT-MAPPED REGISTER S55

Type:

Register

Format

AT [cmds] S55=n [cmds]

Unit:

Bit mapped

Bit

Value

Function

0

1

Disable 8S-2D mapping

1

2

Disable 16S-4D mapping

2

4

Disable 32S-2D mapping

3

8

Disable 64S-4D mapping

 

 

 

 

 

 

 

 

BIT-MAPPED REGISTER S56

Type:

Register

Format

AT [cmds] S56? [cmds]

Unit:

Bit mapped

Bit

Value

Function

0

1

Disable non-linear coding

1

2

Disable TX level deviation

2

4

Disable pre-emphasis

3

8

Disable precoding

4

16

Disable shaping

6

64

Disable V.34

7

128

Disable V.FC

BIT-MAPPED REGISTER S67

Type:

Register

Format

AT [cmds] S67 [cmds]

Unit:

Bit mapped

Bit

Value

Function

0

1

Disable non-linear coding

1

2

Disable TX level diviation

2

4

Disable preemphasis

3

8

Disable precoding

4

16

Disable shaping

6

64

Disable V.34

7

128

Disable V.FC

BIT-MAPPED REGISTER S68

Type:

Register

Format

AT [cmds] S68 [cmds]

Unit:

Bit mapped

Bit

Value

Function

0

1

Enable V.110 fallback

1

2

Use fixed network rate

2

4

Fix the network rate at 64Kbps

3

8

Route incoming analog calls to the data port

4

16

Enable a 45-65 second link delay

5

32

Map B1 LED = B-channel 1 and B2 LED to B-channel 2

6

64

Disable all blinking patterns for B-channel LED’s

BIT-MAPPED REGISTER S69

Type:

Register

Format

AT [cmds] S69 [cmds]

Unit:

Bit mapped

Bit

Value

Function

0

1

Disable Plug and Play signaling

 

 

DSR RECOGNITION

Type:

Register

Format

AT [cmds] S25=n [cmds]

Default:

5

Range:

Unidentified

Unit:

.1 second

Description:

Sets DTR recognition time

DSR SIGNAL

Type:

Register

Format

AT [cmds] S24=n [cmds]

Default:

150

Range:

Unidentified

Unit:

.02 second

Description:

Sets the duration between pulsed DSR signals when the modem is set to &S2 or &S3.

FLOW CONTROL CHARACTER - XOFF

Format

AT [cmds] S23=n [cmds]

Default:

19

Range:

Unidentified

Unit:

ASCII

Description:

Sets the character used to represent XOFF

FLOW CONTROL CHARACTER - XON

Format

AT [cmds] S22=n [cmds]

Default:

17

Range:

Unidentified

Unit:

ASCII

Description:

Sets the character used to represent XON

 

GUARD TIME DURATION

Type:

Register

Format

AT [cmds] S43=n [cmds]

Default:

200

Unit:

.02 second

Description:

Sets the duration of the guard time for the remote access sequence

INACTIVITY TIMER

Type:

Register

Format

AT [cmds] S19=n [cmds]

Default:

Unidentified

Range:

Unidentified

Unit:

Minute

Description:

Sets the duration in minutes for the inactivity timer

 

REMOTE ACCESS

Type:

Register

Format

AT [cmds] S41=n [cmds]

Default:

0

Description:

Sets the number of allowable remote access login attempts, thus enabling or disabling remote access.

REMOTE ACCESS ESCAPE CHARACTER

Type:

Register

Format

AT [cmds] S42=n [cmds]

Default:

126

Range:

Unidentified

Unit:

Unitdentified

Description:

Stores the ASCII decimal code for the remote access escape character.

TRANSMIT BUFFER

Type:

Register

Format

AT [cmds] S38n [cmds]

Default:

0

Description:

Sets the time before a forced hang up and clearing of the transmit buffer when DTR drops during an ARQ call.