V.25bis Operation

		V.25bis Operation (Selected models only)

The modem may be controlled through the use of the V.25bis command set. The ITU-T recommendation V.25bis defines modem behaviour only while the modem is disconnected from the line, or while the modem is off hook and attempting to establish a connection.

Communication between the DTE and the modem is half duplex. The DTE is the "master" and the modem is the "slave". When the modem is in data transfer mode after the connection has been established, it is no longer in V.25bis mode.

The modem accepts commands in either synchronous mode HDLC (NRZ or NRZI), BSC or asynchronous mode. The message fields used in each of these cases contain either a command or an indication. The modem operates in accordance with command/response behaviour as defined in ITU-T recommendation V.25bis. A command is invalid if the message field contains less than 3 characters and is rejected with an INV indication.

When the modem receives a valid command, it ignores another command until it sends an indication back to the computer, or if the call attempt is successful until the connection is terminated.

Synchronous signalling (Selected models only)

The modem automatically detects and switches from HDLC framing to BSC framing.

The modem can accept a valid command frame that follows an invalid frame if there is a pause of at least 2 bit-times between the end of the invalid frame and the start of the valid frame.

For every command received, the modem responds with exactly one indication if the connection is not completed, and no indication if the connection is completed.

HDLC frames
HDLC frames, commands and indications are HDLC frames with data fields called messages.

FLAG	7Eh
ADDRESS	FFh
CONTROL	13h
MESSAGE	Data. Must be an integer number of 8-bit characters between 3 and 60.
FCS	16-bit cyclic redundancy check based on the polynomial: X16 + X12 + X5 + X1

Frames may be preceded and followed by any data including additional flags. A frame with an address, control or FCS field error is invalid. The modem automatically detects whether each command frame is NRZ or NRZI encoded and follows suit for its indication. Command and indication frames contain inserted zero bits as required by HDLC. The receiving entity (modem or computer) strips out these extra bits.

BSC frames
The format for synchronous character oriented format shall be in accordance with ISO 1745.

SYN	16h
SYN	16h
STX	02h
MESSAGE	Data. Must be an integer number of 8-bit characters between 3 and 60.
ETX	83h

Frames may be preceded and followed by any data, including additional SYN characters. A frame with a parity error is invalid. The modem does not recognise or send any of the short "ack" type messages used in character oriented protocols.

Asynchronous signalling

The modem uses V.25bis asynchronous signalling of the following format:

MESSAGE <Return> <LF>

The modem ignores commands with incorrect parity.

The modem performs an auto baud/auto parity/auto length function on the first characters received following a power-on reset or CNLZ. If the modem receives a <Return> <LF> sequence, it is able to calculate the speed, parity and word length. If the modem only receives a <Return> character, it calculates the speed but retains the existing parity and word length values. If the modem receives neither of these characters, it continues to use the existing values for speed, parity and word length. If no previous speed, parity and/or word length information is available, the modem defaults to 7-bit even operation at 9600 bps. The auto length/auto parity facility is capable of 7-bit or 8-bit characters of even, odd or no parity with one stop bit. The modem then accepts and sends characters according to this selection of speed, parity and word length. The adaption algorithm is also re-initialised by a DTR OFF-to-ON transition.

Switching between V.25bis and AT command modes

The factory setting of the modem is AT command mode. Register S80 bit 5 allows selection between V.25bis and AT command modes as follows:

Bit 5 = 0	AT selected
Bit 5 = 1	V.25bis selected

Thus the command ATS80=33 <Return> enables V.25bis commands and the command CNLS80=1 <Return> disables V.25bis commands. A power-on reset or a software reset (Z) must be performed before the modem actually acts upon any change made to register S80 bit 5.

Once this setting is changed it remains in force continuously irrespective of any other commands such as &F or &W.

V.25bis commands

The following V.25bis commands are implemented and parameters applicable to each are noted following the description of the command.

CIC Connect incoming call :

The modem goes on-line in answer mode, cancelling any DIC command previously issued. If no incoming call is present, the modem issues the invalid (INV) message.

CRI Call request with number and identification :

The modem operates as though the CRN (call request with number) command is issued. However, should a suitable modem fail to answer the call, the calling station identification information is transmitted three times using V.23 1200 bps before disconnecting the line.

x...x	Dial string (0-9 * # T P < =":" & ^)
:	Separator (inserted between dial string and identification)

CRN Call request with number :

The modem goes on-line, dials according to the dial string entered and attempts to establish a connection.

0-9	Digits 0-9
*	The "star" digit
#	The "gate" digit
T	Select tone dialling. Affects current and subsequent dialling.
P	Select pulse dialling. Affects current and subsequent dialling.
<	Short dial pause. The modem pauses before dialling the digits following the <.
=	Long dial pause. This period is twice as long as the short pause. Pauses of combined duration exceeding 12 seconds must not be used.
:	Wait for dial tone. The modem waits for the dial tone before dialling the digits following the : (colon).
&	Hook flash. The modem goes on hook for the period specified in register S29.
^	Disable calling tone generation. Applies to current dial attempt only.

CRS Call request with memory access :

The modem goes on-line, dials according to the telephone number stored in the number store requested and attempts to establish a connection.

1-20	Telephone number store

DIC Disregard incoming call :

The modem, although configured for automatic answering, disregards the incoming call. If there is no incoming call, or automatic answering is not enabled, the modem issues the invalid (INV) message. This command applies only to the current incoming call and does not affect subsequent calls.

PRN Program number :

The modem stores a telephone number in the specified number store.

1-20	Telephone number store
:	Separator (inserted between number store and dial string)
x...x	Dial string (0-9 * # T P < =":" & ^). An empty string clears the number store referenced.

RLN Request list of stored numbers :

The modem returns the telephone number or list of telephone numbers according to the supplied parameter.

1-20	Telephone number store. If no store is specified, a full list of all stored telephone numbers is returned.

CNL Local configuration :

This is an extension to the V.25bis command set. Any AT command string may be entered as a parameter of this command (e.g. CNLS0=2).

V.25bis indications

The following indications may be returned by the modem in response to commands when in V.25bis mode. Parameters applicable to each message are noted following the description of the response.

CFI Call failure indication :

The modem sends this message when a call fails to connect. A parameter is appended to give the reason for the failure.

AB	No dial tone detected or the carrier wait time (specified by register S7) expired.
CB	Local circuit busy (telephone off hook).
ET	Engaged tone (busy) detected.
NS	No telephone number stored at the number store requested for dialling (call not placed).
NT	Ringing detected then ringing stopped. Carrier wait time expired with no answer tone detected.
RT	Ringing detected. Carrier wait time expired with ringing still detected.

CNX Connect :

The modem returns this message when a connection is established. The message may be followed by a value indicating the connection speed (e.g. CNX 2400).

INC Incoming call :

The modem returns this indication when an incoming call is detected on the telephone line.

INV Invalid :

The modem sends this message if the command line contains a syntax error or the modem is unable to execute the command.

LSN List of stored numbers :

The modem returns this message in response to the RLN command.

VAL Valid :

This indication is returned when the modem successfully executes the command issued and no other response is applicable.