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Page 1 Document TR30.4.1/89-01001R4
January 1989
ELECTRONIC INDUSTRIES ASSOCIATION
AD HOC SUBCOMMITTEE TR30.4.1
Project: PN-2188, Asynchronous Facsimile DCE Control
Standard
Source: Data Race, Inc
Subject: DTE-DCE Interface Description for Facsimile
Operations with Minimum DCE Facilities ("Class 1")
1. Introduction
At the September 15, 1988 meeting of the TR30.2.3
committee, Les Staples of Data Race was requested to produce
a working paper for the "Class 1" DTE-DCE interface for
Facsimile Operation embodying the features and facilities
agreed upon by the committee. At the November meeting in
Las Vegas, the first draft of that paper was submitted.
Many useful suggestions and comments were made at that
meeting. A second draft was presented at the January 1989
committee meeting. The committee has been renamed TR30.4.1.
A large number of good suggestions were incorporated,
resulting in this, the third draft of that paper.
Several changes to the original paper have been made to
facilitate a simple 3-wire electrical interface, to
eliminate the need for the DTE to be able to detect and
handle the BREAK signal, and to eliminate the requirement
for the DTE to be able to force the DCE to Command Mode.
The attendees at the January, 1989 meeting were in general
agreement on the workability of this paper in this form.
The major changes made at the January 1989 meeting were:
. requirement for the DCE to support XON/XOFF
. removal of NULL padding ability during FAX transmit
. add of DCE capability to detect the Final bit in
the C-Field of an HDLC frame it is transmitting and
to use the Final-bit detection to cause a return to
Command Mode.
. add of OK response code each time the DCE returns
to Command Mode for consistancy.
. add of CONNECT response code each time the DCE exits
Command Mode while On-Line for consistancy
Page 2 TR30.4.1/89-01001R4
2. Description of Interface
The interface described is an extension of the
asynchronous modem control "AT" command set currently under
study by the TR30.4 committee and uses the format proposed
by the TR30.4 committee. Every effort has been made to keep
this level of interface as simple as possible with minimum
requirements for processing placed on the DCE. The ability
to efficiently operate with the current T.4 Recommendation
as well as with a future error-protected facsimile
transmission protocol, such as that specified in the Revised
Recommendation T.4, AP IX-24-E, was also a consideration.
An object of this paper is to allow a single port rate
setting throughout the facsimile operation. Port rate
setting methodologies are left to PN-2120.
Bi-directional flow control is specified in all modes to
prevent loss of data. The DCE must be capable of using in-
band (XON-XOFF) and, optionally, electrical lead signaling
(RTS-CTS). Methodologies for specifying to the DCE the flow
control method to be used are left to PN-2120.
Page 3 TR30.4.1/89-01001R4
2.1 Electrical Interface - Data and Control Signals
The described interface assumes a serial asynchronous
connection between the DTE and the DCE consisting of, as a
minimum, Circuit BA (Transmitted Data), Circuit BB (Received
Data), and Circuit AB (Signal Ground). Circuit CA (Request
To Send) may optionally be used by the DTE to flow-control
the DCE, and Circuit CB (Clear To Send) may optionally be
used by the DCE to flow-control the DTE. Circuit CD (Data
Terminal Ready) may optionally be used by the DTE to control
the Off-Hook and Command Mode states of the DCE. The
specific use of these leads are left to PN-2120.
2.2 Firmware Interface - Commands and Registers
2.2.1 Modulation Control Register
The described interface uses a single register to
determine the desired modulation type. This register may be
set and examined by the normal ATSr commands. Register 65
has been chosen for this description, but another currently-
unused register may be used in its place. Values in the
register select:
Value Modulation
3 V.29 - 7200 BPS
4 V.29 - 9600 BPS
5 V.27 - 2400 BPS
6 V.27 - 4800 BPS
10 V.21 - 300 BPS
11 TONE - 2100 Hz (CED)
12 TONE - 1100 Hz (CNG - EOM)
13 V.33Bis 12000 BPS*
14 V.33Bis 14400 BPS*
15 V.xx 19200 BPS
21 Reserved for DTMF tone detect
*These speeds are not required but the values are reserved
for future implementations.
2.2.2 Dialing and Answering
2.2.2.1 Dialing
The normal ATD<dial string> is used to dial a facsimile
call. If the DCE is set for CED Tone mode, normal DCE call
progress detection and reporting will be used. When the DCE
detects the configured tone, it will send the CONNECT
response code to the DTE, followed by the OK response code
indicating that the DCE is in Command Mode.
Page 4 TR30.4.1/89-01001R4
2.2.2.2 Answering
The normal ATA command or Automatic Answering is used to
cause the DCE to come off-hook in answer mode. In facsimile
operation, when Register 65 is one of the defined the
defined facsimile modes, the CONNECT result code will be
immediately sent to the DTE and the DCE will enter Command
Mode. The DTE would normally immediately order the DCE to
transmit the 2100 Hz CED tone.
2.3.2 Facsimile Transmission and Reception
Facsimile transmission and reception is controlled by
four commands which are specific to facsimile operation.
Since the data set over the interface is asynchronous,
containing start and stop bits, it is necessary to set the
DTE-DCE interface to a higher data rate than the line
modulation rate. Since the maximum line modulation rate
specified in Recommendation T.4 is 9600 BPS, it is
convenient to set the DTE-DCE interface rate to 19,200 BPS.
The DCE performs buffering and flow-control as described in
the remainder of this document. An object is to allow the
entire facsimile operation to be accomplished at a single
port rate. Methodologies for port rate setting are left to
PN-2120.
Page 5 TR30.4.1/89-01001R4
2.3.2.1 Facsimile Transmit
The command AT+FT is used to cause the DCE to transmit
using the modulation selected by the content of the
Modulation Control Register. The DCE should be Off-Hook
when this command is issued by the DTE. The DCE will return
the CONNECT response code and immediately send the proper
training sequence in the selected mode (if any) followed by
data.
The DCE buffers data in this mode. The configured
flow-control method will be used by the DTE as necessary to
pause the DTE.
When the DCE's transmit buffer becomes empty, the DCE
will stop transmitting, lower transmit carrier, and return
to command mode, returning the OK result code. The DTE may
force the DCE to command mode by making Circuit CD (Data
Terminal Ready) false, if so configured. When the DCE
returns to command mode, it will lower its transmit carrier.
If the mode chosen is TONE, the DCE will immediately
begin to send the configured tone when the command is
issued. If the DTE sends any subsequent data to the DCE,
DCE will return to command mode. If the data from the DTE
which aborts the receive process is a valid command, the
command will be executed correctly.
Page 6 TR30.4.1/89-01001R4
2.3.2.2 Facsimile Receive
The command AT+FR causes the DCE to enter receive mode.
The DCE receives in the modulation mode specified in the
Modulation Control Register. The DCE will wait up to the
time set in Register 7 for carrier to be detected. If the
signal fails to be detected within that time, the modem
returns to Command Mode and sends the NO CARRIER response
code.
If the selected mode is detected (Receive Line Signal
Detect in the proper mode), the DCE will send the CONNECT
result code to the DTE.
If a signal is detected, but it is not the selected
mode, the DCE will send a CERROR (Connect Error) result code
to the DTE. This result code may optionally contain a
numeric code indicating the type of signal that the DCE has
determined that it is detecting.
If the modulation selected is TONE, the CONNECT result
code will be returned when the selected tone is detected for
the period of time set in Register 9 , the Carrier Detect
Response Time register, (0.1 second increments, default 0.6
seconds). The DCE will then return to command mode.
In data modes, the DCE will return to Command Mode upon
loss of carrier for the period of time set in Register 10,
sending the normal NO CARRIER result code.
The DCE will obey the configured flow control from the
DTE.
If the DTE sends any data to the DCE other than XON or
XOFF while the DCE is in a Receive mode, the DCE will
return to command mode. If the data from the DTE which
aborts the receive process is a valid command, the command
will be executed correctly.
The DTE may force the DCE to command mode by making
Circuit CD (Data Terminal Ready) false, if so configured.
Page 7 TR30.4.1/89-01001R4
2.3.2.3 HDLC Transmit
The command AT+FTH causes the DCE to transmit frames in
HDLC protocol using the modulation mode chosen in the
Modulation Control Register. If, when the command is
executed, the mode set in the Modulation Control Register is
not a valid synchronous mode, the response code ERROR will
be returned by the DCE and the DCE will return to command
mode.
The DCE will return the CONNECT result code and
transmit training followed by flags until the first byte of
data is sent by the DTE. When the buffer becomes empty the
DCE will append the Frame Check Sequence and a closing Flag
to the frame and return to Command Mode, sending the result
code CONNECT to the DTE. The DCE will continue to send
flags until one of the following actions is taken by the
DTE.
. The DTE sends additional data. This action causes
the DCE to send another frame.
. The DTE forces the DCE to Command Mode via Circuit CD
(Data Terminal Ready) or the Escape Sequence.
. The period of time in Register 10, the Carrier Detect
Response Time, elapses from the time when the DCE
reported the CONNECT result code without the DTE
sending additional data to the DCE.
The DCE detects the FINAL bit in the C-field of the
frame.
When the DCE stops transmitting and returns to Command
Mode and sends the the DTE the OK result code or the NO
CARRIER result code in the case of S10 expiration.
The DCE performs HDLC zero insertion and Frame Check
Sequence generation while in this mode.
The DCE buffers data in HDLC transmit mode. The DCE
will use the configured method of flow-control to pause the
DTE as necessary.
Page 8 TR30.4.1/89-01001R4
2.3.2.4 HDLC Receive
The command AT+FRH causes the DCE to receive frames in
HDLC protocol using the modulation mode selected in the
Modulation Control Register. If, when the command is
executed, the mode set in the Modulation Control Register is
not a valid synchronous mode, the response code ERROR will
be returned by the DCE and the DCE will return to command
mode.
If the selected mode is detected (Receive Line Signal Detect
in the proper mode), the DCE will send the CONNECT result
code to the DTE.
If a signal is detected, but it is not the selected
mode, the DCE will send a CERROR (Connect Error) result code
to the DTE. This result code may optionally contain a
numeric code indicating the type of signal that the DCE has
determined that it is detecting.
The DCE strips Flags and receives and buffers frames.
The received data, starting with the first non-Flag byte and
continuing through the last FCS byte will be transferred to
the DTE. The DCE performs HDLC zero-deletion and error
checking. When the last byte of FCS has been sent to the
DTE, the DCE may optionally inform the DTE of End-Of-Frame
by generating a BREAK signal. After the FCS bytes,
following the optional BREAK, the DCE will inform the DTE of
the status of the frame reception by returning a result code
of OK, if the frame was received correctly, or ERROR, if the
frame was received in error.
The DCE will obey the configured form of flow control
from the DTE.
After sending the Result Code indicating the frame
reception is complete, the DCE continues to receive and
buffer data in the selected mode. If the DTE issues another
HDLC Receive command, the buffered data will be sent to the
DCE and HDLC reception will proceed. If any other command
is issued by the DTE, the DCE will stop the receive process.
Any buffered data will be discarded and the command will be
obeyed.
The DCE will return to Command Mode upon loss of
carrier for the period of time set in Register 9, sending
the normal NO CARRIER result code.
Page 9 TR30.4.1/89-01001R4
In this mode, the DTE may force the DCE to command mode
with the Escape Sequence or by making Circuit CD (Data
Terminal Ready) false, if so configured.
2.4 Optional Features
2.4.1 Data Buffering
In order to allow a computer, such as a multi-user one,
which may not always be able to quickly respond to avoid
having under-run conditions when transmitting a facsimile,
significant buffering in the DCE may be desirable. The
ability to buffer up to 3 seconds of data for transmission
(less than 4 kilobytes of buffer) may be provided as an
option.
2.4.2 DTMF Detection
A desirable feature of a multi-function DCE which is capable
of facsimile and various forms of data communications would
be to be able to detect Dual Tone Multi-Frequency digits to
tell a program what service the caller desires.
In this operation, the calling DCE would respond to a
secondary dialtone emitted by the answering DCE and send one
or more digits. These digits would result in unsolicited
result codes to the DTE.
Page 11 TR30.4.1/89-01001R4
2.4.2.1 Suggested DTMF implementation.
Examples:
Dial string at caller: ATDT<number>@x (where x is a
digit).
Command sequence to answering DCE:
Upon Detection of RING: ATH1+FT (The H1 causes the DTE
to come Off-Hook. The @ in the dial string causes the DCE
to wait for silence before continuing. The +FT with the
proper tone setting in register 65 causes the DCE to send a
short tone. The DTE then goes into DTMF detect mode.
If the DCE detects a DTMF digit it sends the DTE an
unsolicited response code consisting of an ! (exclamation)
followed by the digit received. The DCE is in a form of
Command Mode. Any command sent to the DCE will cause it to
exit the DTMF detection mode and be obeyed.
Many of the currently available facsimile data-pumps
cannot quickly detect a DTMF digit without additional
hardware. They are, however, capable of detecting the
column tones, allowing at least the digits 1, 2, and 3 to be
detected.
A better syntax may be determined by the TR30.4 group which
will satisfy this function.