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1994-11-13
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Date: Thu, 16 Jun 94 04:30:06 PDT
From: Advanced Amateur Radio Networking Group <tcp-group@ucsd.edu>
Errors-To: TCP-Group-Errors@UCSD.Edu
Reply-To: TCP-Group@UCSD.Edu
Precedence: Bulk
Subject: TCP-Group Digest V94 #120
To: tcp-group-digest
TCP-Group Digest Thu, 16 Jun 94 Volume 94 : Issue 120
Today's Topics:
Standard Digital Radio Interface Proposal
Send Replies or notes for publication to: <TCP-Group@UCSD.Edu>.
Subscription requests to <TCP-Group-REQUEST@UCSD.Edu>.
Problems you can't solve otherwise to brian@ucsd.edu.
Archives of past issues of the TCP-Group Digest are available
(by FTP only) from UCSD.Edu in directory "mailarchives".
We trust that readers are intelligent enough to realize that all text
herein consists of personal comments and does not represent the official
policies or positions of any party. Your mileage may vary. So there.
----------------------------------------------------------------------
Date: Wed, 15 Jun 1994 20:38:15 -0500 (CDT)
From: Jeffrey Austen <JRA1854@tntech.edu>
Subject: Standard Digital Radio Interface Proposal
To: tcp-group@ucsd.edu
Here is an article which was just published in PSR. Comments?
A Proposal for a Standard Digital Radio Interface
Jeffrey Austen, k9ja
2051 Clearview Dr., Cookeville TN 38501, USA
Internet: jra1854@tntech.edu
PacketBBS: k9ja@wa4uce.#midtn.tn.usa.noam
Introduction
Just about everyone who has ever used packet radio has had to deal with
what should be a simple task: that of properly connecting radios and
terminal node controllers (TNCs) together. Unfortunately, many people
have learned that it is not very simple. Not only do the proper signal
connections need to be determined between each radio and TNC but the
correct audio levels must be set in order for the system to work well.
This problem is compounded for persons with multiple TNCs or multiple
radios. Every time a radio or TNC is changed, the system must be
readjusted for proper receive and transmit audio levels, as well as
proper delay times to accommodate the key-up time of the transmitter.
These problems are exacerbated by the existence of differing connectors
for different models of radios and TNCs. All of this can be attributed
to the fact that the interface between the equipment uses analog
signals despite the fact that packet radio is a mode of digital
communications. For operation at speeds greater than 1200 bits per
second (b/s) most radios do not even provide a connector for the
appropriate signals. Operators of digipeaters or remote sites are
burdened with the task of hauling around extra test equipment and
adjusting radios on-site instead of performing these adjustments in a
convenient location such as a laboratory or home station. Emergency
operation is difficult because it is almost impossible to properly
connect various equipment quickly in the field unless the exact
configuration is known beforehand.
In this article a proposal for a digital radio (DR) interface is
developed. This interface is designed to support all current packet
modulation methods and speeds and any which can be reasonably
anticipated for future use. It provides "plug and play" operation
between any digital radio and TNC (from here onward the term TNC refers
to a TNC or any other device, such as computer or packet switch, which
processes the data being communicated). It can be easily incorporated
into most of the current equipment and it allows the use of a single
radio in multiple packet modes without changing cables or making any
adjustments.
Requirements
The requirements of the interface are as follows:
- connect any DR to any TNC;
- be transparent to the data stream;
- operate over a wide range of speeds;
- operate with both synchronous and asynchronous modulation modes;
- operate in both full- and half-duplex modes as well as in transmit-
only and receive-only systems;
- provide good immunity to electromagnetic interference (EMI);
- be tolerant of variations in the equipment: not require any
adjustments when equipment is changed;
- operate over cable distances from zero to at least 10 meters;
- be usable for all existing digital communications modes and for all
anticipated modes in the future;
- operate at all existing speeds and at all reasonable future speeds,
at least up to 2 Mb/s;
- have a single standardized connector so that connection is "plug and
play;"
- sense when cable is disconnected or when the DR is powered down;
- make use of existing standards, where possible; and
- allow easy migration from the current system.
Development of the Interface
In a digital communications system the digital information is
communicated by representing the information as an analog signal. For
the interface the simplest representation should be used for the
information being sent: this is a serial bit stream. To do this, it is
necessary to move the "modem" out of the TNC and into the DR. This
change has a benefit of making the dual use (voice and data) of the
radio easier to accomplish. A front panel switch could easily select
between voice and one or more data modes; for example, a 2-m radio
might be built to support voice, 1200 b/s packet and 9600 b/s packet.
Many standards have been developed for use in data communications.
Some standards which are related to the needs of this interface are
EIA/TIA-232, EIA/TIA-422, EIA/TIA-423, CCITT V.10, V.11, EIA/TIA-449.
There appears to be no standard which provides the necessary
functionality; however, some standards can be incorporated into the
interface.
Examination of the information which must be communicated across the
interface yields the required signals. The fundamental information
which must be conveyed across this interface is receive data and
transmit data. Auxiliary information is necessary to indicate where
each data bit is, when the data is valid, and when the data can be sent
and received. To accommodate both synchronous and asynchronous systems
at varying speeds a synchronous interface is used. The receive and
transmit clock signals originate at the digital radio and are
independent of each other.
To send data from the DR to the TNC the following items are necessary.
- Receive Data: the data from the DR to the TNC.
- Receive Clock: a clocking signal for the receive data, originating
at the DR.
- Receive Data Valid: a signal originating at the DR which indicates
that the receive data signal is valid (similar to carrier detect).
To send data from the TNC to the DR the following items are necessary.
- Transmit Data: the data from the TNC to the DR.
- Transmit Clock: a clocking signal for the transmit data, originating
at the DR.
- Request To Send: a signal from the TNC to the DR indicating that
data transmission is requested.
- Clear To Send: a signal from the DR to the TNC indicating that data
transmission may proceed.
One other signal is necessary to convey the DR status to the TNC.
- DR Ready: a signal from the DR to the TNC indicating that it is
powered up and capable of reception and/or transmission of data.
The Interface Proposal
The signals listed above will be sent using a combination of EIA/TIA-
422 (differential) and EIA/TIA-423 (single-ended) signal levels. The
two data signals and two clock signals, because of the potentially high
speed will use differential signaling, which provides for speeds of up
to 10 Mb/s. These signals will use eight wires of the interface. The
status signals will use single-ended signaling because high speed is
not necessary. These signals will use four signal lines and two ground
lines (one in each direction, per EIA/TIA-423 specifications). To
insure proper operation under fault conditions (either unit is powered
down or the cable is not connected)"fail safe" line receivers must be
used for the four status signals (RDV, RTS, CTS, and DRR).
Much of the delay necessary at the beginning of the transmission are
due to internal delays in the transmitter. This delay is made the
responsibility of the DR rather than the TNC. When CTS becomes active,
data can be sent immediately; after the last bit of data has been sent,
RTS may become inactive. Additional delay may be added in the TNC (as
is done currently).
The physical connector selected is a high-density 15-pin D-series
connector. This connector is small enough to be used on mobile and
portable equipment and yet it reasonably rugged, reliable and
inexpensive. The male connector (plug) is used on the TNC and the
female connector (socket) is used on the DR. Although the same type of
connector is popular for computer displays, the opposite sex connector
is used on the computer so that confusion should not occur. Cables
will act as "extension cords," that is they pass all pins straight
through from the connector on one end to the other end. The shell of
the connector must be used for the shield connection if a shield exists
on the cable; if no shield exists the shells must be connected by a
wire in the cable. All DRs and TNCs must have metallic connector
shells so that shielded cables can be used effectively.
Alternative Interconnections
Although the interface is specifically designed to connect a DR to a
TNC, it can be used to connect two DRs or two TNCs together or it can
be used in a transmit-only or receive-only system. To connect two DRs
together there needs to be a adapter which contains a FIFO large enough
to accommodate the largest packet at the maximum speed differential
between the systems. To connect two TNCs together there needs to be an
adapter which generates appropriate clock signals. The receive and
transmit signals are independent of each other so they can be running
at different speed or be going to different DRs; use in a transmit- or
receive-only system is also possible (a protocol other than AX.25 will
be necessary in this case).
Incorporation Into New and Existing Equipment
This interface can be incorporated into new radio designs by including
the "modem" with the radio and providing a method for switching modes
(e.g., voice, 1200 b/s data, 9600 b/s data). Most TNC designs can be
updated quite easily to incorporate the interface without eliminating
any current features.
Most existing systems can be easily modified to use the new standard.
It appears that the PackeTen, DataEngine, PI, PI2 and PackeTwin cards
and Kantronics DataEngine modems require very little modification as
the appropriate signals are available to easily add the interface; the
only significant change is that with the new interface the modem is
physically housed with the DR, not the TNC. In general, any modem
which performs clock recovery can be easily modified for use with this
interface. Any TNC which provides the "modem disconnect header" can
have the interface added to it by using that connector. A smooth
transition from the current system to using the new interface can be
made by providing adapter kits for common modems and TNCs so that
current equipment will not be obsoleted.
Summary
The interface proposal presented here will solve the problem of
connecting digital radios and terminal node controller or computer
equipment together. It provides a simple, inexpensive, versatile, and
easy-to-use solution. It is applicable to all current packet radio
systems, as well a other digital systems and it does not inhibit future
improvements to packet radio systems, either in the modulation and
coding techniques or in the protocols. While the exact specifications
remain to be finished and tested through implementation, much existing
technology is being used and no problems are anticipated. The author
welcomes suggestions for the improvement of this interface and is
interested in hearing from a few persons who are willing to design and
test interfaces for various modems and TNCs.
------------------------------
End of TCP-Group Digest V94 #120
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