A compatible IRLP Interface Schematic

It has occurred to many that the board could be easily made for very little expense. However there is no official schematic for the interface board.  Maybe you are interested in messing with a Linux based repeater controller. Or are I need of a spare IRLP board for interim node setups and testing.

Per the FAQ, they tell you that you cannot build your own board, this is only because nothing but problems and headaches have come to the mailing list so far from user-built boards.  The mailing list is very busy with other trouble shooting issue and various questions.  They just simply don’t want to have to deal with user-built hardware issue.  So if you know what you are doing, go right a head and build one, but please do not ask for help on the mailing list regarding it.

The male DB-25 on the IRLP board connects with a supplied short male to female DB-25 extension cable to the printers parallel printer port.  The male DB-9 strap that connects to the main board has all the radio connections for interfacing to ones repeater / radio.  The board is designed to mount into two spare PCI computer slots, as  the  Version 3 board has an on-board 4 conductor male molex connector for connection to the computer power supply.  The 5 volt DC lines actually power the board.  There is also PTT lockout jumper allows DTMF commands to be received when the transmitter is transmitting, for duplex environments (not the default position).

As seen here, Kyle, K0KN has designed a schematic for people in these situations, to emulate the IRLP board.  Otherwise you might find some people willing to part with theirs on the IRLP mailing list.

This schematic has nearly every feature and capability of the IRLP v3 board, that sells for $105, and can be constructed for about $25.  An important financial consideration is that many manufacturers of personal computers and laptops consider parallel to be a legacy port and no longer include the parallel interface.   (David Cameron, VE7LTD (IRLP System Designer)  reported 12/06 & 11/07 that he is working on USB based solution, or a non parallel port IRLP interface card as these are being faded out in modern computers, even more so in embedded systems.)

Parts List:

QTYICsQTYResistors QTYMisc
1HCF-4081 (Dual 4 Input And Gate)7330 ohm11N914 Diode
1MT8870 (Mitel CMOS Integrated DTMF Receiver) *1820 ohm7LEDs
Capacitors51 k ohm 1male DB-25 (right angle PCB mount)
10.1 uF42.2 k Ohm1male DB-9 (right angle PCB mount)
10.01 uF63.3 k ohm
110 k ohm variable13.579 MHz reference crystal
1100 k ohm
12.2 m ohm

* (The MT8870 is becoming hard to find. In 2001, Mitel was split into two parts.  The original company retained the semiconductor division. It was renamed  Zarlink Semiconductor. It is unknown to me if the original MT8870 exists under a  new part number (MT8870DE1), or if it just no longer being produced.)

More information on the IRLP board wiring, interfacing to your radio, audio wiring and other installation notes can be found here: http://www.irlp.net/new-install/

The IRLP board is a very simple circuit, the most difficult part being the on-board DTMF decoder.  The board is powered from the 5 volt lines (red and black) of a standard computer molex connector.

The DTMF decoder is comprised of a MT8870 (or similar) DTMF decoder IC and HCF4081 (or similar) and-gate IC. The MT8870 uses digital counting techniques to detect and decode all 16 DTMF tone-pairs into a 4-bit code  Two chips are needed because the MT8870 has latched outputs, and the IRLP software looks for short pulses at the parallel port pins 10,12,13,15 in order to acknowledge a DTMF digit. MT8870 pin 15 provides a pulse when any valid DTMF digit is decoded, so this signal is used on one input of each gate on the HCF4081. The other gate input is from MT8870 pins 11,12,13,14. The output of the HCF4081 (pins 3,4,10,11) connect to the parallel port and provide the pulsed input that IRLP needs.  From there basic Linux I/O port programming is the basis of the various software tools to key the transmitter, read the COR status, and interpret DTMF.

The external DTMF decoder helps to ensure you have dull duplex control at all times, no matter what the state of /dev/dsp.. The decoded digits hit the custom_decode file where you can script/define functions.  It is well designed as the COR can be active high or active low. The COS jumper on the board switches between active high and active low. Also the voltage swing point of the COS can also be adjusted with the LGC SEL jumper.  The COR must swing above and below 2.7 volts in one position and in the other, you can set that swing point to 1 volt.

IRLP uses the following parallel port pins:

3  PTT
4  Aux pin 1 (active high at parallel port)
5  Aux pin 2 (active high at parallel port)
6  Aux pin 3 (active high at parallel port)
10  DTMF 4
11  COS
12  DTMF 3
13  DTMF 2
15  DTMF 1
18-25 Ground 

A few things to point out and clarify:

If you build your own you obviously don’t get a an IRLP node number to coincide.  This comes when you fill out an order form and payment.  The IRLP board is $105. They also suggest a $40 donation.  The IRLP software can be downloaded to save some cost.  But if you wish, when you order the IRLP board you can get a pre-made ISO IRLP software disk that is provided by VE7LTD (David Cameron) for a cost of an extra $30.

Date: Sat, 27 Dec 2008 17:55:47 -0800
To: [email protected]
From: Stephen Rodgers <[email protected]…>
Subject: Re: [rtpDir] Re: new to the project – questions

Yes, that is the usual way to get an IRLP node, but there’s now another way as well. Use ACID with chan_irlp using the irlpsetup.sh script, then send $40.00 via Paypal to Dave Cameron with your newly assigned node number in the comments section of the payment, and he’ll take care of adding your node number to the PGP public key ring.


On Dec 27 17:19:46 2008: JI1BQW, Kay Ishikawa wrote:
You have to purchase an IRLP board to be assigned a node number and added in the authentiation key ring. Without those, you will not be able to connect to other IRLP nodes or reflectors, even if you install  the IRLP software.

http://www.irlp.net/guideline-FAQ.html:  “IRLP PGP keys are only assigned to users that support IRLP, either by purchasing IRLP hardware or by making a donation to the project. “  These guidelines were last revised by David Cameron, VE7LTD (IRLP System Designer) January 28, 2009 acknowledging advancements in other voice over IP (VoIP) systems.  (Per: http://www.mail-archive.com/[email protected]/msg51571.html)

When questioned in November 2006, about Asterisk and Allstarlink, David Cameron, (VE7LTD) IRLP system designer said they are currently not working or supported pieces of IRLP, although they may be in the future.   It seems that that migrating IRLP to an Asterisk based platform will be pushed by the obsolesce of the PC’s parallel port.  Asterisk code has been developed for USB sound “pocket”/ fob sound devices based on the CM108 chipset for inexpensive do-it-your-self radio interfaces as well as enhanced commercial manufacture of them.  The realization of a need for an IRLP redesign in the future, is probably what prompted the change in IRLP guidelines and acknowledgement of advancements in other voice over IP (VoIP) systems.

In October 2012 per the  Per request of Dave Cameron, VE7LTD, support and functionality for IRLP connectivity is officially dropped from the app_rpt project.

It’s the opinion of many that IRLP is literally infested with politics and the want to make money off of Ham radio.  The result of this is simple. In Las Vegas, Nevada a lot of us are SHUTTING DOWN their IRLP nodes. They are no longer needed. Allstar simply outperforms IRLP hands down.

Running tlb or app_rpt as an IRLP node isn’t “officially” supported. To do it in a politically correct way requires an experimental node number assigned by Dave Cameron.

> On 2/1/09, ve7ltd <[email protected]…> wrote:

Experimental node numbers assignment:

In order to obtain and use an experimental IRLP node number, I

1) Your name
2) Your callsign
3) Your IP address (static) or fully qualified hostname (no-ip.org
etc) **SEE NOTE**
4) What IRLP codec you would like the IRLP nodes that connect to
use, and if full duplex
5) Whether you want DTMF tones to be muted or not.
6) What current IRLP nodes you own or maintain
7) What your planned use of the experimental node is (software
used, etc).

Send this information to <[email protected]>,
not to my address.

In return, I will add you to the list of experimental nodes,
which is contained in the exp-x-reference file.

In order to participate in the experimental node system,
participating nodes must add the following flag to their
environment file:


Nodes who do not add this will get a “node does not exist” when
they try and call.

The IRLP files that have changed or been added:

scripts/decode – added the ability to split out the 0000-0999 as
experimental scripts/exp-x-reference – the file that contains all 
the IP/host info scripts/experimental_call – the script that sets 
up the calls

These will be automatically updated tonight, or you can manually
force an update.

The call that is made is the same call that is made to the echo
reflector. No verification of the identity of the node, no ping,
and no TCP connection is made.

Limitations –

1) Only one assignment per callsign at first. Once the demand has
been determined, others will be allowed if available.
2) No updates to the entries will be made once set, so verify the
info is correct.
3) There will be no status reporting on the IRLP status page,
except that IRLP nodes will show they are connected to 0010 for 
4) IRLP reserves the right to remove these experimental
at any time, for any reason
5) No requests for specific numbers – they will be assigned in

**NOTE** – If you think that your node’s IP will change, get a
dynamic DNS hostname setup. I will not be updating the exp-x-
reference file after setup.

Dave Cameron

Another issue is that the binaries that talk to his IRLP hardware board  are not open source.   However, the basis of the IRLP board signaling uses Linux parallel port programming.  

IRLP system designer David Cameron, VE3LTD had developed a similar (commercial) system to IRLP that is being used commercially, and this to may be part of his proprietary design reasoning  http://www.roip.com/  The hardware (the radio interface card) is the same as the amateur product. The software is somewhat different but definitely is not free.

Another reason the board design is unpublished is that the board is the IRLP designers only source of funding to keep things alive.  Also to cut down on the number of questions/problems from the non-technical savvy.  While neither of these are bad, it still doesn’t let technical savvy people do much more with.  This closed architecture doesn’t seem to be in the true spirit of amateur radio if you ask me.  The system architecture is simple and powerful.  One can have a lot of fun and learn a lot in the process by playing with it.  

However, Adi Linden, VA3SLT and others have taken the code that Dave Cameron released and built upon it for a home brew repeater controller.  The code Adi produced remains open source and is available here. (An updated version here.) The code still lacks considerable documentation and proper acknowledgment of its origin and contributors.  All the features one would expect to find in a repeater controller are supported:

  • Basic COS and CTCSS controlled repeater operation
  • Interoperability with IRLP
  • Configurable courtesy beeps for different repeater states
  • Intelligent CW repeater ID
  • Fan control

IRLP uses modified version of Speak Freely to stream audio to the remote station.  Speak Freely was originally written by John Walke of Switzerland.  While it was developed in 1995-96, it has roots back to it’s preceding program from 1991.  A 2004 end of life announcement declared the final release of the program.  Since then, an afterlife development program has been registered on SourceForge.

Speek freely has two major command line tools; sfmike and sfspeaker.  imike and ispeaker are the IRLP modified versions that have COR and PTT support.

IRLP is an interesting simplistic approach to streaming radio audio over the internet.  However IRLP has largely been replaced by Allstar /Asterisk app_rpt.  

As mentioned earlier, IRLP is not open source.  And the IRLP board is not an open design either.  Neither however are impossible to figure out.  The core problem for many is there is a lot of politics and operational considerations on the Internet side of IRLP.  Who gets connected to who and who doesn’t. What service allows connections and what doesn’t.  Many folks are interested in experimenting with interoperability solutions to link other VOIP and digital radio systems together. 

The open atmosphere of Allstar makes this in most situations an individual decision rather than the decision of some hierarchy.  With IRLP you either had a choice of connecting to one person or to a reflector run by an administrator who dictated how things were conducted. Individuals had little choice.

Allstar gives you that choice. 

(*Note IRLP & ROIP are not registered trademarks per the Trademark Electronic Search System (TESS).  But both are developments of David Cameron, VE3LTD.  Echolink is a trademark of Synergenics, LLC., and Asterisk is a trademark of Digium.) 


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