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  Using the Motorola GR1225 as a Repeater
By Don L. Blanchard - WA7GTU
IRLP Node 3574
Edited and HTML'd by Mike Morris WA6ILQ
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Note from the editor:

The R1225 is a continuous duty radio (about the size of a MaxTrac) that has two antenna connectors and will work in full duplex mode, and 25w continuous duty at that! The GR1225 is that radio in a box with a power supply plus two options: a duplexer and an advanced controller (the sales brochure is here). The GR1225 was designed to be the continuous duty model in the the GR300 / 400 / 500 line of prepackaged repeaters, all of which were intended for low-duty-cycle full duplex applications like a shopping mall security department repeater, or other low-end non-public-safety repeating applications. The GR1225 configuration parameters allow it to be programmed as a repeater or as a non-repeating full duplex radio and that, coupled with the continuous duty design makes it ideal as a point to point link, and much better than a Maxtrac or GM300 mobile, neither of which can duplex, or run continouous duty. The R1225 / GR1225 also has a fairly decent internal controller where the GR300 / 400 / 500 series required the RICK box to do that. The optional advanced controller could be one of four, and an autopatch was available (see the sales brochure link above).

The Internet Repeater Linking Project is a worldwide informal group of amateur radio repeaters that are linked using VoIP (Voice over Internet Protocol) systems. More information on IRLP is available at http://www.irlp.net. Each repeater in the IRLP network is connected to the internet through a PC running the IRLP system program, with runs on the Linux operating system. The computer is treated as if it was a link radio, with audio in, audio out (via the sound card), COR and PTT (via pins in the printer port). The IRLP interface board that plugs into the printer port has the necessary interfacing circuitry, including a hardware touchtone decoder. The board can be jumpered for simplex or full duplex operation, and if you have a full duplex environment you really want to set the jumper to match. A "Node" in the IRLP system is one computer and repeater system.

The ideal IRLP situation is where you have an internet connection at the repeater site and can connect the node computer to the repeater controller directly (as if it was a second radio). Since very few repeater sites have an internet connection many connections use the second-best option: a point-to-point radio link (on UHF, 900 MHz or 1200 MHz link frequencies) between the repeater site and a location where the node computer and internet connection is actually located. Others use the third-best option: they set up the node computer and radio as if it was just another user on the repeater.

This article describes how WA7GTU set up a Motorola GR1225 radio as a local repeater, with an IRLP node connected to it. The same GR1225 if programmed differently could also function as a "ground level" end of a point-to-point link, i.e. set up as a non-repeating full duplex base station.

There are pages at this web site that cover the RSS and RIB in detail.

IRLP Node 3574, located in Cedar City, Utah has been operating with a Motorola Radius GR1125 Repeater on the 444.900 ‑ 449.900 MHz frequency pair for a couple of years. There have been a number of inquiries as to how it was interfaced with requests to make the information available on the Internet. The following document is an attempt to answer those inquiries. It is believed that the mobile GM300 is basically the same radio and the following information should apply to both models, however the GR1225 will run full duplex, whereas the GM300 is a "normal" PTT radio.

There are several models of the GR1225, and the product line includes models that cover both the 146 to 174 MHz and 444 to 474 MHz frequency ranges with differing power levels. The 146-174 models can be "stretched" down to 145 MHz, and the 444-474 models can be stretched to 440 MHz.

The first thing that you need to know is that the GR1225 has to be programmed using the appropriate Motorola RSS software, so you either need to know someone with the hardware/software or have good rapport with your local Motorola dealer. The hardware/software is of fairly old vintage and hopefully it is still available in your area. The radio programming software is DOS based and will not even work on today's faster computers. The programming is done through the microphone plug with a computer interface called a RIB. There are diagrams of these interfaces on the Internet, but getting the software is a problem. Please see the RSS page in the Motorola section at this web site for details. The second thing that you need to know is that the 450 MHz version of the GR1225 will program no lower than 444.000 MHz without modification. The 3574 node repeater is operating within this frequency range, so no attempt was made to look into what modification would be required to go lower in frequency, but it probably can be done.

The GR1225 is a small self contained repeater (see photo below) that can be programmed to 16 different frequencies. The 3574 node repeater is also programmed to a second frequency of 444.500 - 449.500 MHz which is another frequency pair coordinated for this area. The built in duplexer handles this second frequency reasonably well with very little degradation, but this frequency is only there as a backup and adding it to the programming took less than a minute. The repeater operates from a built in switching power supply and there is a fan integrated into the housing for cooling. It often operates for hours at a time and there have had no temperature problems. The fan is temperature controlled and does not come on very often. The model in operation is a 10 watts version. The author also has an older 25 watt version as a spare, but has not yet tried it on IRLP. It is all programmed, but some of the connections on the Accessory Connector need to be rewired.

Below is a reproduced page from the GR1225 Service Manual explaining a bit about the available connections in the accessory connector on the rear of the radio. The fuctions of a number of the pins are programmable.

Expanded Accessory Connector R1225 Transceiver Service Manual

Programmable Pins

Pins 4, 6, 8, 9, 10, 12, and 14 on the accessory connector are programmable. The function of each pin can be assigned using RSS. Information on the available functions and how to program them is contained in the RSS help screens and in the files in the Appendices section.

Pin 4 is an output only. It provides an active high to the 13.8 V dc supply (0.25 Amps maximum), otherwise it is pulled low via 10k ohms.

Pins 6, 10, and 9 are inputs only. They are normally pulled high inside the radio to 5 V dc via 4.7k ohms. To activate any of these input pins, it should be pulled low to within 0.7 V dc of ground.

Pins 8, 12, and 14 may each be programmed as either an input or output. If programmed as an input, the pin is pulled high to 5 V dc via 4.7k ohms. To activate the input, it should be pulled low to within 0.7 V dc of ground. If programmed as an output, the pin is normally pulled high to 5 V dc via 4.7k ohms. When enabled, the output goes active low. Maximum sinking current is 50 mA.

As mentioned above, a number of the accessory plug pins can be programmed for various functions. Several are electrically bidirectional but are programmed as either in or out. The manual says that Pin #8 defaults to COR, but on mine it was not, so that function had to be programmed in addition to the operating frequencies, PL, timeout timer, and identifier parameters (callsign and 10 minute spacing). The repeater also came with an existing interconnect cable and all that was required was to move two of the pins in the accessory plug and put the connectors required to interface with the IRLP board on the other end. See the Expanded Accessory Chart below.

The table below is from section four of the June, 1997 version of the R1225 manual, part number 6880905Z53-O.

Section 4
Expanded Accessory Connector

The following is a description of the pin functions on the Expanded Accessory Connector for the R1225 Transceiver module. Refer to Figure 4-1 above for pin locations in the connector housing.

Pin Description Application
1 External Speaker (-) Connect an external 8-ohm speaker to pins 1 and 16.
CAUTION: This is a bridged-type (floating) output. Neither pin 1 nor 16 is ground or can be grounded.
2 External Microphone Audio Input impedance: 500 ohms.
80 mV rms at 1 kHz for 60% deviation.
this path is enabled when external mic PTT is keyed.
3 External Push-To-Talk in or out Pull this pin low (less than 1.8 vDC) to key the transmitter and enable the external mic audio path. This pin is pulled low via a diode when front panel mic PTT is pulled low to allow sensing of mic PTT by an accessory. This pin is pulled high to 5 V dc via 9.6k ohms.
4 Programmable Output Defaults to Null. Provides an active high to 13.8 V dc battery supply. Maximum current is 0.25 Amps. Refer to the "Programmable Pins" section below.
5 Transmitter Flat Audio Input Input impedance: 35k ohms. 150 mV rms for 60% deviation. May be programmed to bypass limiter using RSS.
6 Programmable Input

Defaults to Null. Refer to Refer to the "Programmable Pins" section below.

7 Ground  
8 Programmable Input/Output Defaults to COR carrier detect. Refer to the "Programmable Pins" section below.
9 Programmable Input Defaults to Null. Refer to the "Programmable Pins" section below.
10 Programmable Input This pin has two functions: Null and Repeater knockdown (enbale/disable). This pin is defaulted to Null Input, but may be programmed for repeater setup / knockdown by the RSS. In Repeater Mode of operation, activating this pin knocks down (disables) the repeat function and places the unit in Base Station Mode. The front panel "RPT EN" pushbutton is overridden by pin 10.
11 Rx Audio Output Output impedance: 560 ohms. 330 mV rms (at 1 kHz if de-emphasized) at 60% deviation. Minimum load resistance: 5k ohms. Default is de-emphasized and squelch muted. May be programmed for flat and unmuted using RSS.
12 Programmable Input/Output Defaults to Null. Refer to the "Programmable Pins" section below.
13 Switched A+ Sense Positive 13.8 vDC source for accessories that is hot when the radio is turned on. Note that the maximum current available is 0.5 Amps.
CAUTION: Accidentally shorting this pin to ground with the transceiver turned on can damage the PC board plating.
14 Programmable Input/Output Defaults to Null. Refer to the "Programmable Pins" section below.
15 Internal Speaker (+) If jumper JU501 is removed, jumper this pin to pin 16 to enable the internal speaker.
NOTE: If the HLN3145 Public Address and Speaker A/B Switch kit is used, jumper JU501 must be removed if it is desired to mute the internal speaker when the switch is in position B.
16 External Speaker (+) Connect an external 8-ohm speaker to pins 1 and 16.
CAUTION: Bridged-type (floating) output. Neither pin 1 nor 16 is ground or can be grounded.

The call sign identifier for IRLP can be programmed via software in the IRLP box, but this wasn't adequately researched prior to getting the radio programmed, so for node 3574 it is programmed into the radio itself. There is one inherent problem with using the built in radio identifier and that is if the repeater is keyed up before the identifier completes its cycle, it will start over again when the repeater is un-keyed. This is usually not a problem locally, but it is a problem when it is connected to a reflector and there is not a sufficient pause between transmissions for the identifier to drop out. Programming the call for higher speed helps in this case. There is also a programmable transmitter timeout timer which is currently set to 3 minutes and it gets timed out on occasion. A little longer time period might be preferable.

In my system the GR1225 Carrier Delay set to Off. This means that if you "key up" you will not get a response back unless the identifier comes on. Our PL is set to 100 Hz. The use of PL on IRLP is HIGHLY recommended.

Just remember that if you need to make any changes to the configuration of the radio (i.e. the programming), it means another trip back to whomever has the RIB (the programming hardware) and the Radio Service Software (RSS), so try to get it right the first time. The author has a good relationship with the local Motorola dealer and was fortunate enough to wind up with a complete set of service manuals for the GR1225, but does not have the required software, progamming box or cable for programming the radio. If you end up with a GR1225 the two manuals that you will need are 6880905Z53 (the R1225 radio) and 6880904Z90 (the GR1225 cabinet, power supply, etc). The Radio Service Software has its own manual.

The figure below illustrates how the GR1225 repeater is interconnected to the Version 3.0 IRLP Board. Note that the IRLP board has its own DTMF decoder and the audio feed from the radio goes to both the computer LINE IN connection and also to the IRLP board pin 8.

If you are connecting a GR1225 to a Version 2 IRLP board you will need to use the two diagrams below to identify the changes. Note that NOTHING in the pinouts is the same between the two versions.

The version 2 board pinout.
The version 3 board pinout.

A final note, please do not request copies of the Service Manuals from the author. Many of the schematics are very large and with both books the stack is about 3/4 of an inch thick. Pretty much all of what you need to know is in the information above.

Don L. Blanchard - WA7GTU - Nov. 2006

Three notes from WA6ILQ on the GR1225 units:

  1. The R1225 is a 16 channel radio. Adding the local "test pair" to the channel list takes an extra minute as you are programming it, and depending on your circumstances might be a good idea.

  2. The stock GR1225 power supply is the HPN90333A and is made by Duracomm. It is a switching supply and weighs about six pounds. The output is 13.8 volts at 15 amps continuous and 17 amps intermittent duty. The optional HLN9455A Battery Revert Kit adds a power fail/switchover/battery charger module that connects a user-supplied sealed battery to the unit, keeps it charged, and switches to it during a power failure. Don't bother buying one, just get the schematic for it and build it yourself.

  3. One of the options in a GR1225 is an internal duplexer (part number RFE4000A, made by Celwave as their part number 633-2A-2N). It is a notch-only duplexer. The HKN9235 Internal Duplexer Cable Kit (about $40 from Motorola Parts) consists of two cables that have N-type male connectors on the duplexer end and mini-UHF male connectors on the radio end. The cables that Moto supplies are not fabricated to meet any technical requirement (i.e. length), they are simply made long enough to fit with only a little slack.

    The cheap plain-jane RG-58/U cable used in the HKN9235 Internal Duplexer Cable Kit causes some serious desense - more than 2 dB, in fact. RG-58/U coax is just too leaky to be used for duplexer jumper cables of any type.
    If you are ordering a GR1225, forget the duplexer. Pick one up off of eBay and make up your own cables, using only double-shielded RG-400/U cable, and install the correct silver-plated connectors on each end - and without any adapters.

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This page originally posted on 21-Nov-2006

Photos and article text text © Copyright November 2006 by Don L. Blanchard, WA7GTU
Artistic layout and hand-coded HTML © Copyright 2006 and date of last update by Mike Morris WA6ILQ.

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.