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  Interfacing the
Motorola MTR2000 To
External Controllers

Originally Written And Published
By Robert E. Shepard
(Withdrawn At His Request)

Revised For Repeater-Builder
By Robert W. Meister WA1MIK
(Because It's Useful Information)
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I have to say this was one of the tougher controller interfaces I've had to deal with, although it did not have to be. Determining which wire went where was not a problem as much as dealing with a lot of quirks along with incomplete and occasionally conflicting information from many sources. Hopefully, most issues will be covered here lest the next person have a similar headache. This particular installation was to a Computer Automation Technologies CAT-250 controller. However, the information should be valid for almost any external accessory.

Start Prepared:

Before beginning modifications to any station, it is highly recommended that you first test the station operation as is. Do not assume that the station, its features or options, are working properly simply because they are physically present.

Required Items:

The Station Interface:

The physical connections to the station are straightforward, save for obtaining the 96-pin connector, which for me was initially quite a hunt. The connector has 3 rows, A, B, and C, each with 32 pins. A word of caution here: the 3rd party male connector will most likely have the pin numbers labeled on the connector body. Do not go by these numbers! The back of the station's female connector is clearly labeled and for reasons unknown, does not agree with the male connector's manufactured labeling. The image below depicts the MTR2000 system (female) connector as viewed from the rear of the station


Note: The matching male connector (cable connector) numbering-wise, is a mirror of the female (station connector) when viewed from the rear (as it would be connected) and identical when viewed from the pin side (pins facing you). It may be that mirror effect of the pin numbering is because the male connector I used was designed as PCB mount. In any event, when in doubt, use the pin orientation of the station connector depicted here, which matches the station labeling.

The image below depicts the recommended pins for basic repeater operations along with some additional accessory power and grounds. Again, the view is facing J5 (system connector) from the rear of the station.


J5 System Connector, Pin Descriptions:

The following describes the connections to the station and their functions.

C02 - RX unsquelched - TTL output, Active High. Active when the receiver audio is active (unsquelched). If the station is programmed for CTCSS or DCS (i.e. PL or DPL), this line only activates when a signal with the proper code is received.

B04 - Carrier Detect - TTL output, Active High. Just as it says, active when a carrier is present, regardless of the state of the receiver's squelch setting or presence of CTCSS or DCS.

C10 - PTT - TTL input, Active Low [1]. Applying a ground to this pin will activate the station's transmitter.

C17 - Discriminator Audio Output. Use coupling capacitor.

A17 - Auxiliary Transmit Audio Input [2]. Use coupling capacitor.

A20, B20, C20 - +5Vdc [3].

A32, B32, C32 - +14.2Vdc [4].

A19, B19, C19, A27, B27, C27, A31, B31, C31 - Station Ground [5].

The following additional I/O pins may be useful depending on your particular operation.

A10 - VSWR_Fail, Active Low. Pulled up transistor output (10K to +5Vdc).

A04 - AC Fail, TTL output, Active Low.

Pin Notes: [1]: PTT must be mapped in RSS to "Wire line and Auxiliary Audio" for external controllers. Otherwise the audio path will not be enabled from the auxiliary transmit audio input and no transmit audio will be heard.

[2]: The Auxiliary Transmit Audio level must be set in the RSS. This may or may not be set depending on previous programming. By default, the value is zero (off), and no transmit audio will be heard until this is corrected. This level is fixed on stations with old firmware. You can find this in the Service menu.

[3]: Total 5Vdc current through all pins (summed) not to exceed 500mA

[4]: Total 14.2Vdc current through all pins (summed) not to exceed 1.0A

[5]: Total ground current through all pins (summed) not to exceed 1.5A

Cable/Pin Connector Tricks:

If you're like me, trying to solder wire onto a densely packed, multi-row connector is a living hell. I came up with a little trick that works perfectly on this 96-pin connector. On the ends of the wires, I attached a female pin for a DB-25 type connector. This allowed me to slide the cable (pin actually) onto the 0.5" long PCB mount pins of the male connector. Once in place, I simply soldered the female pin to the connector pin, after which I slid on some heat shrink tubing to isolate and strengthen the connection.

I recommend the removal of any unused pins from the male connector. The density is such that a simple bump or cable tug could bend a pin (or pins) into a shorted situation, resulting in the "smoke theory", which on the MTR2000 would be an expensive boo-boo.

I was initially going to put heat shrink over all unused pins but that was tedious, not to mention the pins can be removed with a good pair of needle nose. The one drawback is that with fewer pins, the connector does not remain as snug as it does when fully populated. Use your best judgment or secure the connector with screws. Personally, I'd rather chance having it fall out than have any shorted pins on the MTR2000.

Operational Quirks:

Having a talking controller, one usually would like to hear the repeater when it actually is talking. What little information I had discovered on interfacing to the MTR2000 was usually in reference to multi-tone, community repeater type operations. As such, all of those articles recommended setting the MTR2000 to use carrier squelch for TX and RX.

As a big proponent if using some form of CTCSS, I wanted full time encode/decode operations. Since the CAT-250 controller does not provide CTCSS encode or decode and the MSR2000 does, I figured why buy yet another device. I programmed the MTR2000 to use its own internal encoder/decoder. This created an operational quirk that I personally find to my liking, but others may be annoyed with. As such I am still looking into a work around for those people.

The quirk is that the station will only transmit CTCSS/DCS while it is actively receiving it. This produces the situation for a user running CTCSS/DCS on their receiver, who will only hear actual repeated transmissions of other users. System messages and the CWID are still being transmitted, but without CTCSS/DCS, so users can only hear them if listening in carrier squelch. I am 100% positive this issue lies with the MTR2000 itself, and perhaps there is a programming work around. Alternatively, I may also have, in the case of the CAT-250, a way of not only enabling the system messages with TX CTCSS/DCS, but also the ability to turn it on and off via remote command. To be sure, the system messages and the CWID are always transmitted; the question is simply whether or not CTCSS is sent with them.


I've interfaced controllers to many different commercial and amateur stations. Unlike the older vintage commercial gear such as the Motorola MICOR and GE's MASTR series, these newer radios are not quite as easy to modify. However, with a little bit of patience and reading, it is possible to emulate in software, what use to be done with the soldering iron.

The information used in this document was put together from many sources, most of all my own experience in configuring this setup. It is my hope that this document will help to get your operation up and running a lot quicker than was my initial experience.


Acknowledgements and Credits:

This document originally Copyright 2004 by Robert Shepard (June 11, 2004).

Motorola, MTR2000, PL, DPL, RSS, and a bunch of other terms are trademarks of Motorola, Inc.

Contact Information:

The person responsible for re-publishing this article can be contacted at: his-callsign [ at ] comcast [ dot ] net.

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This article originally written 11-Jun-2004.
Converted to HTML, revised and reposted on 27-Aug-2011.

Original article text © Copyright 2004 by Robert E. Shepard.
Article diagrams and hand-coded HTML © Copyright 2011 by Robert W. Meister WA1MIK.

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.