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  Motorola MSF 5000:
The GRONK Radio Standardized Interface Method

By Matt Lechliter W6KGB of the GRONK Radio Network

HTML'd by Mike Morris WA6ILQ
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Note from WA6ILQ: Matt's experience has been with UHF and High Band MSFs. He's never had to interface a 900 MHz station. Due to the differences in the MSF (the reduced deviation and added Hear Clear circuitry) the information below will serve as a good starting point for 900 MHz stations.


Our standard interface modifications for the MSF5000 stations are detailed below. The result is compatibility with GRONK and Cactus specifications. Most internal functionality is bypassed since it cannot serve this purpose. However, extensive modification is undesired should factory-stock operation be desired later. A simple programming change or even the flip of an existing switch can restore the station to internal repeat in the event of external controller failure or other calamity. This keeps your system on the air (perhaps in a degraded mode) and keeps the channel poachers at bay.

Let’s be honest. The MSF 5000 is an over-engineered station concept. It began Motorola’s departure from commonality between base-station and mobile infrastructure design. Lines like the Motrac, MICOR and Mitrek/MSR2000 all shared basic design between mobile and station. Their packaging was the greatest difference. The MSF5000 was the first station design unproven in field-mobile applications. It’s not what we’re used to, yet a great design nonetheless if managed well.

MSF stands for “Maximum Station Flexibility”. As Motorola’s hot new item in the early 1980s during the first computing booms, it was to support many modules all connected by common bus architecture (MUXbus) to perform many different tasks and interfaces. These modules, naturally, were options from Motorola.

Beginning as an EPROM-programmable version, it was later upgraded. The new “Analog Plus” (“GFB”) and “Digital Capable” (“CXB”) stations were PC-programmable and some supported Motorola’s SECURENET encryption packages. Gradually as they’re replaced, these stations of both versions (Analog or Digital-capable) have become available to us hams, but their makeup is unlike other radios previously encountered. The two versions shall henceforth be referred as “CLB” for Analog EPROM type or “CXB” for Digital type. Note that the “GFB” was a exportable (non-SECURENET) “CXB”, the “RUB” was a Canadian “CXB”, and the “RLB” was a option-limited “CXB” (limited by marketing, the hardware was the pretty much the same). For the purposes of this document they are all equivalent to the ”CXB”.

Interface to the non-Motorola world came through the “WildCard” option. These were bridges between the MUXBUS control signals and some audio pathways. The WildCard module even included a small breadboard-like area that could be used to build small circuits on the WildCard or to simply bring wires out to external devices. WildCard modules aren’t the most plentiful goodies in the world. Obtaining one is half the battle. Documentation of the item is the other half. The author has a module but as yet not obtained any docs.
Note from WA6ILQ: There are several vintages of "Wild Cards" including the QLN2914A (which was known as "Option C232AA, C232AB, C232AG or C233AB when factory installed). The manual on that one is 6881114E40-C.

While WildCards would work, there’s a better and simpler mousetrap. Thankfully, it’s fairly straightforward in either CLB or CXB stations. Ham applications rarely need anything offered by the station’s internal controller and associated options. It falls short for most needs and reaches beyond others. Plenty for some, however those of us implementing MSFs in “California Linking” architectures (as they’ve come to be called), need little from the radio besides exactly that: a raw full duplex radio. The remainder is handled via external controllers. Our controllers are painstakingly designed by ourselves, and over the various generations are purpose-built to our architecture and needs. This modification procedure below suits exactly that.

The CLB Stations:

CLB (Analog) Stations are easy to interface. A handful of signaling points and test points gives you everything. The difficulty with CLBs is having them programmed. This requires the R-1801A suitcase programmer (with the 4KB option), the appropriate R-1801A EPROM set that programs the MSFs and the MSF PROM Programming Adaptor. All of these are scarce birds. Some folks offer programming for a price, while other kind souls do freebies with the cost of chips and/or postage as their only “fee”. Note that 2732 family EPROMs are getting scarce, you should expect to have to provide one to be programmed - and there was one MSF PROM Programming Adaptor for the 12v 2732s and another for the 21v 2732s). Make sure that you find out what adapter the gentleman has and provide the proper voltage 2732 chip.

So, let’s explore where we apply solder. Listed below are the connections:

ConnectionFunctionNotes / Description
TP4 Transmit Audio tap Flat TX audio point of injection (non-pre-emphasized)
TP3 Receive Audio tap Direct unprocessed/ungated audio from the receiver quadrature detector (non-de-emphasized).
TP9 Station Local PTT (Push-to-Talk) This is an active-low or low-true signal used to key the transmitter. Typically, it is unaffected by an internal time-out timer, but be sure to double-check your programming when having codeplug EPROMs made. It is also available at the front Mic/Control connector.
A.G. Audio Ground Tied to Logic Ground (L.G.)
TP6 / TP7 Carrier Squelch Indicator Internal noise squelch logic output, 2 circuits available. More notes in the text below.
U829-15 Private Line (PL) / Digital Private Line (DPL) Indicator RX1 Audio Gate Control.

The internal audio processing circuitry of the station is bypassed completely. This is common of “California Linking” methodologies regardless of radio make or model. We refer to it as “Flat Audio”. May I note neither this term nor this concept were invention of Jeffrey "Shorty" Stouffer (K6JSI). The terminology and practice extends back to the early 1960s and linking efforts of that time in Southern and Central California.

The points chosen are conveniently available on the station control board. All but one are “Test Points” as their “TP” designators suggest. Motorola designed this station even for us without even knowing it. Quite a pleasant realization it was. TP4 injects audio directly with a data-quality tap point for transmission. TP3 is the equivalent in the receiver chain: raw, unsquelched audio. An audio compensation network is suggested but not required between detector and controller. Generally, two 10K ohm resistors (1/4W) and a 0.068uF in parallel with the second resistor (controller side) provide the proper shaping. This will vary with the cable type and length used.

Next is an oddity to the MICOR, MSR and MSF stations. Two noise-squelch circuits exist; one serves as ‘Repeater Squelch’, the other as ‘RX1 Squelch’ (or 'Receiver Squelch'). These two settings are elements of the “Receiver Qualifier” primarily used in internal repeat operation. One controls the actual gated audio while the other is a logic reference for “holding in” the repeater’s transmitter (repeater PTT).

TP6 is ‘RPTR Unsquelched Indicate’ and TP7 is ‘RX1 Unsquelched Indicate’. TP7 is the preferred signal, the audio control signal. Unlike TP6, TP7 attempts to follow the MICOR squelch concept of hysteresis to provide decent squelch action. Not exactly the MICOR squelch itself, but close unless you’re a purist. These signals are active-high or high-true signals. Use a 2N7000 buffer or at minimum a diode to protect the internal circuitry from damage. Most parts on the control board are unsocketed – not fun to change. Don’t attempt it unless you have a decent solder-sucking iron.

The next interface point is where most folks give up. While it’s easy to use an outboard PL decoder like Spence Porter’s TS-32/P (ComSpec) or the newer TS-64, that takes the fun out of it. An MSF handles this already, right? Why not use it? The trick is finding where. Not easy at first since there isn’t a testpoint or other discrete line saying “Here I am!” – until one looks closer. Part of the codeplug programming allows specification of audio gating control from the receiver. It can be either CSQ only, CSQ + PL/DPL or PL/DPL only. The last option is key. Specifying option ‘C’ (Coded Squelch) turns the RX1 Audio Gate Control line into a PL Decode Indicate line. This line is found at U829 pin 15 on the controller board, an active-high/high-true signal. You’ll have to tack-solder to the IC’s leg, but it’s worth the effort. If done cleanly, no harm and no foul. Buffer it with a 2N7000 or diode as with the CSQ indicator, and there you have it: separate COR (TP6) and PL decode lines.

CLB Codeplug Programming:

The station codeplug can be programmed as a repeater or base station. Either option will allow use of the preceding procedure. When programmed as a Repeater, the “Acc. Dis” or Access Disable switch is used to disable internal (in-cabinet) repeating. The radio then acts like a base station. Making a Base Station codeplug works fine also. The aim of a Repeater codeplug is merely to alleviate having to switch codeplug EPROMs if the external controller is removed for service. Up to your tastes.

C74CLB7106A (base) or C74CLB7106AT (repeater)
FieldData Notes / Description
Station transmit frequency (your TX freq) Frequency in MHz
Station receive frequency (your RX freq) Frequency in MHz
Transmit Code (your tone) Specified by Motorola PL code or DPL number, i.e. 4Z (136.5) or D411
Recieve Code (your tone) Specified by Motorola PL code or DPL number, i.e. 4Z (136.5) or D411
LINE-TOT x.x Wireline Timeout Timer (in seconds)
LOCAL-TOT 0.0 Local PTT Timeout Timer (entering a value of zero disables the timer)
RPT-TOT x.x Repeater (internal) Timeout Timer
RPT-DOD x.x Drop Out Delay (hang timer or carrier delay timer) Used only on the in-cabinet repeater
PTT PRIORITY LR (Local then Repeat) PTT Priority, Local first and Repeat second suggested
RPT. ACT. SC Rpt. Activate Qualifier (initial keyup, SC = CSQ and PL / DPL)
RPT. HOLD C Repeater Hold-in Qualifier (during hang time)
RX AUDIO Receiver Audio Gate Control Qualifier (must be C)
TX AUD/DAT. NO  Not used in our application
LOC. AUD/DAT. NO Not used in our application
RPT. AUD/DAT. NO Not used in our application
AUTO I.D. (your callsign) Internal Morse Identifier Callsign (in-cabinet repeat only). Entered as text, for example "WB6SLR"
ALARM OTA ENABLED Alarm Tones Over the Air (audible notifcation of PA problem, etc.)
ALARM WIRE DISABLED Alarm Tones Over the Wireline (Not used in our application)

The above fields are as used by the R-1801A suitcase programmer in building a codeplug. Other parameters can be specified, but are beyond this article. Consult the R-1801A owner to determine more finitely what you need.

The CXB (and similar) Stations:

Like the CLB, a CXB is fairly straightforward. Our approach is the same but different. A few changes in the design make use of an external board almost necessary for noise squelch, but our choice of PL indicate signals increases. Programming is also far easier by using a PC and RSS instead of the R-1801A. The MSF 5000 RSS is very useful in that it can mimic a Diagnostic Metering Panel (type with a display matrix) to observe MUXBUS activity. DMPs are nearly a must-have for MSF owners. Unlike the CLB, a CXB can be multi-channel easily. For those where this serves a need, a definite asset. Programming them up isn’t terribly difficult in the RSS.

Here we have the CXB connections:

ConnectionFunctionNotes / Description
TP4 Transmit Audio tap Flat TX audio point of injection (similar to a CLB)
TP3 Receive Audio tap direct unprocessed/ungated audio from RX quadrature detector.
TP9 Station Local PTT (Push-to-Talk) also avail. at front Mic/Control connector.
A.G. Audio Ground tied to Logic Ground (L.G.)
U802-58 Carrier Squelch Indicator internal noise squelch logic output to ASIC.
U810-9 Private Line (PL) /Digital Private Line (DPL) Indicator RX1 Audio Gate Control.
TTRC Systems Connector P2900-9, Junction Box DB-25 connector pin 9 Red/White Wire TTRC Spare Output bit, assign to MUXA3B2 in RSS for PL-only operation.
JU-18 (default strapping) Center RX1 Audio Gate Control Selector Programmed Qualifier default or CSQ Only)
Use of the internal squelch is possible, but not recommended. Unlike the CLB, no test-points exist for either of its circuits. (RPTR or RX1) Tacking a line onto the ASIC or NPN driving it are possible, but not worth the trouble. A good, clean MICOR squelch circuit will perform better, be it Link-Comm’s RLC-MOT or a do-it-yourself version. Sneaking an RLC-MOT into the Control Tray with the SSCB is certainly doable, cleanly at that. Feed it with audio from TP3.
U802-58 Input to the I/O ASIC from the RX1 Squelch circuit. This is a high-true signal. If used, be sure to buffer it. Take extreme care not to blow the ASIC with ESD. Q1510 is an SMD NPN transistor and drives the ASIC from the squelch circuit. It is on the bottom side of the board. Not something to tamper with unless you have a good feel for SMD work.

My strong suggestion is to forget the CXB internal squelch altogether. Stick an RLC-MOT (or your home-brew equivalent) in with double-sided tape on an open part of the SSCB tray. Wire it with TP3 as RX detector tap, and use as base-level-set and buffer for RX audio to your control system. It provides its own audio gating if needed by your controller. Without component changes however, the recovered audio is sufficiently flat for most applications. De-emphasis can be added if desired. Refer to your RLC-MOT documentation for those details.

The PL Indicator signals are three:

  1. U810-9 RX1 Audio Gate Control programmed appropriately, TTRC Systems Connector Pin 9 Output programmed to reflect MUXA3B2 and JU-18 in its default position (marked by a white brace on the board next to it). The easiest to access near the rest of our interface points is U810-9. Pin 9 is a high-true output which you’ll need to buffer with a 2N7000, VN10KM or similar device to feed your controller. The RSS must be programmed so that the Receiver Audio Control is ‘C’ for Coded Squelch Only. This is in the Mode or Channel screens of the RSS.
  2. Option #2 is the TTRC method. The Spare Output pin must be programmed to output MUXA3B2, MUXBUS Address #3, Bit #2. This is RX1 PL ACT. It should then be buffered by a 2N7000/VN10KM device to drive your controller. Contrary to what first look might say, RDSTAT is not a desirable output. It is two RX Qualifier bits ANDed together and will give CSQ only or CSQ + PL, not PL exclusively. The CSQ signal is that of RPTR Squelch as well, rather than RX1 Squelch (faster).
  3. Option #3 is only available on later SSCBs - look to see if JU-18 exists or not. The jumper provides a high-true output that can drive a 2N7000 or VN10KM buffer to your controller for PL indicate. Like the use of U810-9, the RX Audio Control must be programmed as ‘C’ for this to operate correctly. This also hooks directly to the microprocessor rather than through an inverter like U810-9, meaning increased risk of serious damage to the SSCB by ESD or other mishaps.

Something to remember, a useful bit of information…. The SSCB CPU & ASICs are interchangeable with the TTRC CPU & ASICs. The 68HC11 CPUs of both contain the same ‘bootstrap’ program to reference their external EPROMs. Any old 68HC11 won’t work. So, in your spare time, cruise eBay and pick up one or two TTRC Logic Core boards. They’ll have all 3 chips. Be very careful when inserting or extracting PLCC chips, however. The sockets are easily damaged. Always use the proper extraction tool! DO NOT USE A SCREWDRIVER.

In closing, using the MSF in amateur applications is easier than it looks at a first glance. Basically you program it as a simple in-cabinet repeater, then disable the repeat function with the "Access Disable" (“Acc. Dis”) switch. Once that is working to your satisfaction you connect in your external controller - be it a Scom, NHRC, Link (RLC), or a Palomar. If your controller dies or has to be disconnected for any reason you simply flip the switch to re-enable the in-cabinet repeat.

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Article text © Copyright 2010 by Matt Lechliter W6KGB.
Hand-coded HTML © Copyright 2010 by Mike Morris WA6ILQ.

This page originally composed on 18-June-2010

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.