Breakout adapter board for GTM900 modem modules

Mychaela Falconia mychaela.falconia at
Mon Jan 20 04:35:42 UTC 2020

Hello FC community,

While we don't have much in the way of new development activity, the
principal goals of FreeCalypso are still here and will never go away;
one of these goals is to make our published-source modem solution
available to anyone who desires it.  In other words, if someone
desires a standard GSM/GPRS modem that can run our FreeCalypso modem
firmware, I consider it my duty to provide as many options as possible
for such users.

At the present time there are 3 hardware options that can run our FC
firmware: (1) our own FCDEV3B, (2) Huawei GTM900-B discovered last
year by Comrade Songbosi and (3) a recently discovered modem module
that has been codenamed Tango.  When it comes to our own FCDEV3B, I
currently have about 10 boards which I would be happy to give away
free of cost to worthy recipients, but so far no one has claimed even
one of them.  I have to say that I don't really understand why no one
has taken some of these boards off my hands yet: perhaps it is the
stipulation that they may not be used for the antisocial purpose of
running OBB software, or perhaps people just aren't comfortable with
asking to receive on a no-cost subsidy basis something that actually
costs about $500 at break-even.

Given this situation of no one taking up my offer of subsidized FCDEV3B
boards, and considering other possibilities where someone may desire a
published-source modem module that is physically smaller and suitable
in terms of form factor for integration as a component into larger
systems, I have been exploring ways to make other options more
accessible as well, particularly the GTM900 option, and so I have
produced this little board:

A bare GTM900 module is not particularly usable by itself: one needs
to break out the 40-pin FPC/FFC interface and connect power and at
least one of the UARTs at the very minimum, whereas a more complete
setup would involve connecting both UARTs, a SIM socket, PWON and
RESET pushbuttons, earpiece and microphone.  Songbosi did provide a
"dumb" breakout adapter that turns the 40-pin FPC/FFC interface into
a 40-pin 2.54 mm header, but it was very inconvenient, not really
conducive to using the GTM900 in an FCDEV3B-like manner, so I sought
out to produce a better (more specialized) breakout adapter board for
these GTM900 modules, and the result is the MMTB1 linked above.  The
linked tarball contains schematic, PCB and gerber files, and I have
physically produced one of these boards to prove the design as good.

This GTM900-B modem module is certainly nowhere near as nice as our
own FCDEV3B or Tango: FCDEV3B is triband, Tango is fully quadband, but
GTM900-B supports only 900 & 1800 MHz bands, no "American" bands at
all.  There also exists another variant called GTM900-P which
supposedly supports 850 & 1900 MHz bands (while NOT supporting the two
"EU" bands), but I am not currently able to test that hypothesis
because I have reached the limit of how much money I can dump into
this passion, and I am no longer willing to buy a GTM900-P for testing
with my own money.  Therefore, if someone would like to see this
GTM900-P potential option properly tested and supported, they would
need to buy a GTM900-P module at their expense and send it to my
address in California - then I will test it in my MMTB1+CMU200 setup
and add the necessary support as appropriate.  The other commonly
encountered variant is GTM900-C, but it is known to NOT work: it has
the LoCosto chipset inside instead of Calypso, so it is a no-go unless
you wish to start your own FreeLocosto project and spend a few years
bringing it up to a state comparable to where FreeCalypso is today.

But despite its limitations, this GTM900-B module seems to be the
cheapest and most accessible option at the present time: FCDEV3B is a
development board not meant for integration as a component into larger
systems, Tango is not currently a viable option because I am not able
to disclose its real name until and unless someone pays for my SRS
(for anyone considering that option, this Tango module is the absolute
best Calypso modem ever, and the available stock is many thousands of
pieces), thus we are left with just GTM900 for now.  If I am not
mistaken, Comrade Songbosi still has the GTM900-B version available in
large qty for just $15 apiece.

If someone is designing some larger system such a ZeroPhone-style
smartphone, desires to use a GSM modem module that runs or can run
firmware with fully published source code and chooses the GTM900 as
that modem module, they would need to design their system to interface
directly and natively to GTM900's 40-pin FPC/FFC connector.  But if
someone needs to test this GTM900 modem by itself first, just to see
it working before committing to a design, they would ideally need a
breakout adapter board like my MMTB1 to exercise the GTM900 by itself
on a lab bench.  Therefore, it would be really nice if these MMTB1
breakout adapter boards could be made easily and inexpensively
available to individual hobbyist tinkerers.

Now that I have published the design files, anyone can produce these
simple MMTB1 boards (the design of this MMTB1 is in the public domain,
unlike FCDEV3B).  If I am to produce them here in California, I would
need a minimum order of 20 pcs, and the price will be $50 apiece.  The
dominant cost component is assembly labor: the PCB is about $4 apiece,
the components are about $16 total per board, but assembly is $25
apiece in the qty of 20 pcs.  Doing smaller quantities would be
absolutely uneconomical: to assemble just one board I was charged $250,
so at that price you would be better off going for FCDEV3B - speaking
of which, a batch of 20 FCDEV3B boards (once again the minimum
production batch size) would cost $300 apiece if done as a single

If someone wishes to order a batch of 20 FCDEV3B boards at $300 apiece
as a commercial order, I would be willing to waive the strict
prohibition against using those boards for the purpose of running OBB
software if the purchaser would be willing to sign a legal liability
waiver.  The purchaser would need to (officially, with a paper trail)
understand and agree that OBB software running on Calypso hardware (be
it Motorola or FreeCalypso) has been demonstrated and proven to put
out radio transmissions that are in violation of GSM specifications
and very capable of causing interference and disruption to GSM and
other cellular networks, and therefore anyone who runs OBB on an
FCDEV3B in open air (no Faraday cage, no cabled setup w/o antennas)
would be not only causing interference and disruption to cellular
networks, but doing so knowingly and willingly, which implies a
significantly higher level of criminal culpability.

Back to MMTB1, because it is just a passive breakout adapter and does
not contain any actual GSM radio functionality (all of the latter is
in the Huawei-made GTM900 piece), MMTB1 can be produced and sold
without caring how it will be used and with what software - if someone
wishes to turn their GTM900-B modem into a rogue transmitter by way of
OBB software, it would be 100% their culpability - there is no
conceivable way how a maker or distributor of a GTM900 breakout
adapter board could be held liable.

It would be nice if someone like Harald/Sysmocom could make these
MMTB1 breakout adapter boards available in a webshop (perhaps in the
form of a kit together with a GTM900-B module and the needed FFC
jumper), either by having me produce a batch or producing one
themselves.  As I already said, if I were to produce these MMTB1
boards, the assembly labor will be the dominant cost component, so
perhaps someone else could produce them cheaper.

Hasta la Victoria, Siempre,
Mychaela aka The Mother

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