155 lines
8.5 KiB
Plaintext
155 lines
8.5 KiB
Plaintext
MiniSport Laptop Hacker - Vol #21. April 1994
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To discourage pecuniary interests, Copyright (c) 1994 Brian Mork
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>>> ADMIN
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The MLHacker series is formatted to print out on 8.5"x11" sheets manual-
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ly fed into an Epson LQ-850. I print at 8 lpi, which gives 82 lines per
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page. My goal is to make each MLHacker one to two pages, to fit front
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and back on one page. Back issues are available via Internet e-mail and
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ftp archives and direct from my data line listed below. File download
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privilege is given on the first call. Use the F)ile option and L)og
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into the \public\computer directory. No cost 'cause I just like doing
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it. Do something you're interested in for free and pass it on!
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The Air Force has me committed to some massive amounts of overseas
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flights during April and May. For this reason, I expect MLHacker will
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take a hiatus during that time.
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>>> AUTOMOBILE POWER SUPPLY
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Something has been bothering me ever since issue #12 of MLHacker. I
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mentioned how you could tap into your car cigarette lighter, step the
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voltage down to 9 volts and use that to power your Minisport. So far,
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true. My bad recommendation was the placement of a noise-killing capac-
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itor and inductor.
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Yes, put a big inductor in series (old lamp ballast works fine) to kill
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ignition glitches. But don't put the capacitor on the car side of the
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inductor. Put it on the *computer* side of the inductor. The problem
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is this: When you switch off the computer, the magnetic field sustained
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by the inductor collapses, forcing continued current flow. But the com-
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puter is off! Being off is a high resistance. As the charge collects
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on this high resistance, the voltage peaks very high. To some extent,
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the internal Minisport battery, which is effectively in parallel across
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the power line, begins to conduct (i.e. charge up) as the voltage rises,
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so there is some limit on the voltage. But I was wrong to suggest put-
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ting the capacitor where the inductor could feed off from it. It's much
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better to put it on the computer side, where it will dampen the inbound
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current. An additional improvement would be to put a diode across the
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inductor (cathode, banded end toward the car). It will then short out
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the inductor spike upon power-down.
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Some folks have indicated they run the Minisport straight from 12 volts.
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I'm not sure I'd recommend that, but I'd be interested in anybody's suc-
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cess doing this. Battery? Direct from car? One MLH reader is working
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to modify a drill battery pack to power his Minisport. I hope to talk
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him into writing up notes on the project.
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>>> INSIDE THE BATTERY PACK
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Jim sent me a dead battery pack to rip apart. Aha! The final link to
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the puzzle. MLH #12 discusses the wall adapter/battery pack connection.
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MLH #14 discusses the battery pack/computer connection. Issues #17 and
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#18 discuss the switching power supply board inside the Minisport. Fi-
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nally, here's the stuff from inside the battery pack. In the process of
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documenting this, I discovered another error from issue #12--my descrip-
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tion of the small pin on the 3-pin charging connector. I believe the
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information provided here to be the correct interpretation.
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BATTERY PACK INTERNALS
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Round Power Supply Connector Tab Connectors
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------------------------------ ----------------
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9V, 2A BOTM --------------------------------------------- CTR (red)
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6V, 1A TOP -------------------------------------------+-- AFT (yellow)
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8.3V O.C. LITL ----TC1---+ +---:|:|:|--TC2--:|:|---+
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GND SHIELD --------------+----+---------------------------FORW (black)
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There is also a tiny chip capacitor between the 9V supply rail and
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ground. A parallel surface mount spot (for a resistor?) is empty on the
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circuit board inside the one battery pack I've inspected.
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The charger adapter has four connections: the shield, two similar pins,
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and a third pin smaller than the other two, thus the titles BOTM, TOP,
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and LITL. The tab connections on the bottom of the battery pack corre-
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spond to the three slide connectors visible inside the minisport when
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you remove the battery pack. The colors correspond to the wiring color
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visible inside the computer when the bottom is removed.
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TC1 is a thermal cutoff unit, tightly mounted to the battery cell clos-
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est to ground. TC2 appears physically similar. While charging this
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battery pack, I monitored the voltages on the power supply connector.
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BOTM was 9.2V, TOP was 7.6V, and LITL was zero volts. I applied a heat
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gun onto TC1, and at 0:12 the voltage at LITL jumped up to 8.3 volts.
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At 0:40, TC1 closed and the voltage at LITL went back to 0 volts.
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It seems clear that the design of this system is to overvoltage the bat-
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teries (7.2v onto a 5x1.2 => 6.0v battery pack) and when the batteries
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get warm, the thermal cutoff ungrounds a signal back to the charging
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unit. To confirm that the charger responds to the signal, I loaded the
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battery pack with a 20 ohm resistor and hooked up the charger, giving
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7.2 volts across the resistor (about 350 mA drain, 2.5 watts). I would
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expect that if the charger shut off, the voltage would drop a little bit
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as the battery pack picked up the entire load and started discharging.
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Sure enough, after about 20 seconds of heating TC1, the voltage across
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the resistor dropped to 6.8 volts.
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Heating TC2 has expected results. While charging, I measured 7.0 volts
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across an unloaded, charging battery. At 0:20 of heating, an audible
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click occurred and the voltage dropped to 2.9 volts. I turned off the
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heat gun and at 1:14, I heard another click and the voltage comes back
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up. While *not* charging, the voltage goes from 6.5 to 0.0 with nearly
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identical timing. I think the 2.9 volts observed while charging was
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residual output from the charging unit.
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>>> BAYCOM VS POOR MAN'S PACKET
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Poor Man's Packet software works fine to receive packets on the Minis-
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port using the hardware described back in MLHacker #9. It does not work
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on transmit. If you try to send a packet, the transmit tones just lock
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up at 1200 Hz or 2200 Hz. This was confirmed using another ham's
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(thanks Ron) PMP hardware rigged to run off COM1 (normal DB-9 out the
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back). PMP is written in C, including the AX.25 bit packing routines.
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I think these are just not fast enough when run on the 8 MHz Minisport.
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Baycom uses a separately loaded driver to talk to the hardware. It's
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small and efficient, probably written in assembly. I got a copy of Bay-
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com software and using Ron's Baycom hardware, which was rigged to oper-
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ate on a parallel port, everything worked fine.
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Conclusion? The Minisport needs bit packing routines written in assem-
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bly. Baycom offers this; PMP does not. I thought the PMP hardware was
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designed to be bit compatible with Baycom (e.g. RTS does the PTT func-
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tion, and so on). Apparently, this is not so. When I redirect the Bay-
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com to COM2, it does not work with the PMP hardware I built and de-
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scribed in MLHacker #9.
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I don't have the time right now to do an assembly rewrite of the PMP
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software, so I want to figure out how to use the Baycom software. Any-
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body out there more familiar with Baycom and what bit lines it uses for
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what? Eventually I'll poke around and find out what's going on, but
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maybe somebody could save me the trouble.
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>>> FUTURE PROJECTS
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I saved up money for several months and purchased a Fluke 97 digital
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storage scope / DMM "do everything" box. It's roll mode data monitor
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worked great for probing the battery pack. Expensive, but a *superior*
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bench top accessory. I'm interested in anybody using one of these.
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Mine has an optical data interface and that will, I'm sure, eventually
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be hooked to my Minisport.
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I also picked up a 670 nm laser diode assembly, including a photodetec-
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tor and stepper motor. The light beam comes up through the shaft of the
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stepper motor, allowing the outbound beam to be aimed onto one of eight
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mirrored segments, and reflected forward. Inbound light is collected by
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all eight segments simultaneously and focused onto the on-axis photode-
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tector. I wonder if I can hook this up to the computer to do a laser
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light show. Or maybe a proof of concept optical radar. Hmm... :-)
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Please provide feedback! * Direct data 1-509-244-9260
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* ARO Net KA9SNF@wb7nnf.#ewa.wa.usa
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* Internet bmork@opus-ovh.spk.wa.us
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73, Brian * 6006-B Eaker, Fairchild, WA 99011
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