New Project wp34s micro usb flash cable revisited

[MarkHaysHarris777]

(02-08-2015 06:59 AM)matthiaspaul Wrote:  If you have opened the converter already, you might check if the RTS/CTS pins (and in case of the FT232RL or PT2303HX also the DTR/DSR/DCD pins) are connected with each other or not. If they are still open (that is, not connected to anything else), you could short them. With this patch in place, it is no longer necessary to disable hardware handshake when opening serial connections through USB - it will work both ways. While this isn't normally a problem when using dedicated software it'll make things easier when setting up a connection in a terminal program.

BTW. Based on a photo of Harald's USB board, he didn't connect RTS and CTS as well, so this patch also applies to his board.

PS. Where do the 5.1V and 3.4V figures come from? Shouldn't this read 5.0V and 3.3V?

hi Paul, nice to meet you... I have never actually looked closely enough at the PL2303HX pinouts on *any* of these boards / I have not had any difficulty using terminal, usually minicom on gnu/linux, but then I configure things as a null modem and disable all the hardware negotiating. I can tell you that the board I'm using is 'flaky,' by that I mean it does not work with MySamba 100%. ... about 80/20. If I retry it will work. I don't know whether its the timing in Marcus' stuff; but I suspect that the unit needs to be in the right state (115200,N,8,1) or it won't work. I open up terminal and make the settings, then close terminal without reset.... seems to be more reliable; but its not 100%. I'll take a look at the pinouts on the PL2303HX and let you know. Interesting.

I didn't think Harald's board had any advantage (but I was wrong). I thought it was bass-ackwards until I realized that the board ALSO has the IR driver circuit on it... solder the IRD directly to his board which has the driver transistor and current limiter. He actually did a very nice job with that thing. But, I still don't really like the physical mod/ oh well.

The voltage levels on my board are called 5.0 and 3.3; however, they measure at 5.1v and 3.4v / both depend on the regulators in the PC of course, but the 3.4v on the board is being regulated on-board, and its fairly robust. I have to admit that in the process of experimenting with my flash-box I had a solder bridge on the 3v side (nothing worked and the adapter plug got VERY hot ) I figured I toasted it. NOT SO; saved just in time. It was not damaged, which means there is overload protection built into that thing as well. Its an impressive little board as well.

I was thinking of disassembling the board (mounting the board inside the 34s, and wiring my own micro usb connector female side in the backplate under the battery cover. Calculator won't lay flat, but I don't really care... I have to hold it anyway to press the control buttons for update on the front and still be able to get to the reset hole in the back (I won't be laying it flat). The way I'm thinking about it will keep the connector clean. On the other hand, Barry's idea is just so dang simple, its hard to resist (three wires and one phone jack).

<sigh>
Cheers

[matthiaspaul]

(02-08-2015 02:04 PM)MarkHaysHarris777 Wrote:  nice to meet you...

Nice to meet you, too.

Quote:I have not had any difficulty using terminal, usually minicom on gnu/linux, but then I configure things as a null modem and disable all the hardware negotiating. I can tell you that the board I'm using is 'flaky,' by that I mean it does not work with MySamba 100%. ... about 80/20. If I retry it will work. I don't know whether its the timing in Marcus' stuff; but I suspect that the unit needs to be in the right state (115200,N,8,1) or it won't work. I open up terminal and make the settings, then close terminal without reset.... seems to be more reliable; but its not 100%.

From your description that problem does not seem to be related, but regarding the right state in general, communication also won't work, if the data connection would attempt to use hardware handshake, a setting which can be overlooked easily when configuring it manually. If the RTS/CTS lines are left floating on the converter side, hardware handshake cannot work, so any communication requesting it will fail. Once these unused signals are shorted, it will work regardless of the hardware handshake setting. I have seen colleagues pulling their hair while unsuccessfully trying to establish a 3-wire serial connection when all they had to do was to disable hardware handshake - therefore, in my own designs, I have come to the habit of always looping back the handshake signals when they are provided but not needed, so it becomes a no-brainer when setting up the software later on.

Quote:I didn't think Harald's board had any advantage (but I was wrong). [...] I realized that the board ALSO has the IR driver circuit on it... solder the IRD directly to his board which has the driver transistor and current limiter.

Hm, judging from the photo of Harald's board in this thread:

http://www.hpmuseum.org/cgi-sys/cgiwrap/...186#223186

he isn't using a driver transistor. The anode of the IR LED is directly connected to a port pin of the ARM CPU, and the cathode is connected to GND via a 390R resistor (R5) in 0805 package.

He's using a SOT-23 dual Schottky diode (D1) in common anode configuration like the BAS40-06 to route the 3.3V from the converter chip to the calculator's VCC input and derive a switching signal for the transistor (T1), which, when a positive voltage level is asserted, opens the connection between the power coming from the combined plus side of the coin cells and the calculator's VCC input, thereby avoiding a reverse current into the non-rechargeable batteries, which would ultimately result in battery leakage. By default, the transistor's control input (gate) is pulled down to GND by a 3.3k resistor (R4).

I don't know if this was discussed in old threads already (if so, I haven't found them), but at first glance, Harald's design might seem to be more complicated than necessary, after all, a common-cathode dual Schottky diode should do as well when combining the 3.3V provided by the converter IC (when bus-powered) and the ca. 3.0V from the built-in batteries.
However, while the voltage drop on the diode is no issue for the USB power path, and, in such a low-power design, the voltage drop can be reduced down to ca. 0.1V to 0.2V for the battery power path as well by choosing a suitable ultra-low-drop Schottky diode, even this low difference is quite a bit in terms of maximum battery utilization - I guess, it would already reduce the time the calculator can run on a single set of batteries by a couple of days. Also, all such ultra-low-drop Schottky diodes typically suffer from a rather significant reverse current up into the low milliampere region, so, to a limited extent, an exhausted battery might still be reserve-powered when USB is plugged in, and, if a dual cathode diode would be used, the battery could be drained out through the converter chip as well. This makes the "ad-hoc" approach of just using diodes to combine the power paths less desirable, and is probably what led him to use a transistor instead.
A bipolar transistor wouldn't make much sense here, though, that's why I assume that Harald is using a FET (I cannot decipher the marking code in the photo - something like "NHW" or "HW", perhaps?), thereby taking advantage of the near-zero voltage drop between source and drain when fully closed, much like in a real switch or "ideal diode".
However, if he's using a FET, the design is missing some ESD protection as one of the FET's pins is directly connected to the batteries' plus pole and will be touched by humans.
In a perfect design, the two coin cells shouldn't be directly connected as well, as this will reduce their combined capacity due to drift currents between the cells if they are not both identical (they never are due to tolerances and discharge history). Ideally, they should be handled as two independent inputs into the circuitry, so R4 and T1 (as well as the missing ESD diode) would have to be doubled for the second battery, very slightly increasing the costs.

Quote:The voltage levels on my board are called 5.0 and 3.3; however, they measure at 5.1v and 3.4v / both depend on the regulators in the PC of course, but the 3.4v on the board is being regulated on-board, and its fairly robust.

I see. While this is still within tolerances, most likely the meter is sligthly off - at least the 3.3V supply is typically matched quite narrowly.

Greetings,

Matthias

[MarkHaysHarris777]

(02-08-2015 07:01 PM)matthiaspaul Wrote:  I don't know if this was discussed in old threads already (if so, I haven't found them), but at first glance, Harald's design might seem to be more complicated than necessary ...

... for some reason, when any of use design a circuit, depending on our goals, some of the circuit becomes unnecessary or too complicated. I've done that lots of times.

It would be nice to see Harald's schematic, and have a bit of a theory of operation on the board from his perspective; I'd like to know his thoughts (that might have existed at one time, but I have not seen it). Thanks for reverse engineering the board from the photo! I had not tried to do that yet, although I have seen the photo too... I'd like to play with the board a bit; but I'm still inclined to use the board in the FTDI adapter, if I do that at all. Barry's three wire design with a small phone jack is a very simple tempting idea.

I will compare my PL2303HX boards. I have a couple that actually convert | adapt USB to DB9 RS232 serial connector. I have two of them that only bring Tx, Rx, GND, and Vcc out to plugs for the Arduino board; so the RTS CTS (&c) pins are missing. For all I know they are missing on the DB9 connection too. Thanks for your insights. I'm gonna keep reading and thinking about this.

I needed the flash-box as a project station to get a few dozen of these 30b(s) initially erased | flashed (group project, everyone does not need to build a flashbox) . But, everyone should have their own update cable; and that should be as simple (and inexpensive) as possible, with minimal modification to the 30b housing if possible. At least, that's what I'm thinking now. Should everyone have one of Harald's boards installed? I don't know yet, but others have said yes. Frankly, I don't know where to get one of Harald's boards yet just to play with it a bit... maybe contact him directly ( the doc really doesn't say ).

cheers,
marcus

[BarryMead]

(02-08-2015 08:42 PM)MarkHaysHarris777 Wrote:  It would be nice to see Harald's schematic, and have a bit of a theory of operation on the board from his perspective; I'd like to know his thoughts (that might have existed at one time, but I have not seen it). Thanks for reverse engineering the board from the photo! I had not tried to do that yet, although I have seen the photo too... I'd like to play with the board a bit; but I'm still inclined to use the board in the FTDI adapter, if I do that at all. Barry's three wire design with a small phone jack is a very simple tempting idea.

I ordered one of Harald's USB boards, and modified one of my calculators that way.
From my experience here are the PROS/CONS of Harald's board vs 2.5mm Headphone jack method.

ADVANTAGES TO HARALD'S BOARD:
1. Inexpensive link cable (standard USB to micro-USB)
2. Includes IR link to printer for those who own a printer.
3. Better ESD protection as standard USB inputs are designed for external connections.
4. While USB cable is connected 3V power is supplied by USB port no drain on Coin Cells.
This can make flashing more reliable as you never have to worry about your coin cells going dead
during a flashing cycle.

ADVANTAGES TO 2.5MM HEADPHONE JACK:
1. Less wiring (only 3 wires) (Harald's USB board requires 6 to 7 wires)
2. Simpler case modifications (No cutting on the case mounting struts)
3. No removal of surface mount Inductors LB1 and LB2 required.
4. Direct Calculator to Calculator transfer of Programs and Data is possible.

I hope this helps you make your decision, Barry