My WP-34S : USB to Serial cable prototype

[jebem]

Hi all,

As electronics is one of my hobbies, I decided to prototype my own USB to Serial cable flash box, so I can re-flash from scratch the HP-20B/30B into a famous WP-34S RPN scientific calculator anytime I need to.

1) First of all, thanks for all the good people posting here on this subject, I have learned a lot from You!
These are some of the main threads I have used and above all the most important source of information from the WP-34S creators:
WP-34S project repository
Homemade ARM-based Calculator Programming Cable, by Katie Wasserman
WP34s Programming Cable, by several contributors
WP 34S Programming Adapter Plug Parts - FS, by Dave Frederickson


2) My prototype flash box includes:
- One 3.3V regulator power supply for extra energy for calculator during the flash operations;
- Two push buttons (Erase and Reset), each one with a LED to provide visual feedback;
- USB 5Volt Power On LED status;
- Two additional LED's to monitor the TX and Rx serial data transfer;

The flash box circuit includes an 3.3V low voltage drop regulator based on the LM1117-3.3 IC as a source voltage to the calculator power supply, Reset and Erase functions of the serial cable.

One word on the calculator internal Reset logic:
When the calc is powered On, the normal state of the Reset pin is High (3V).
When the Reset action is taken, its state goes down (0V) and stays down even after releasing the Reset action, for as long as the batteries have good voltage.
The only way to turn the Reset state to High again is by powering On the calc.

So, the Reset indicator LED circuit needed a more elaborated circuit, as we do not want to overload the calculator internal Reset circuit that is maintained to 3.3V thru a 100Kohm resistor. As I had a BS170 MOSFET at hand, i have used it as a switch with very high input impedance load, followed by a NPN BC547 acting as inverter/driver.

Schematics are included in one of the pictures below.
Update on 23-June:
Added a picture with the draft of the PCB layout (Top view, where the red copper lines are underneath).
The Part's Listing are included as well (see below).

3) The USB to Serial converter is based on a Prolific PL2303HX.

The cable itself is named as "USB auf RS232 TTL serielles Kabel Adapter Modul PC-PL2303HX Chipset Computer T5", and it was purchased in TAS.

Windows 7 x64 has automatically detected the Prolific PL2303HX device, installed the required driver, and it runs without any flows!
I have tested in two different computers with no issues at all:
- One ASUS AMD systemboard with USB2 interfaces;
- One Toshiba Portégé Z930 Laptop on both USB2 and USB3 ports;

Windows 7 x64 Device Manager information:
Prolific USB-to-Serial Comm Port (COMnn)
Driver Provider: Prolific
Driver Date: 17-10-2013
Driver version: 3.4.62.293
Driver file: ser2pl64.sys

4) Testing the flash box:
The calculator is not used at this stage.

We need to simulate the calculator internal reset load, so I have connected a 100Kohm resistor across J31 (Vcc) and J32 (RST).

Also, I have connected a jump wire between J35 and J33 to create a loopback connection. This allows the testing using the Hyperterminal tool for Windows.

5) The Pogo connector to plug in the back of the calculator was hand made from a kit got from Dave Frederickson. Unfortunately Dave doesn't has these kits anymore. The kit included the 2mm IDC ribbon cable assembly, the spear point pogo pins type PA50-B (16mm length), and one brass strip approximate 0.25 x 2 x 0.16 inches to make a clip.
We need a small piece of double-sided foam tape (2.5mm thick) to attach the clip.
The pictures below includes the build plan details.

6) The following (shaky) video shows the prototype in action.
The serial interface is connected in loop back mode.
I am running the Hyper Terminal on Windows to send and receive ASCII chars from the keyboard.
http://youtu.be/X10TVjPdGmE

7) Current status:
This flash box works nicely to re-flash the HP20B or 30B calculators, providing all the visual aid during the Erase and Flash procedures.


8) Parts Listing

8.1) Semiconductors:
IC1 - LM1117-3.3V low drop regulator TO220 (I used the SMD version, but this TO220 case is easier to solder)
T1 - BC557C PNP transistor
T2 - BC557C PNP transistor
T3 - BS170 N-channel FET transistor
T4 - BC457C NPN transistor

PWR LED - LED Red
TxD LED - LED Red
RxD LED - LED Green
RST LED - LED Green
ERASE LED - LED Yellow
(or any other color scheme you like)

8.2) Capacitors
C1 - 10uF, 35V Tantalum
C2 - 10uF, 35V Tantalum

8.3) Resistors (all 1/8W or 1/4W, with 5% tolerance)
R1 - 470R
R2 - 1K
R3 - 1K
R4 - 39K
R5 - 330R
R6 - 39K
R7 - 330R
R8 - 560K
R9 - 2M2
R10 - 68K
R11 - 470R
R12- 470R

8.4) Miscellaneous
- Veroboard with a minimum of 29 x 8 holes to build the circuit (7cm x 2.5cm);
- A plastic box (8.5cm x 5.5cm x 3.0cm) Velleman WCAH2855;
- Thin isolated wire of different colors for internal wiring;
- 6-pin Pogo connector of your choice and interconnect cable (6-wires);

8.5) USB interface
- One USB-to-Serial TTL adapter based on PL2303HX;


Thanks all for the contributions and comments.

[olr]

Hi,

i'm also waiting for pogo pins to get my cable ready (no extra LEDs though). I'm not an expert in electronics but i have some questions/remarks.

• You are connecting 3.3V to Vcc of the calculator. This is probably neccessary when the batteries are removed. But what if not?
• The RESET pin is already high(3V). I think a reset of the calculator is triggered when the RESET pin is connected with GND?
• My USB-TTL converter is also based on Prolific PL2303HX and TX/RX are operating at 3.3V and it has an 3.3V output pin, that might be used.
  

Usually FTDI chips are recommended. However i flashed my HP20b successfully with a PL2303HX-based converter.

olr

[jebem]

(04-12-2014 06:40 AM)olr Wrote:  

• You are connecting 3.3V to Vcc of the calculator. This is probably neccessary when the batteries are removed. But what if not?
• The RESET pin is already high(3V). I think a reset of the calculator is triggered when the RESET pin is connected with GND?
• My USB-TTL converter is also based on Prolific PL2303HX and TX/RX are operating at 3.3V and it has an 3.3V output pin, that might be used.
  

Usually FTDI chips are recommended. However i flashed my HP20b successfully with a PL2303HX-based converter.
olr

Hi Olr,
Thanks for your comments!
Well, this is just a prototype, not the final version... it is really a work in progress. And I never flashed a HP-30B before.

As for your excellent remarks:

- I expect to use the external 3.3VDC to supply the calculator as well, removing the cell batteries first. Of course none of this is needed, one can just use the cell energy to do all the operations, including the Erase function as others have reported here in the MoHPC.
But I do not know how much current the Erase and flash operations can consume and I do not want to deplete cell batteries one after another.
As I said, I am waiting for the pogo connectors and then I will be able to do some readings.
But I have made some current readings on my HP-30B and it can consume in excess of 20mA, and that is a lot for cell batteries (!), for instance when one runs the diagnostics (ON + PMT, then scroll down to "TSys" setting and change it from "Off" to "On"...).

- As for the RESET: Good point! I have just checked (again) my own HP-30B, and You are right, in normal conditions the RST connector has +3VDC.
When I did my previous readings it was at Zero Volts, so I assumed the RST was working in the same way as the ERASE.
In fact, if one press the RST button in the back, the RST connector voltage goes down to Zero Volts (as expected) but then it stays a Zero Volts! The only way to restore the normal condition is to Power On the calculator (or remove the batteries). That was probably why I was reading Zero Volts initially.
I will correct my diagram and publish it in the initial post.


- As for your last point: The Prolific PL2303HX having an 3.3V output.
Yes, I saw that possibility as well, however I checked the datasheets and apparently the current supply available on this 3.3V supply is really low.
This 3.3V supply (Pin 17 - VO-33) typically consumes up to 20mA (as per RS-232 standards?), so probably can supply that much, but it already has to supply current to the TX serial line, so I do not want to take my chances overloading it with additional Erase duties.

- And many Thanks for sharing your experience with this Prolific IC, instead of the more "recommended" FTDI one.

Now, I cannot wait for those pogo pins...

[Marcus von Cube]

My setup is the following:

• The original cable by HP with the blob and two push buttons
• An FTDI USB to 3.3V TTL serial
• External 3V power supply
  

The problem with the original design is that it uses the calculator 3V VBatt supply on the connector to feed the level shifters in the blob. This draws power even while the calculator is turned off. Add to this the SAM-BA boot loader which runs at full processor speed without any user visible sign of life and you know why this is a battery killer setup.

So I removed the electronic guts in the blob and just left the switches connected. They connect directly to the pins in the calculator without any electronics involved. Sometimes things are simpler than anticipated...

The FTDI cable is connected inside the blob to the RX, TX and GND pins on the calculator end. The power supply is connected to GND and VBatt, all inside the blob. Before I connect a calculator to this setup, I just remove the batteries.

MySamba runs in a virtual XP setup on my iMac. The virtual machine "owns" the FTDI USB device. There is a small setup program available at FTDI which allows to modify the characteristics of the FTDI chip in the cable. I used it to change the USB identification string to "HP Calculator Programming Cable". This makes it easier to identify the USB device to attach to the virtual machine.

[walter b]

(04-13-2014 08:46 AM)Marcus von Cube Wrote:  So I removed the electronic guts in the blob and just left the switches connected. They connect directly to the pins in the calculator without any electronics involved. Sometimes things are simpler than anticipated...

The FTDI cable is connected inside the blob to the RX, TX and GND pins on the calculator end. The power supply is connected to GND and VBatt, all inside the blob.

Sometimes a circuit diagram tells a thousand words, doesn't it?

d;-)

[jebem]

Thank You Marcus and Walter for your knowledge sharing!
It is really gratifying to be able to learn from others here.

Marcus, Your explanation makes all the sense to me. I believe have followed a "liberal" approach of your past posts on this subject, among others ideas shared in this Forum.

My apparent circuit complexity is related to the LED indicators, as I really want to have visual feedback of the operations going on. Just a matter of taste, I would say.
These indicators do not impose a significant electrical load on the Tx, Rx or Reset signals, so I wouldn't expect troubles from this additional circuitry.

Now, I really do not understand why on earth a simple letter with 18 "pogo" pins inside takes so long to be delivered from Canada to Portugal.
Usually I use German suppliers for faster delivery inside EU, but in this case I couldn't find one for these pogo pins.
Three weeks pasted since they marked my package as sent in eBay and counting...

[anetzer]

(04-13-2014 10:17 AM)jebem Wrote:  Now, I really do not understand why on earth a simple letter with 18 "pogo" pins inside takes so long to be delivered from Canada to Portugal.
Usually I use German suppliers for faster delivery inside EU, but in this case I couldn't find one for these pogo pins.

Hi,

conrad.de has them listed under "Federkontakt" or "Präzisions-Prüfstift für Leiterplattenprüfung".

The one I link to looks like this:
link to picture

A casing is listed as well.

They have them listed as "available immediately", which means they ship within the week.

I would be very grateful, if you could compile all your findings in a comprehensive documentation that can be executed by us, the electronically uninitiated. A parts list with order numbers and photos of all the critical parts and phases would be especially useful!

Thank you,

a.n..

[jebem]

Hi ANetzer,

1) Thank You so much for your information on the "pogo" pins from a European local supplier.
I wil have a look and perhaps order some pins from them if my Canadian order doesn't show up in good time.

2) As for project documentation, I can only promise that I will document it the best I can in the small spare time I have.

So, once I can successfully flash my HP-30B, I will publish:

- The final manual drawing schematics;
- Part's list;
- The components layout design over a prototype board like the one I am using.

Please keep in mind that this is a hobby - and hobbies consume time and money - in the end, if one just looks to how much the serial cable did cost to make in this way, for sure it will cost (much) more than just buying a commercial solution.
It depends on what materials and tools one have in house, and what ones have to be acquired to get there.


3) Electronics knowledge:

A minimum knowledge of practical electronics is recommended.
At very least, as it is critical for the project's success, one needs good soldering skills. There are plenty of instructions on the net if someone is not trained on this matter. Training is mandatory here, by "killing" a few spare components just to get the soldering skills.
For instance it is easy to destroy LED's while soldering - one very well know trick to protect them is to use tweezers attached to the terminal base before soldering it.

We need at least two different types of soldering machines, one of low power (like 15 Watt) for small delicate components that requires a very fine soldering tip, and another one with medium power (like 25 Watt) for bigger components that require more thermal moment.
Or for deep pockets there are temperature controlled soldering stations that can help here.
The secret is simple - we do not want to kill the components by overheating them - so we need power enough to maintain the correct temperature in order to make a good soldering as fast as possible (obtaining a good solder melting and minimizing the heat transfer to the component's internal parts during the operation).
My experience with soldering delicate electronic components (like LEDs, small transistors, IC's, and of course the SMD components) tells me that more than 2 seconds with the soldering gun on the component lead is half way to kill it.

[jebem]

I have just received one nice kit to build a pogo connector from Dave Frederickson.
Thanks, Dave for letting me know about your kit offer.

I would recommend others members looking to build a serial cable to contact Dave here in MoHPC private messaging system to check for this kit availability.
http://www.hpmuseum.org/forum/thread-609...light=pogo

This kit includes several components namely the spear point pogo pins type PA50-B, the brass strip to make the clip, the 2mm IDC plug on a color ribbon cable assembly, and 2mm machined socket strip.
Picture below.

[walter b]

(05-16-2014 11:35 AM)jebem Wrote:  I would recommend others members looking to build a serial cable to contact Dave here in MoHPC private messaging system to check for this kit availability.

According the link you mentioned, all that material was gone by 6th of May.

d:-)

[Dave Frederickson]

(05-16-2014 12:21 PM)walter b Wrote:  According the link you mentioned, all that material was gone by 6th of May.

Correct. Jose got the last one. I thank Jose for the recommendation, but the "kits" were never intended to be a sideline business. I was simply trying to offset my expenses in building my own cable. The reference in the first post should be removed.

Dave

[jebem]

(05-16-2014 03:28 PM)Dave Frederickson Wrote:  

(05-16-2014 12:21 PM)walter b Wrote:  According the link you mentioned, all that material was gone by 6th of May.

Correct. Jose got the last one. I thank Jose for the recommendation, but the "kits" were never intended to be a sideline business. I was simply trying to offset my expenses in building my own cable.
Dave

Thanks, Dave!
I was lucky

[Michael Kathke]

I used some standard SIL/DIL IC socket pins to build up a custom 6-pin socket. Let's have a look.





Glued together:


I had to open the slot a little:


Fits perfect:




For the programming cable I used the PCB from h.pott@gmx.net:



More pictures of my WP-34S somewhere in the old forum...

[walter b]

(06-16-2014 08:57 PM)Michael Kathke Wrote:  I used some standard SIL/DIL IC socket pins to build up a custom 6-pin socket. Let's have a look.

Picture 1.

Picture 2.

Glued together: picture 3.

I had to open the slot a little: picture 4.

Fits perfect(ly): picture 5.

Picture 6.

For the programming cable I used the PCB from h.pott@gmx.net: picture 7.

More pictures of my WP-34S somewhere in the old forum...

Great pictures (pun intended). I found your text by quoting only. The last figure caption may be a bit off since you don't show the PCB AFAICS. But great work and instructive photographs anyway.

d:-)

[Brad Barton]

(06-16-2014 09:08 PM)walter b Wrote:  I found your text by quoting only.

I'm able to see all of the text in line with his pictures. Is your browser hiding the text for some reason?

Nice work Mr. Kathke. Soldering those center pins looks like it took a bit of skill.

[jebem]

(06-16-2014 08:57 PM)Michael Kathke Wrote:  I used some standard SIL/DIL IC socket pins to build up a custom 6-pin socket.

Nice soldering job, Michael.
Simple solution, and effective, as long as we do not plug the connector 180 degrees around

[anetzer]

Jebem,

Do you have any high resolution pictures of the board in the switch box? It might be interesting how you laid it out. I'd also like to be able to control, how posts and poles of certain parts are connected.

Thank you,

a.

[jebem]

(06-22-2014 02:20 PM)anetzer Wrote:  Jebem,

Do you have any high resolution pictures of the board in the switch box? It might be interesting how you laid it out. I'd also like to be able to control, how posts and poles of certain parts are connected.

Thank you,

a.

Hi, Anetzer,

Thanks for your interest!
The WP-34S project is really nice to follow and it is a good way to spend our spare time.
Well, this switch box project was a prototype, so my switch box interior design is really
of no big interest, as the components were placed as I did the project iterative experimentation process.
It looks ugly and messy, so no one will want to copy that "layout" at all

The print circuit board (PCB) was built on a small piece of veroboard , but again,
I have put the components without any kind of layout planning.

To develop the presented circuit schematics (published in the OP), I followed what usually we do when prototyping.
We start by collecting information, then design the initial electronics circuit on paper according to the project's specifications,
do some simulation (I didn't, as this is too simple to require simulation), then we build the physical circuits
by following the best practices concerning soldering and component relative positioning when electromagnetic interference is a issue
(that's not the case in this project, as we deal here with very low frequencies and currents),
do comprehensive testing, experiment with circuit corrections/optimizations to fit the design specifications,
and then go back to the design phase.
After a couple of design cycles I got the final schematics.

As I stated in another post, when I have the time I will build a list of materials and a PCB layout, but I can't promise anything.

But because you asked, despite being ashamed to display my prototype internals,
here are the pictures (from one of my web publication sites: http://suna-jebem.blogspot.pt/ ):

Please feel free to criticize, comment, improve the circuit and ask any questions.
We learn by sharing our thoughts and this MoHPC portal is invaluable as such.

[jebem]

I have added a picture to the OP with a hand draft of a possible PCB layout.

The Part's Listing was added as well.

[anetzer]

(06-23-2014 11:03 PM)jebem Wrote:  I have added a picture to the OP with a hand draft of a possible PCB layout.

The Part's Listing was added as well.

Thank you very much!

Andreas