I am in the final stages of designing a tube curve tracer for eventual commercial sale and I need to decide what tube bases to support. The actual connections to the pins are programmable, it is just the physical sockets that are in question.
It will definitely support the standard Octal (KA8) and 9 pin (B9A) bases, as well as the UX4 4 pin base. I was also planning on supporting the 7 pin (B7G) base. However, I plan to use CMC machined pin sockets and I found that they don't make a 7 pin socket. So, I'm wondering whether 7 pin tubes are that popular. Less standard sockets can always be added with a separate adapter, but I am trying to minimize the cost and size of the baseline unit.
So, are there a lot of designs still using 7 pin tubes?
Thanks,
Chris
It will definitely support the standard Octal (KA8) and 9 pin (B9A) bases, as well as the UX4 4 pin base. I was also planning on supporting the 7 pin (B7G) base. However, I plan to use CMC machined pin sockets and I found that they don't make a 7 pin socket. So, I'm wondering whether 7 pin tubes are that popular. Less standard sockets can always be added with a separate adapter, but I am trying to minimize the cost and size of the baseline unit.
So, are there a lot of designs still using 7 pin tubes?
Thanks,
Chris
I compiled a list from previous posts of "favorite tubes" and I did notice that there were several 5 pin ones, so that is a possibility. The plate cap is not a problem, since the connections between the test electronics and the sockets are configurable with high voltage sheathed 4mm banana plugs. All you need to do is have an extra cable that terminates in a cap.
I too am designing a computer-controlled curve tracer (smoking amp's posts in that other thread were a huge inspiration) and intend to make it avaliable as a DIY project with parts not costing more than $50-100 (total, PCB, MCU and transformer included in parts cost estimate). I plan to use computer application for data display (with opto-isolated communication, of course).
Instead of complicationg my life with various socket types I decided to use screw terminals instead so one can wire any sockets in any fashion, be it through rotary switches, relay multiplexers or simply by using alligator clips or some similar method.
Depending on your target customer base (and sale price) you should perhaps consider using fewer socket types by default but keeping your options open instead of blowing costs through the roof by supporting every avaliable socket type out there ?
Our target price is $599, so the customer base is not as large as that for a $100 kit. But hopefully it will have enough features that people will think it's a good value. A lot of the cost is actually in the physical package and high voltage connectors/cables.
The tentative specs are:
500V@200ma Plate, 500V@200ma Screen, +/-100V@100ma Grid, 2.5-30V@15W Filament, with support for directly heated cathodes. All of the supplies are progammable and voltages are measured with 12-bit A/D converters. It uses high voltage sheathed banana jacks to configure the connections between the supplies and the sockets. As in your case, the interface to the computer will be optically isolated. I'd hate to blow up my laptop when I'm testing it.
We're still in the process of designing the PCB, so it will be a while before the prototype is ready for testing. However, at some point this spring we will be looking for beta testers, if anyone is interested. Also, we'd love to hear any feature requests.
Chris
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Those price ranges sure sound a bit optimistic.
For the el-cheapest approach, I would just make a step generator circuit board that one could use with a scope for display. (4 bit counter IC, D/A, HV amp stage and gain control, with a 60 Hz zero crossing comparator/trigger to clock the 4 bit counter) A separate screen supply from the B+ supply allows one to measure "plate" current at the cathode with a small grounded resistor. B+ supplied by rectified, but un-filtered, HV xfmr. Voltage divider off plate for scope input. A collection of plate resistors to jumper in for loads (or see later, a programmable or switch controlled load resistor circuit board). A camera photographs XY displays on scope for capture.
Then for fancy (more expensive) designs with opto-isolated PC control, some nice features (beyond the expected data capture, display and programmable voltages) would be a tube power limit parameter to constrain tube dissipation during scans. A programmable load resistor with near continuous range would save $ (over a bank of power resistors) and be much better utility wise: EDN PDF (bottom article)
The Tek 576 tracer I have is very limited by its small selection of load resistors and voltage ranges. It also suffers from non-contiuously variable display scale factors and step voltages, which I am fixing now, but this makes calibration/readout more difficult. All this is easily done in a PC controlled setup. Tek also included some operator safety features, which generally get over-ridden due to their inconvenience. But a ground fault tripper unit might be a good feature (can be added as an external unit easily anyway).
The usual plate curve display is de-rigueur, but with computational power available, some other displays/readouts would be very useful. Mu curves, gm and Rp plots the most obvious ones (with selection of either grid voltage or plate current or plate voltage as the horizontal axis or the basis for curve steps).
Then a gain plot with a specified load resistor and B+ and bias. This would sweep grid voltage thru a specified signal range (voltage ramp) while displaying gain at each point. Then a set of these curves could be done with discrete steps of load resistance (and a re-bias to some plate current or tube diss. for each). This would indicate the best operating point for a tube quickly (flattest gain curve in the set).
Then tube tester functionality with a gm test at specified current and voltage.
Then gm tests at a group of op. points with some result storage features for tube matching (maybe a super-imposed graph for each tube, for the set of tubes) to make a selection constellation display, where the user can easily pick out the closest tubes.
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The gain plot scheme I mentioned above would also make a nice el-cheapo stand-alone circuit board unit for use with a scope. A small 1 KHz sine wave signal would be summed with a slowish ramp signal for the grid (adjustable ramp limits). Then a frequency (1 KHz) selective (filtered then rectified) voltage readout of the gain from the cathode current sampling resistor. The ramp signal controls the horizontal on the scope, the measured gain signal controls the vertical on the scope.
User adjusts grid bias or plate resistance to find the flattest curve.
A set of such curves could be displayed using either the grid voltage stepper board also or load resistor stepper (below). These would get a count signal from the ramp generator to count up a step each time.
Optionally, a stepped, programmable load resistor could be provided also (maybe a separate PC board for this, so it could be used with the earlier grid voltage stepper board too) which would step thru a range of load resistances with each slow ramp scan (4 bit counter to control the D/A, or a presettable 16 bit counter with modulo 16 count reset). Some adjustable DC grid bias output (second D/A off stepper counter), derived from the Rload stepper, would be needed to adjust the DC grid bias as the load resistor changes.
For the el-cheapest approach, I would just make a step generator circuit board that one could use with a scope for display. (4 bit counter IC, D/A, HV amp stage and gain control, with a 60 Hz zero crossing comparator/trigger to clock the 4 bit counter) A separate screen supply from the B+ supply allows one to measure "plate" current at the cathode with a small grounded resistor. B+ supplied by rectified, but un-filtered, HV xfmr. Voltage divider off plate for scope input. A collection of plate resistors to jumper in for loads (or see later, a programmable or switch controlled load resistor circuit board). A camera photographs XY displays on scope for capture.
Then for fancy (more expensive) designs with opto-isolated PC control, some nice features (beyond the expected data capture, display and programmable voltages) would be a tube power limit parameter to constrain tube dissipation during scans. A programmable load resistor with near continuous range would save $ (over a bank of power resistors) and be much better utility wise: EDN PDF (bottom article)
The Tek 576 tracer I have is very limited by its small selection of load resistors and voltage ranges. It also suffers from non-contiuously variable display scale factors and step voltages, which I am fixing now, but this makes calibration/readout more difficult. All this is easily done in a PC controlled setup. Tek also included some operator safety features, which generally get over-ridden due to their inconvenience. But a ground fault tripper unit might be a good feature (can be added as an external unit easily anyway).
The usual plate curve display is de-rigueur, but with computational power available, some other displays/readouts would be very useful. Mu curves, gm and Rp plots the most obvious ones (with selection of either grid voltage or plate current or plate voltage as the horizontal axis or the basis for curve steps).
Then a gain plot with a specified load resistor and B+ and bias. This would sweep grid voltage thru a specified signal range (voltage ramp) while displaying gain at each point. Then a set of these curves could be done with discrete steps of load resistance (and a re-bias to some plate current or tube diss. for each). This would indicate the best operating point for a tube quickly (flattest gain curve in the set).
Then tube tester functionality with a gm test at specified current and voltage.
Then gm tests at a group of op. points with some result storage features for tube matching (maybe a super-imposed graph for each tube, for the set of tubes) to make a selection constellation display, where the user can easily pick out the closest tubes.
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The gain plot scheme I mentioned above would also make a nice el-cheapo stand-alone circuit board unit for use with a scope. A small 1 KHz sine wave signal would be summed with a slowish ramp signal for the grid (adjustable ramp limits). Then a frequency (1 KHz) selective (filtered then rectified) voltage readout of the gain from the cathode current sampling resistor. The ramp signal controls the horizontal on the scope, the measured gain signal controls the vertical on the scope.
User adjusts grid bias or plate resistance to find the flattest curve.
A set of such curves could be displayed using either the grid voltage stepper board also or load resistor stepper (below). These would get a count signal from the ramp generator to count up a step each time.
Optionally, a stepped, programmable load resistor could be provided also (maybe a separate PC board for this, so it could be used with the earlier grid voltage stepper board too) which would step thru a range of load resistances with each slow ramp scan (4 bit counter to control the D/A, or a presettable 16 bit counter with modulo 16 count reset). Some adjustable DC grid bias output (second D/A off stepper counter), derived from the Rload stepper, would be needed to adjust the DC grid bias as the load resistor changes.
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I agree the $599 will be tough to achieve, but in large enough quantity it should be doable. The question is how much the quantity goes down as the price goes up. My feeling is that as the price rises from this level, the market starts shrinking a lot.
The circuit directly measures the plate current with a 500V capable high side current sensor, so you can track the current continuously and shut down if exceeds a preset limit.
This is all just a matter of sofware once you have the data. Instead of going through all of the expense of programmable loads, couldn't you just get an extremely dense set of curves, use them to create a model for LTSpice, and then simulate everything for a particular tube?
I had thought of the overlay matching feature, but I hand't thought about putting up several tubes at once. That certainly sounds doable.
Chris
"This is all just a matter of sofware once you have the data. Instead of going through all of the expense of programmable loads, couldn't you just get an extremely dense set of curves, use them to create a model for LTSpice, and then simulate everything for a particular tube?"
Yes, certainly, if you have all programmable supplies, then the load resistors can be skipped. I was thinking of the more common case where a rectified xfmr is used for plate supply.
I wouldn't make the Spice model first, just compute the curves from the dataset, more accurate. Spice model afterwards. If the interface is fast enough (USB) one could consider real time measurements for a Spice interface also.
Not to overly complicate things, but a very useful extension would be to use two tracer units in tandem for testing differential/LTP pairs also, or at least provide an extra cost option to provide an inverted grid signal as well (one Op amp and HV gain stage with offset/bias adjust).
The gain tracer idea I mentioned earlier is also oriented towards more complex circuits than just one tube. One can inject the test signal anywhere and measure the response somewhere else in a multi stage ampl. with some additional parameter (voltage or resisance) swept thru a range. Which is where a programmable resistor is useful. This could even be used for testing PSRR with a current xfmr to insert the signal. Not trying to compete with an Audio Prec. One here. That would require 24 bit converters everywhere. But this is still useful for checking circuits that are not under feedback loop control/accuracy. (Well, even in a feedback controlled amplifier, only the input and output are kept to high accuracy, intermediate stage signals can still be quite distortive.)
Yes, certainly, if you have all programmable supplies, then the load resistors can be skipped. I was thinking of the more common case where a rectified xfmr is used for plate supply.
I wouldn't make the Spice model first, just compute the curves from the dataset, more accurate. Spice model afterwards. If the interface is fast enough (USB) one could consider real time measurements for a Spice interface also.
Not to overly complicate things, but a very useful extension would be to use two tracer units in tandem for testing differential/LTP pairs also, or at least provide an extra cost option to provide an inverted grid signal as well (one Op amp and HV gain stage with offset/bias adjust).
The gain tracer idea I mentioned earlier is also oriented towards more complex circuits than just one tube. One can inject the test signal anywhere and measure the response somewhere else in a multi stage ampl. with some additional parameter (voltage or resisance) swept thru a range. Which is where a programmable resistor is useful. This could even be used for testing PSRR with a current xfmr to insert the signal. Not trying to compete with an Audio Prec. One here. That would require 24 bit converters everywhere. But this is still useful for checking circuits that are not under feedback loop control/accuracy. (Well, even in a feedback controlled amplifier, only the input and output are kept to high accuracy, intermediate stage signals can still be quite distortive.)
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On the real time Spice interface idea, a substantial speedup scheme is readily performed in the Spice to tracer driver interface software. A initial Op. point request from Spice would have that and its neighborhood measured by the tracer as well. Then subsequent Spice data calls would be answered by driver software interpolation within the data range until the range gets exceeded. Then another data measurement and it's neighborhood gets performed on the tracer. Etc. Might give a 10x to 100x speed improvement for 20 lines of code.
One could even do predictive monitoring in the driver to determine where the Op point is drifting and request tracer data ahead of time, before the range is exceeded. Zero latency maybe then.
One could even do predictive monitoring in the driver to determine where the Op point is drifting and request tracer data ahead of time, before the range is exceeded. Zero latency maybe then.
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Unfortunately, the way that the power supplies operate, it is not possible to quickly move to a random combination of voltages, so it wouldn't be practical to do a realtime interface. That is one of the tradeoffs that was made to keep the cost down.
However, you could do a sensitivity analysis on the various voltages and record enough data to allow you to do an accurate interpotation from the actual curves. That should get you pretty close to what you would see with the tube in a real system, assuming that you plugged in all the inter-electrode parasitics into the model as well.
However, you could do a sensitivity analysis on the various voltages and record enough data to allow you to do an accurate interpotation from the actual curves. That should get you pretty close to what you would see with the tube in a real system, assuming that you plugged in all the inter-electrode parasitics into the model as well.
This sounds like it's getting close to "Physical Modeling". Just have Spice call the tracer and have the tracer make a measurement and return the value to Spice.
Hmmm
I kind of like the approach using a transformer and some resistors, diodes, and caps. maybe the diac to make it a one-shot and capture the curves with my storage scope...
Oops, I see you're ahead of me. anyway, I think there could be some simple adapter board with 2 rows of banana plugs and a tube socket in the middle. You solder jumpers from the socket pins to the banana plugs to customize it, then the tracer base has matching rows of banana jacks.
I recommmend supporting higher anode current (to at least 500mA) and try to squeeze more anode voltage up to the connector ratings (750V?). Capability for g2 curves would be a plus for some folks. (thinking power tubes here)
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Mine is obviously going to have to settle for certain compromises to keep the part count down and consequently to keep the circuit simple so it is definitely not going to challenge any commercial products. I mainly deal with noval tubes so in order to cover my needs my targets are the following:
- up to 300-400V anode voltage due to choice of active components (500V mosfets are very cheap
)
- 10 mA and 100 mA maximum cathode current ranges
- grid voltage: 64 steps in 0.5V increments (could be referenced at any of the 8 points along the range, but I imagine most will choose to operate grid in -0V to -31.5V range in order to avoid grid meltdowns), limitation coming from the voltage regulator and opamp supply rail caps so this range could be extended by employing (more expensive) higher voltage parts
These two parameters (and anode dissipation) will be adjustable in software, so tracer will terminate tube's cathode connection (driving the cathode up and thereby pushing device under test into cutoff) if maximum of any of those three parameters is exceeded.
Software will take 500 measurements along each grid curve (with 400V anode supply that would come out to less than 1 volt per step which is the kind of granularity that is easily on par with graphs in datasheets) and allow oversampling at least 1000 and 2000 points per curve (less than 0.5V per step on average) which should be more than enough even for low voltage devices such as JFETs and low voltage tubes. In theory it should be possible to oversample the signal with significantly higher rates but I see no point in that, however it would merely require a change of software should the need ever arise.
ADC sampling resolution is 10 bits, resulting in ~0.1 mA/div at 100 mA range (used for majority of tubes) and ~0.01 mA/div at 10 mA range (for ECC83 and the likes of it).
I intend to use poor man's "ramp" generator in the form of unfiltered rectified AC which means readings will be more concentrated around Va(max), but I find that to be an acceptable tradeoff for simplicity and cost. A real ramp generator of lower frequency could easily be substituted in order to improve the low voltage performance.
PC connection will be optically isolated in order to prevent any nasty surprises that could occur from defective resistors, blown mosfets or anode-to-grid arcovers in tubes. The downside ? A bunch of transformer secondaries will be required but that still beats fried computer due to one faulty tube ...
Why PC ? Easy printing of graphs and export of data for further processing.
I'm confident I will reach my pricing range. The majority of the work is in software anyway, hardware is relatively simple.
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I have everything planned so I just need to finish my schematic and create the PCB. Then it's programming time on both MCU and PC fronts. My business is very slow so I have more time for private projects ... A mixed blessing
-=-=-=-=-=-=-=-=-=-=-
So before I proceed I would really appreciate some feedback from you guys regarding my idea. Would you find such a tool usable with given specifications or would you want to see some of its parameters (voltage and current ranges, etc.) changed and/or improved from the get-go ? For instance, adding another current range only costs a couple of bucks and some PCB real estate ...
Do you have any other suggestions for changes that I hadn't considered at all ?
- up to 300-400V anode voltage due to choice of active components (500V mosfets are very cheap
)- 10 mA and 100 mA maximum cathode current ranges
- grid voltage: 64 steps in 0.5V increments (could be referenced at any of the 8 points along the range, but I imagine most will choose to operate grid in -0V to -31.5V range in order to avoid grid meltdowns), limitation coming from the voltage regulator and opamp supply rail caps so this range could be extended by employing (more expensive) higher voltage parts
These two parameters (and anode dissipation) will be adjustable in software, so tracer will terminate tube's cathode connection (driving the cathode up and thereby pushing device under test into cutoff) if maximum of any of those three parameters is exceeded.
Software will take 500 measurements along each grid curve (with 400V anode supply that would come out to less than 1 volt per step which is the kind of granularity that is easily on par with graphs in datasheets) and allow oversampling at least 1000 and 2000 points per curve (less than 0.5V per step on average) which should be more than enough even for low voltage devices such as JFETs and low voltage tubes. In theory it should be possible to oversample the signal with significantly higher rates but I see no point in that, however it would merely require a change of software should the need ever arise.
ADC sampling resolution is 10 bits, resulting in ~0.1 mA/div at 100 mA range (used for majority of tubes) and ~0.01 mA/div at 10 mA range (for ECC83 and the likes of it).
I intend to use poor man's "ramp" generator in the form of unfiltered rectified AC which means readings will be more concentrated around Va(max), but I find that to be an acceptable tradeoff for simplicity and cost. A real ramp generator of lower frequency could easily be substituted in order to improve the low voltage performance.
PC connection will be optically isolated in order to prevent any nasty surprises that could occur from defective resistors, blown mosfets or anode-to-grid arcovers in tubes. The downside ? A bunch of transformer secondaries will be required but that still beats fried computer due to one faulty tube ...
Why PC ? Easy printing of graphs and export of data for further processing.
I'm confident I will reach my pricing range. The majority of the work is in software anyway, hardware is relatively simple.
-=-=-=-=-=-=-=-=-=-=-
I have everything planned so I just need to finish my schematic and create the PCB. Then it's programming time on both MCU and PC fronts. My business is very slow so I have more time for private projects ... A mixed blessing
-=-=-=-=-=-=-=-=-=-=-
So before I proceed I would really appreciate some feedback from you guys regarding my idea. Would you find such a tool usable with given specifications or would you want to see some of its parameters (voltage and current ranges, etc.) changed and/or improved from the get-go ? For instance, adding another current range only costs a couple of bucks and some PCB real estate ...
Do you have any other suggestions for changes that I hadn't considered at all ?
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"This sounds like it's getting close to "Physical Modeling". Just have Spice call the tracer and have the tracer make a measurement and return the value to Spice."
Well, except the PC CPU running the Spice is operationg in the GHZ range. The tracer is probably operating in the Hz range. And it'll really be Hz'ing if it's connected by RS-232.
But the power supplies in the tracer don't need fast random access. Spice is typically running a sine wave, and performing 1000s or more data points per cycle. With the mega-zillions of calculations going on in the Spice, it's probably not doing more the a few Hz signal wize though. The tracer just tracks the sine wave in almost real time, with the interpolation driver software doing the fast stuff.
The driver software just needs to keep the tracer slewing in the proper direction. And predictive software can get the measurement started before its needed, as long as the interpolation data can hold off long enough.
I used to program fuel controls for jet engines. Thats what they did. The early ones ran on G-- awful slow discrete 8 bit processors. Every trick in the book got used to speed it up and keep memory size down. (4 kilbytes!) We could have written MS Windows in less than a MB, and had it run a million times faster. Modern software has become the very model for entropy.
Well, except the PC CPU running the Spice is operationg in the GHZ range. The tracer is probably operating in the Hz range. And it'll really be Hz'ing if it's connected by RS-232.
But the power supplies in the tracer don't need fast random access. Spice is typically running a sine wave, and performing 1000s or more data points per cycle. With the mega-zillions of calculations going on in the Spice, it's probably not doing more the a few Hz signal wize though. The tracer just tracks the sine wave in almost real time, with the interpolation driver software doing the fast stuff.
The driver software just needs to keep the tracer slewing in the proper direction. And predictive software can get the measurement started before its needed, as long as the interpolation data can hold off long enough.
I used to program fuel controls for jet engines. Thats what they did. The early ones ran on G-- awful slow discrete 8 bit processors. Every trick in the book got used to speed it up and keep memory size down. (4 kilbytes!) We could have written MS Windows in less than a MB, and had it run a million times faster. Modern software has become the very model for entropy.
I see what you are saying. Having run a few SLOW tube simulations myself, it should have occured to me. As long as the power supplies can slew fast enough to keep the tracer in sync with the simulation, and do what you mention with respect to look-ahead and/or caching around the current operating point in the driver, you could interface Spice with a real tube.
Sounds like I should define a control API for the tracer so that someone with a few more braincells than me can tackle the problem.
I don't know how you guys are doing the PC interfacing. But I bought a little module from USBmicro a year ago that has a one chip interface to USB on a tiny PC board, which plugs into a DIP socket to provide a 16 bit parallel interface. Has a Cypress Semi chip on it. Maybe newer/ better ones out by now since I see an 2004 date code on it. A CY7C63743 part. Comes with user instruction data. I think it was like $30, but not sure now. Can check what they have these days at USBmicro ODN. Jackinnj suggested it a while back.
I haven't used it yet, since I got some RS-232 controlled power supplies off Epay that look to be just a software effort to use. But the Cypress part would provide a fast interface that could be opto-isolated to a uP. I suppose many uP's have USB directly ported now too. But then how to opto-isolate them except for on the other side of the chip. Which could be a problem if the chip has onboard D/A and or A/D capability.
The Xantrex power supplies I bought off Epay have dual micros in them. One handles the communication protocol. The second processor has a fast opto link to the 1st and does the actual control of the power supply. To control a programmable resistor circuit, I could use a spare Xantrex control board that's available, or go with the USBmicro interface.
I haven't used it yet, since I got some RS-232 controlled power supplies off Epay that look to be just a software effort to use. But the Cypress part would provide a fast interface that could be opto-isolated to a uP. I suppose many uP's have USB directly ported now too. But then how to opto-isolate them except for on the other side of the chip. Which could be a problem if the chip has onboard D/A and or A/D capability.
The Xantrex power supplies I bought off Epay have dual micros in them. One handles the communication protocol. The second processor has a fast opto link to the 1st and does the actual control of the power supply. To control a programmable resistor circuit, I could use a spare Xantrex control board that's available, or go with the USBmicro interface.
I'm not familiar with that particular Cypress USB chip, but their EZ-USB series chips are very popular. They have a built in 8051 compatible CPU. And you are right, there are a ton of other USB MCUs.
The particular USB chip that we are using is from FTDI. It's basically a parallel FIFO interface. All the low level control is in programmable logic on the board.
You can get a $18 board with the FTDI chip on it at SparkFun Electronics. See SparkFun Electronics - Breakout Board for FT245RL USB to FIFO . It's basically an 8 bit parallel interface with a FIFO in each direction. They have very good software support, with lots of example code. They also make an FTDI USB to RS232 that you could use to talk to your XANTREX power supplies. See SparkFun Electronics - Breakout Board for FT232RL USB to Serial . Xantrex makes good stuff.
Chris
The particular USB chip that we are using is from FTDI. It's basically a parallel FIFO interface. All the low level control is in programmable logic on the board.
You can get a $18 board with the FTDI chip on it at SparkFun Electronics. See SparkFun Electronics - Breakout Board for FT245RL USB to FIFO . It's basically an 8 bit parallel interface with a FIFO in each direction. They have very good software support, with lots of example code. They also make an FTDI USB to RS232 that you could use to talk to your XANTREX power supplies. See SparkFun Electronics - Breakout Board for FT232RL USB to Serial . Xantrex makes good stuff.
Chris
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