Characteristics of HP-IL Transformers?

[J-F Garnier]

(09-06-2020 06:11 PM)J-F Garnier Wrote:  The input path uses two transformers in cascade, this was designed in this way for the ESD protection.

Carefully reading again the HP Journal article on HP-IL (p.15), it turns out that the reason for the two-transformer design of in the input path was more dictated by EMI considerations:
The design of a receiver circuit to handle [externally generated EMI] transients turned out to be a major undertaking. [..]
Because of the difficulties in manufacturing very small transformers with shields, it was found that the two transformer approach is more cost-effective. [..]
The design results have been very rewarding. With the design approach outlined here, prototype systems that failed the susceptibility test at 100V transient amplitudes were able to handle 500V to 1000V transients and did not exhibit any transmission failures below a field strength of 5V/meter.

However, for retro/hobby projects, we don't need such EMI performance, and a single transformer solution is very acceptable.

J-F

[J-F Garnier]

(08-25-2020 02:59 PM)J-F Garnier Wrote:  I had the project to experiment with pulse transformers on the PIL-Box, [..] but didn't do it yet (too many projects...).

Well, stimulated by this thread, I found the time to make some experiments with the PIL-Box.

Using the measurements done by Hans (thanks!), I selected this miniature, low-cost pulse transformer from muRata: the 78601/1C , 2mH, 1:1 ratio part.

Et voilà, now I have a transformer-coupled PIL-Box :-)
   

J-F

Addendum Nov.2024: link to the PIL-Box transfo mod (minor correction).

[hth]

Somewhat unrelated, but I had a project idea of making an HP-IL box based on the BeagleBone black. It is a low cost Linux computer with an interesting I/O RISC co-processor. My idea was to use the co-processor to handle the low level stuff and then do the emulation of HP-IL devices at the Linux side.

That computer fits into one of those metal mint candy boxes and would be able to be standalone (no need to connect to a laptop of stationary computer). Then you could always ssh into it to configure things or even provide IL devices for getting on the internet, perhaps treat the Linux machine as a kind of co-processor to the the HP-41. My thinking is that it would be a lot faster as it would bypass the USB channel, allowing the ARM processor to directly access the HP-IL frames in memory.

Lots of ideas, too little time, LOL

[Dave Frederickson]

(09-16-2020 04:06 PM)J-F Garnier Wrote:  Using the measurements done by Hans (thanks!), I selected this miniature, low-cost pulse transformer from muRata: the 78601/1C , 2mH, 1:1 ratio part.

Et voilà, now I have a transformer-coupled PIL-Box :-)

I've noticed that my PIL-Boxes are sensitive to my desk lamp being turned on or off. Besides adding isolation, will the pulse transformers make the PIL-Box more robust with regards to electromagnetic susceptibility?

Dave

[rprosperi]

(09-16-2020 04:06 PM)J-F Garnier Wrote:  Et voilà, now I have a transformer-coupled PIL-Box :-)

Thanks JF!

Can you please explain the virtues of transformer coupling for us non-EE's in the audience? Put another way, why/how is this better (I presume you wouldn't bother to make this and announce it if it wasn't better) than a standard PIL-Box?

[J-F Garnier]

(09-16-2020 09:06 PM)Dave Frederickson Wrote:  I've noticed that my PIL-Boxes are sensitive to my desk lamp being turned on or off. Besides adding isolation, will the pulse transformers make the PIL-Box more robust with regards to electromagnetic susceptibility?

(09-17-2020 12:22 AM)rprosperi Wrote:  Can you please explain the virtues of transformer coupling for us non-EE's in the audience? Put another way, why/how is this better (I presume you wouldn't bother to make this and announce it if it wasn't better) than a standard PIL-Box?

The transformer coupling ensures balanced signals: common mode (i.e. present on both wires) input transients are rejected - to some extend - and output signals have no common mode and generate less perturbations.
So it will probably improve the rejection of external EMI, although not at the same level than the HP design with the two-transformer input (see here). The capacitive coupling is quite limited on this respect.

But for the typical PIL-Box usage (personal use at home), it will not make a big difference.

There are two other PIL-Box limitations that are improved by the transformer coupling:
- you will be able to connect several PIL-Boxes/PIL-IO boards on the same loop,
- you can drive longer HP-IL cables.

Note that I released this information for possible DIY PIL-Box modification, I don't plan to make a new PIL-Box version.
My motivation was rather to demonstrate how to use HP-IL transformer coupling with modern components.

J-F