I
suppose I really ought to find more time for this project
It's almost 3 years since I started - sadly I have essentially zero time for hobby electronics these days.
As it happens I found a WPMII at a reasonable price on eBay which rekindled interest - rather different board based on a µPD78F1168
Hi.
I tested the communication: The messages were just reflections or something ... So I read my own message back in a distorted way. (you probably need some other component in the circuit, like a capacitor or something)
Yeah, it sounds like there are reflections meaning your phototransistor is picking up output from your IR LED - but none of the other circuits (see below) include anything to suppress "local echoes" so, presumably, if you get the two SHF7221/7250's close enough together and well aligned it is not a problem.
Sadly several of the pages referenced a couple of years ago no longer exist - though the web archive has a partial record, including this page
https://web.archive.org/web/20160619033006/http://openv.wikispaces.com/Adapter+Eigenbau. The last but one is probably pretty similar to the hardware previously discussed in this thread but the last one is also interesting - utilising a USB interface but the TTL-level RS232/SFH7221 interface side of things is very simple.
As a starting point I reverse engineered the schematic for the WPM controller I bought (I chose to do that board because the serial I/O is on fixed pins on the 80C32 microcontroller so it was easier to start at those pins).
The red dots are where there are odd vias that I have not been able to trace. They usually start out on a trace on the top of the board but don't connect on the bottom of the board (it is 4-layer) and the ground fill prevents me from seeing if there is a trace going somewhere on an inner layer but it's quite possible that they
don't go anywhere - the circuit seems complete without adding anything.
The enable is one of the data outputs from a 74HC4094 serial->parallel shift register, I assume this is just driven by the controller, I haven't bothered tracing it further.
So, next step is to figure out what else I need to buy, apart from a £1 SFH7250, to make minimum order at Mouser