Problems with serial to USB on Compukit UK101 - and a solution

[Mike52]

While in lockdown in the UK due to the corona virus pandemic, I pulled out my old Compukit UK101, to see if I could convert it from RS-232 serial input to USB. Following the thread and using the same module as per http://www.osiweb.org/osiforum/viewtopic.php?f=3&t=223 I duly connected it directly to the ACIA and it worked correctly.

Later I decided to fit the switch as per the C1P so that I could select which input to use i.e. Cassette (for nostalgia reasons only), RS-232 to use with an old PC, or USB from my laptop.

I fitted a 1 Pole 3 Way rotary switch and connected it in place of link W10 on the board. There was already a spare RX Data 3 input free on Pin 1 of J2, so the output of the USB module was connected to there. TX Data was taken from Pin 4 of J2.

Cassette and RS-232 inputs worked correctly, but I could not load anything from the USB device, although I could send from the UK101 to my laptop.

Time to get out my trusty Hitachi V203 oscilloscope. Data was appearing at J2 pin 1 and pin 13 of U67, however the output at pin 12 was stuck low.

Replacing U67 made no difference. Looking a little closer to the waveform on pin 13 of U67 revealed that whilst there was data present the dc conditions were wrong it was only reaching a low of 1.8v and a high level of 4.9v. After more checking, it was found that the output of the USB device could not pull the line low enough.

A solution was found by replacing R40,220R by 2K2, and R41,390R by 3K9. (It actually worked with both resistors removed, however I do not like the floating inputs which would happen if ever I removed the USB module).

The moral appears to be that old and new technology do not always work happily together.
Regards,
Mike52

[BillO]

Interesting, thanks for posting.

Why didn't you go directly through pins 6 and 2 of the 6850?

[Mike52]

Hi BillO,

Thank you for your reply and your interest, my thinking on this was as follows.

In the mid 80's I used an old Tandy serial / TTL printer that was connected to this port, (although the input side had never been used). So to save having to cut any more traces on the board, bearing in mind that W18 link had already been cut, I used what was already there.

Had I not wanted to make the input switchable, I would have connected the USB module as per the link I gave in my original post, and your suggestion.

Kind regards,
Mike52

[BillO]

Hi Mike,

I get your drift. I don't currently have a UK101 or even a schematic, but with my OSI boards (600B and 600D) there would be no problem with being able to switch. There are enough jumper options to connect up to. Nonetheless, thanks for your contribution. I may indeed find myself in possession of a UK101 at some future time.