EMU 1 : the saga continues

For some time now, and following all the tests that I have been able to carry out on the EMU 1 motherboard, I have come to the conclusion that I 'must' have a problem with the circuit Zilog SIO/2 serial interface, in other words an 8442 type circuit.

Obviously, I haven't been doing nothing for the entire time since my last post on the subject. I ordered five circuits directly in China from a supplier in whom I have relatively good confidence.

After a few days of waiting, I received my five circuits.

I was quite excited upon receiving these circuits and couldn't resist trying them 'right away' on the EMU 1 motherboard.

Hallo what?!!! Impossible to have the test menu on screen which nevertheless works very well with the original SIO/2 component.

Looking carefully at the logo engraved on the new circuits, I really wonder if they are not fakes!

I don't have the means to resolve the doubt at the moment. I would need a machine using a SIO/2 circuit to check their operation. Exactly, I have one. This is a montage created by Sergey Kiselev, who is called Easy_z80 and which is located at this address: https://github.com/skiselev/easy_z80

Unfortunately, this system uses not an SIO/2, but an SIO/0. The difference? Communications port #2 is not wired the same way. Nothing very complicated in fact, the TX and RX clock is on the same pin on the SIO/0 while on the SIO/2 the clock arrives on two different pins, which allows for a different communication speed in transmission and reception. In addition, the receive pin arrives on pin 29 of the SIO/2 while it is pin 28 on the SIO/0.

The result of this is that I cannot test a SIO/2 on the Easy_Z80 board either!

Impossible to dispel doubt about the functioning of the circuits, and therefore the motherboard of the EMU 1. What a 'mess'!

So I opted for two actions. On the one hand I ordered a set of two other SIO/2s on eBay, but this time in Germany and supposedly coming from an NOS! I also developed a small adapter to allow the use of either an SIO/0 type circuit on the EMU 1 card, or an SIO/2 on the Easy_Z80 system card.

With this adapter, I will at least be able to test the operation of my SIO/2 on the Easy_Z80 card, knowing that Easy_Z80 works very well with the SIO/0 with which it is equipped.

As you can see on the Easy_Z80 card, the SIO is indeed an 8440, a SIO/0 therefore:

I will be able to test the SIO/2 with my adapter, knowing that the two serial ports are used as simple asynchronous communication ports. The /SYNCA and /SYNCB signals are not used, which therefore allows the use of an SIO/2 instead of an SIO/0.

Likewise, these signals are not used on the EMU 1 motherboard. An SIO/0 can therefore be used instead of an SIO/2, with the adapter I made.

The continuation after the next experiments...