6809 + acia 6850

[Wawavoun]

Hello there,

I have a question for the assembly language specialists.

Below we see the 6809 code on the right and the timing diagram of the data bus on the left.

Can somebody confirm that the data which is read in the 6850 by the instruction "LDB ,X" is indeed $0C ?
We can clearly see $E684 then a $C5 (what is that ?) then $0C (the data read at address X ) then we see the $C502 of the operation "BITB # $ 02".

If the data is really $0C logically the program switch to END and the loop starts again and nothing is ever written in the output register of the 6850 ...

Finally if anyone has any idea why the TDRE bit of the 6850 is not to one after a reset that interests me a lot Could that come because the CTS input (coming via a MC1489 line receiver) is floating ?

Have a good evening.
Philippe

[edavid]

Finally if anyone has any idea why the TDRE bit of the 6850 is not to one after a reset that interests me a lot Could that come because the CTS input (coming via a MC1489 line receiver) is floating ?

You answered your own question.

If the MC6850 CTS* is connected to a MC1489, it's not floating.  An unconnected MC1489 input is logic low, so the MC6850 CTS* input will be high, and TDRE will be low.

[Wawavoun]

Sorry if I was not clear !

The cts input of the 6850 is obviously not floating because connected to a MC1489 output.

And yes it is the corresponding MC1489 input which is unconnected and floating...

The cts input of the 6850 is inverted (I dont know the word in english, in french we say "CTS barre")... So I have think cts is active at 0 (and then tdre bit set to low) and inactive at 1..

Regards.
Philippe

[edavid]

Sorry if I was not clear !

The cts input of the 6850 is obviously not floating because connected to a MC1489 output.

And yes it is the corresponding MC1489 input which is unconnected and floating...

The cts input of the 6850 is inverted (I dont know the word in english, in french we say "CTS barre")... So I have think cts is active at 0 (and then tdre bit set to low) and inactive at 1..

The MC1489 inverts the signal.  You need to tie the MC1489 input to a high level.

[Wawavoun]

Hello,

With cts and rts connected together and dcd to +12 V (logic 0) everything work fine !

Thanks all for your advices !

Regards.
Philippe