Use any two small diodes (preferrably Schottky, like BAT54 or similar) and two resistors like 4k7. Pull up both RxD inputs with resistors to their own power supplies, connect diodes' anodes to RxD inputs, cathodes to the other parties TxD outputs. That's all.

You are kidding... Not that I have anything against 4-layer boards, but I can't see any reason for 4 layers with classic microprocessors and even contemporary microcontrollers. 2 layers are more than enough if you don't use BGA packages. I do PCB designs for like 25 years, using Eagle and now KiCAD ...

And why don't you replace the transistor with a jumper?

Well, that's what I did back in 1987 as my MSc - a 16-bit modular microcomputer with 8086, MC68000 and (planned but not finished) Z8000. The system was capable of multiprocessor operation. Every CPU had its own ROM and there was a RAM board shared by all CPUs. 74188 PROM was used as programmable ...

Looks like a perfect idea for an RPI Pico project: the microprogrammable I/O should easily handle the 6502 bus even at 8 MHz or more, and the emulated 6551 could interface to a PC via RPi Pico USB using VCOM.

How about using two diodes and a resistor to form a diode gate, instead of 74xx gates? Timing parameters are not important in this case - CPU cycle is an order of magnitude longer than any gate delays.

So you don't reset your CPU? It may or may not work then. How about RDY line?

Some time ago I described here my SDC_Zero design. It does what the Author of the thread wants to achieve, and a lot more. Plus: one may assemble it in less than 1 hour at the cost of approx. $7. :)
http://forum.6502.org/viewtopic.php?f=4&t=5371&hilit=SDC_65
With SDC_Zero You may single-step, inject ...

I started writing my own 6502 simulator for my new hardware project and after looking at al those decimal ADC/SBC rants I decided to do it the right way. ;) Instead of going "Intel" way (that's what most of 6502 simulators do), I looked at the 6502 schematic and MOS Tech "decimal" patent. There is ...

gbm, I'm a little uncertain, and other may be, too. You seem to imply that the screen shots you posted reveal info based on signals from a real, physical '816... but you didn't actually say so (and the line up top about "Software-Defined Computer" confuses the matter by sounding like a simulation ...

I too am skeptical. Try as I might, I see nothing in the test code that would conclusively prove the device under test is a genuine '816.
I don't know of any other 65xx chip which outputs VPB, VDA and VPA signals, and without these it would not be possible to decode the cycle type info. The CPU ...

Ok, this works on 816, I need to check it on my C02s:
TestCPU:
LDA #0
STA $84
STA $85
LDA #$1D ; 'N' EOR 'S'
STA $83
LDA #$6B ; 'S' EOR '8'
STA $1D
LDA #$4E ; 'N'
RMB4 $83 ; magic $47 opcode
EOR $83
STA io_dr
Update: works on 65C02 and 816.
SDC runs at approx. 500..600 kHz, so no way to ...

Okay, here's a dainty little 20-byte program which can distinguish between four core families upon the behaviour of the $47 opcode, leaving the following ASCII codes in the accumulator:
; N - NMOS 6502
; S - 65SC02
; C - 65C02 or 65CE02
; 8 - 68C816 or 65C802

The $47 opcode is EOR [dp] on the ...

If it executes 65C816 code and produces all those fancy 65C816 signals for cycle type and emulation mode identification, then it must be 65C816. Any other explanation of this phenomenon?
The part number printed (not engraved) is W65C816SP-8. The 3 chips I got had different date codes from around 95 ...

I was happy to find out that my nicely printed, new Chinese R65C02 is a 65C02 (no R, no WDC, no SC), so I am quite lucky with my latest shopping results. ;) Some time ago I bought "UMC W65C02S" which also was an ordinary 65C02, laser remarked.
There is, however, a real, non-remarked bargain at Ali ...