visual6502: Help me understand this internal bus of the 6502

[cerebrum]

Hello,

I'm referring to the bottom right corner of the following:
http://visual6502.org/JSSim/expert.html ... &zoom=12.8

There is a powered diffusion pad(orange) separated from a yellow diffusion pad by the phi2(cclk) clock(purple vertical polysilicon).

So if phi2 is active the yellow should be powered and since it is connected to the idb4(internal databus bit 4) the whole idb4 should be powered as well every time phi2 is active. I searched carefully to make sure that idb4 is not grounded somewhere and it isn't. But if you step through the simulation you will see that at times phi2 is powered yet the yellow pad and idb4 remain unpowered. How can this be?

[BigEd]

Hi Cerebrum
Good question!

You probably already know that if you click on a node you'll see the nodename (and nodenumber) - but if you shift-click on a node you will see the nodegroup, which means all nodes connected by 'on' transistors. In this case:
http://visual6502.org/JSSim/expert.html ... u,dasb,dor
the nodegroup is reported as:
nodegroup from node: 892 idb4 (nodes: idb4,sb4,vcc,alu4,vss)
which tells us that in this half-cycle the pass gates have connected the Internal Data Bus with the Special Bus, and it looks like the Special Bus is being pulled down (on bit4 at least) by the ALU output.

So, the pass transistors connecting the two busses have transmitted a $00 from the ALU to the IDB.

Cheers
Ed

[cerebrum]

Yes, I know about shift click. ib4 is connected to sb4 but I can't even find where this connection is happening. Can you point me to the exact location where ib4 connects to sb4 and where it connects to the ground?

[BigEd]

Sure, it's
transistor: t1859 on node: 1060 dpc25_SBDB
http://visual6502.org/JSSim/expert.html ... &zoom=16.0
for the inter-bus connection

and
transistor: t1210 on node: 129 dpc20_ADDSB06
http://visual6502.org/JSSim/expert.html ... .1&zoom=16
for the connection to the ALU

[cerebrum]

Thanks Ed, now I see it and everything makes sense!

[BigEd]

Great - glad to be able to help.
I'd be grateful if you'd edit the subject of your original post - and therefore the topic title - to reflect that we're addressing a question about internal busses on the 6502.
Cheers
Ed

[cerebrum]

BigEd wrote:

Sure, it's
transistor: t1859 on node: 1060 dpc25_SBDB
http://visual6502.org/JSSim/expert.html ... &zoom=16.0
for the inter-bus connection

How did you find those nodes? Just visual search or is there a trick?

[BigEd]

I used some familiarity and scanned from right to left, having highlighted the two busses so I only needed to look at the area of left-right overlap.

(To find the transistor number itself, I clicked on the gate)

Thanks for changing the topic title!