C128 immediate PowerUp diagnosis using VIC IIe.

[XmikeX]

With respect to diagnosing C128s, is it safe (and meaningful/useful) to test voltages here -> VIC IIe pins/lines 26,27,28 on power-up?  Specifically, I would presume testing these pins/lines would let me know if Z80 is good/bad and if Z80bios rom relocation has/has not occurred.

My rationale (hopefully correct) is as follows:
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Early in the c128 boot sequence the following appears:
"0041:       ED 51             OUT (C),D                write #$FF (11111111) to $D02F."

and this appears just before the C128 is forced into 1MHz mode, oddly enough,
as noted here --> http://www.commodore128.org/index.php?topic=3670.msg18033#msg18033

According to Mapping the 128 and others, $D02F is the Extended Keyboard Scan-Line Control Register (XSCAN). Setting XSCAN bits 0-2 to 1 slams the output lines (K0, K1, K2) to a high (+5v) state.

According to this - http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c128/310378-2-right.gif , K0-K2 correspond to pins 26-28 on the VIC IIe.
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XmX

[Hydrophilic]

It should be safe to test those lines, although I don't know if it would be meaningful.  As I understand, most of CSG I/O chips will have their lines go inactive (+5V) upon reset.  So you may read +5V even if Z80 is defective.

If you have a logic probe, I would test for a signal on Pin 6 of the Z80.  If no signal there (and you know 9VAC is good) then track back to the VIC-II pin 25.  Note this is only active briefly when you power up, and shuts down once the 8502 takes control.

Another idea, based on yours, is a bit later (if no C64 cartridge attached) the Z80 will do this

Code Select Expand


. 006E  3E 7F      LD   A, $7F     ;row 7 active (contains Commodore key)
. 0070  ED 79      OUT  (BC),A     ;$DC00


So you could test CIA#1 (U1) pin 9 is active (0V) and pins 2~8 are inactive (+5V).

Good luck!