If I manually set pin 2 of the flip flop to either high or Low then the Drive Select works.
pin 2 held high, console out DO34,01 changes to Drive B.
It appears to me that the FF is always clocking in a 0 due to timing on the input.
I have two different CPU's in a IMSAI chassis (with front panel). CPU is a Cromemco ZPU and the other is the IMSAI 8080 - both exhibit same behavior.
DO0 definitely changes (see the earlier screen shots) for the different commands 'QO34,00' and 'QO34,01'.
On Sunday, June 8, 2014 7:58:45 PM UTC-4, Thomas Owen wrote:All,
I had posted this to the thread dealing with ordering this card, but decided to post a new topic dealing with the problem I have had with final checkout of the board.
During assembly the board passed all the 'in progress' checks. The final steps were to output to different ports:
I have output the following and everything passes:
QO33,80 ; Configures ports A, B and C as output ports
QO32,2B ; Selects right hand pair of digits
QO30,01; display 01on right digit pair and work through each bit - all ok
QO32,2c; Selects middle pair of digits
QO30,01; Should display 01 on middle digit pair, work through each bit - all ok
QO32,2d; Selects left hand pair of digits
QO31,01; Should display 01 on left digit pair, work through each bit - all ok
All displays are correct.
Now the final step is the Drive Select and that is where I am having trouble:
Board always comes up with 'Drive A' selected
Now, QO34,0; no change
Reset, start again:
QO34,1; no change
I can never select drive B.
Several generous members (thank you David Fry) have made suggestions, and here is where I now stand:
My preliminary check this afternoon shows me that there is a timing issue at U15, the 74ls02. Using my logic analyzer and monitoring the inputs and the output (which clocks the drive select flip flop) I see a big timing difference between SEL_SECOND and WR.
What this means is that the output of the gate never goes high allowing the bit change to effect drive change/selection.
I am going to get some screen shots from the analyzer tomorrow, and if anyone has any suggestions I would greatly appreciate it.