Yes, I am using all 74LS and the board is only running at 2 Mhz. I am going to set up a simple little delay loop using the 2 unused 74LS02 gates and see if that will delay the clock just enough at pin 3 of the flip flop.
On Monday, June 9, 2014 2:43:44 PM UTC-4, David Fry wrote:Thomas,I take it that you are only using 74LS series IC's with no 'F' series in there anywhere, if not then this would seem to confirm timing issue.maybe mixing some 'F' series on the 74F07 and 'L' series 74L02 might buy you some leaway, if not then a resistor/small capacitor as John suggestedwould seem to be the way to go. This would need trimming on the logic analyser for a minimum but safe working delay.regardsDavid Fry
On Monday, June 9, 2014 7:13:55 PM UTC+1, Thomas Owen wrote:UPDATE:
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.