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Re: POC COMPUTER VERSION TWO: A New Direction
Posted: Mon Aug 30, 2021 2:38 pm
by BigDumbDinosaur
Ouch! That hurts.... more fancy coasters for the workshop it seems.
Also, their straight edges make them good for drawing short lines. 
Gotta post pics of the boards anyway though! Can’t leave us hanging.
I suppose. I'll see what I can do.
Re: POC COMPUTER VERSION TWO: A New Direction
Posted: Tue Aug 31, 2021 2:19 am
by jmthompson
The PCBs arrived on Monday, and as I was preparing to get the SMT stuff soldered in place, I discovered a major boo-boo in the layout.
The board is electrically correct but the footprint I used for the 'AC245 transceiver was wrong. It has the right number of lands but is the wrong package width
Welcome to the club.
On my first build, back in 2017, I picked the narrow (7mm) DIP footprint for my 6850 instead of the 15mm one. Fortunately I was able to rig up an adapter with some perf board, a socket, and some headers. Didn't even bother fixing that problem until I upgraded the design earlier this month.
Re: POC COMPUTER VERSION TWO: A New Direction
Posted: Tue Aug 31, 2021 12:29 pm
by plasmo
If it is any comfort, I'm the record holder of the biggest IC footprint mistake: That's a BIG 64-pin 68000 tilting about 45 degree to fit a 600 mil-wide footprint; correct footprint is 900 mil wide.
Bill
Re: POC COMPUTER VERSION TWO: A New Direction
Posted: Tue Aug 31, 2021 1:05 pm
by BigEd
Pythagoras to the rescue!
Re: POC COMPUTER VERSION TWO: A New Direction
Posted: Tue Aug 31, 2021 1:37 pm
by Dr Jefyll
That's a BIG 64-pin 68000 tilting about 45 degree
Bill, I love the stubborn, "don't take no for an answer" attitude! More of your obstinate artistry in
this thread...
-- Jeff
Re: POC COMPUTER VERSION TWO: A New Direction
Posted: Tue Aug 31, 2021 2:32 pm
by barrym95838
If it is any comfort, I'm the record holder of the biggest IC footprint mistake: That's a BIG 64-pin 68000 tilting about 45 degree to fit a 600 mil-wide footprint; correct footprint is 900 mil wide.
Bill
I would just mount a tiny 5V fan at one end and call it extra cooling surface area.
Re: POC COMPUTER VERSION TWO: A New Direction
Posted: Tue Aug 31, 2021 3:46 pm
by plasmo
it is a different way to shrink the size of 64 DIP to save pcb real estate.
I also have a footprint error involving a 64-pin "Shrink DIP". There is nice photo of it somewhere...
Bill
Edit, here is the picture. Notice the row of 32 pins at the bottom of picture; it is not wide angle lens distortion.
Re: POC COMPUTER VERSION TWO: A New Direction
Posted: Tue Aug 31, 2021 8:57 pm
by BigDumbDinosaur
If it is any comfort, I'm the record holder of the biggest IC footprint mistake: That's a BIG 64-pin 68000 tilting about 45 degree to fit a 600 mil-wide footprint; correct footprint is 900 mil wide.
Yikes! That looks downright dangerous. At least you were able to salvage the PCB and complete the assembly.
Re: POC COMPUTER VERSION TWO: A New Direction
Posted: Wed Sep 01, 2021 6:52 am
by Martin A
Notice the row of 32 pins at the bottom of picture; it is not wide angle lens distortion.
Would that be 1.87mm spacing instead of 1.78mm ???
Re: POC COMPUTER VERSION TWO: A New Direction
Posted: Thu Sep 02, 2021 4:43 am
by BigDumbDinosaur
The PCBs arrived on Monday, and as I was preparing to get the SMT stuff soldered in place, I discovered a major boo-boo in the layout...Supposedly, I'll have the new PCBs and stencil on Thursday.
The new boards and stencil arrived. The bus transceiver footprint is now correct. Time to build the contraption and see if she goes or blows.
Re: POC COMPUTER VERSION TWO: A New Direction
Posted: Thu Sep 16, 2021 1:16 pm
by Martin A
I had a chance to put the scope on the board I built based on the POC2 design.
Starting with the clock stretching, both 1 and 2 wait states appear to be inserted as expected. Seen here with 1 wait state selected.
- longer shot of the wait states
The average frequency detected by the scope shows that particular section of code (probably keyboard scanning) has a 5 to 6% slowdown over all as the master clock is 20mhz for a nominal CPU speed of 10mhz. That's a small price to pay compared to running the board ad the speed of the slowest I/O component.
The first test runs were done with a slightly out of spec HC parts, both flip flops have been replaced with newly arrived AC parts and the clock from the flip flop shows a little ringing:
- Scoping the clock side of the damping resistor
On the CPU side of the 120R resistor the ringing is gone, but the waveform is somewhat rounded:
- CPU side of the damping resistor
Not that the CPU seems to mind, the rise and fall rate in the mid range is nice and fast.
Re: POC COMPUTER VERSION TWO: A New Direction
Posted: Thu Sep 16, 2021 4:03 pm
by BigDumbDinosaur
On the CPU side of the 120R resistor the ringing is gone, but the waveform is somewhat rounded...
Looks as though your clock circuit has higher-than-average parasitic capacitance, or your probe is loading down the circuit.
Re: POC COMPUTER VERSION TWO: A New Direction
Posted: Thu Sep 16, 2021 8:16 pm
by Martin A
On the CPU side of the 120R resistor the ringing is gone, but the waveform is somewhat rounded...
Looks as though your clock circuit has higher-than-average parasitic capacitance, or your probe is loading down the circuit.
I wouldn't bet against either to be honest. The important thing is your design appears to be working as intended.
Re: POC COMPUTER VERSION TWO: A New Direction
Posted: Sat Sep 18, 2021 4:47 am
by BigDumbDinosaur
Gotta post pics of the boards anyway though! Can’t leave us hanging.
Here's a pic of the bad board and the good board. The bottom board is the good one.
Re: POC COMPUTER VERSION TWO: A New Direction
Posted: Sat Sep 18, 2021 4:51 am
by BigDumbDinosaur
The PCBs arrived on Monday, and as I was preparing to get the SMT stuff soldered in place, I discovered a major boo-boo in the layout...Supposedly, I'll have the new PCBs and stencil on Thursday.
The new boards and stencil arrived. The bus transceiver footprint is now correct. Time to build the contraption and see if she goes or blows.
Here are a few more pics.
- POC V2.0 PCB Top Layer
- POC V2.0 PCB Bottom Layer
- POC V2.0 PCB w/SMT Components