[Techtoolslist] Z80 Pod - weak signal strength?

[John Robertson]

On 09/29/2014, 6:40 PM, Sprout wrote:
> Relatively new owner, and learning my way around the hardware..    Picked
> up a Z80 pod as part of a larger bundle purchase that was flaky.
> Eventually got it working, and in the process replaced a few 74C245's on
> the secondary PCB with 74LS245's.  Consistently passing self test now.

74Cxx(x) devices handle noise better than 74LSxx(x) as I understand it. 
Best to hunt the 74C245s down. Voltage swings are also a bit better 
allowing for a better noise immunity over the length of the cable.
> Initially, when it was somewhat flaky (and before I appear to have blown
> the '245's by troubleshooting with the pod removed from it's shell - gotta
> be very careful the two boards don't come apart while moving it around :P
> Initial issue was the two boards were poorly seated), I had a couple of
> good game boards where it allowed me to consistently run UUT with no
> issues.  Since then, I'm noticing certain boards will run perfectly with
> the pod (ie Galaxian), and others (Jr. PacMan, Sega Frogger, Ms. Pac
> modified to run Crush Roller) just constantly watch-dog.  My guess is that
> I have poor signal strength between the pod and the PCB's, but wanted to
> see if anybody found this as a common issue, and whether there are any
> components in the pod that may be worth looking at/replacing to address
> this?  The Ms. Pac board has the small satellite board, and I noticed if I
> bypass it and go directly to the main PCB socket, the board is a bit more
> active, which makes me feel I'm on the right track. Jr. Pac doesn't use
> that however, and refuses to budge.
>
> I've verified all the '245's and '374's check out good as per the IC test
> functions on my EPROM programmer, and again - passes self-test
> consistently.  When on the Galaxian PCB, I could read ROM without issue,
> pass all RAM tests without issue - seems to work perfectly.  Can't do
> anything on the others as it's complaining about active lines constantly,
> which are toggling because the boards seem to be watch-dogging as noted.
> If I disable active line monitoring, bus tests pass.

The ROM and RAM tests are run at the Pod's speed, not the UUT's (Unit 
Under Test) so they will often pass even if the Pod won't boot the 
board. Only if you have the Z80QT can you run RAM/ROM tests directly 
under the UUT's CPU at system speed.

I find one has to disable the watchdog to get these boards to boot from 
the pod...
> I also built a new cable (slightly longer than the original) with no change
> in behavior.  This one uses the standard, flat ribbon cable.  Also dropped
> in a Z80B - no change; these boards are all running at 3.072Mhz anyway,
> which shouldn't be an issue...


The cable for each family of pod is a bit special - you have to have the 
correct one for yours. See notes at the bottom of my Fluke page:

http://flippers.com/fluke.html

I haven't taken a cable apart but I suspect there may be some funky bits 
in that rather fat header on some of them. Anyone with access to X-Ray 
equipment care to take a look?

> Thoughts/suggestions?  On a side note - what's the most current version on
> the ROM?  Mine shows part# 536581, version 1.0.  I found the Z80QT pod ROM
> and a few others online, but not this one.  Plan to dump it regardless, but
> would like to upgrade if there's a newer version floating about.
>
> ...
> Thanks,
>
> Brent



The ROM code has been up on the TTL FTP site for over a decade...lots of 
useful info there!

FTP site is: ftp://ttl.arcadetech.org/TTL/Test_Equipment

John :-#)#
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