[DSP-10] DSPx/KDSP10 modification needed

[Bob Larkin]

Going back a week or so, Jiro JQ2LMG posted a note about improper data 
output from the U107/U108 shift registers. These registers are programmed 
serially from the DSP.  Since then there has been experimentation and 
discussion with Jiro, Lyle KK7P, and Steve  N7HPR.  This has resulted in a 
recommendation for a modification. This is particularly important for the 
DSPx board, but could possibly be beneficial for the EZ-Kits and in no case 
should be detrimental!

The 74HC595 shift registers are designed to work in cascade, but this makes 
the assumption that the serial clocks for the two will switch at the same 
time, within a few nanoseconds.  If the rise time of the clock pulse is 
slow, this requirement may not be met. This slowness is apparently 
aggravated by the 3.3 to 5 Volt transition required by the 2185 DSP in the 
DSPx.  If by bad fortune, U108 switches before U107, the data can get 
shifted incorrectly. A hardware fix, that seems to cure this, is a 
capacitor on the U108 serial data output to hold the data for an extra 20 
nanoseconds or more.  This turns out to be easily added to the existing 
DSP-10 main board.

Connect a leaded 470 pF to 1000 pF between pins 14 and 8 of U107. There are 
via holes near each of these pins that can be used to connect the cap.  The 
via connection traces are visible under the solder mask. The cap itself can 
go right over the U107 IC. The leads are not critical in length, but short 
is always good.  The value and type of the cap is not critical.  Ceramics 
would be fine. Values might even be as much as 2200 pF (0.0022 uF).  This 
modification can be made without removing the board, but, be sure the leads 
do not poke excessively through the board.  Obviously, turn off the power 
before doing this!

I would recommend this modification for all DSPx/KDSP10 versions.

Thanks to Jiro, Lyle and Steve for the help.

73,
Bob  W7PUA