[sdiy] SN76477 important info!

[epeasant at telusplanet.net]

Hello, Everyone,

Recent experimentation with this chip has led me to an 
interesting discovery. I have aquired a few 76477 ICs over 
the last couple of years from various sources, with a range 
of date codes. The older date codes on these parts (prior 
to about 1980) correspond to a part number of SN76477, 
while those newer than that are labelled SN76477N. 
Apparently the "N" designation chip is an updated and 
improved version, at least in the mixer section.

In the older type you can only select one of the three 
sources at a time (Noise, VCO, or LFO) by connecting +5V to 
either pin 25, 26, or 27 respectively. If you want more 
than one source playing at a time, you must switch 5V 
between the appropriate pins at a hypersonic frequency 
(multiplexing). This method is mentioned in some of the 
older projects that I have seen for the 76477, but all of 
the data sheets that I have seen are for the "N" version 
only, so I can't check those.

The "N" version of this IC uses a logic table to determine 
which combination of sources is sent to the output; any 
combination of outputs is possible with no external 
circuitry. This is detailed in all of the SN76477N data 
sheets that I have seen.

Interestingly enough, the Bullet SE-01 project pcb contains 
an external oscillator for creating simultaneous sounds 
with the mixer of the older type, and this circuit is 
mentioned in the documentation. But the data sheet they 
also supplied describes the newer type mixer circuit and IC 
instead.

What this means is that if you don't have the "N" suffix on 
your chip, it WILL NOT WORK in most newer projects and 
circuits. This *includes* the Blacet Dark Star Chaos!

As well, one chip that I had in a sealed rat shack package 
labelled SN76477N turned out to be a SN76477 :-( [Thanks, 
Paul ;-) ] So beware, so you don't end up dissapointed with 
a bunch of almost useless chips like I have. 

Take care,
Doug
______________________
The Electronic Peasant

www.electronicpeasant.com