Interesting RK8E fault

[Tony Duell]

On Sun, Dec 9, 2018 at 6:50 AM Josh Dersch via cctalk
<cctalk at classiccmp.org> wrote:


> If anyone has any insights into the inner workings of the RK8E (in
> particular the CRC circuit, since it's used to compare the on-disk cylinder
> address stored in the header with the cylinder selected by the RK8E's
> address register) please let me know.

I think you can ignore the actual CRC logic here. Just treat the CRC
register as a shift register. It is shifted in-sync with the data coming off
the disk, in this case the header word that contains the cylinder address.

Look at page 25 of the RK8-E engineering drawings (Oct72) on bitsavers.
It's sheet D04 (Major Registers PCB).

The header word bits (from the disk) are compared with the contents of the
the shift register one bit at a time by E24c. The output of that goes to E34b
(D input). E34b starts off clear, and remains clear while the bits agree. If
there is a difference in the bits (cylinder address is not right) then
E34b sets. The Q/ output goes low, pulling the S/ input (pin 10) low. This
forces E34b to remain set (The S/ and R/ direct inputs will override the
D input). So after all the bits have been compared, E34b is set if there was
an error.

-tony