Image Map Image Map
Results 1 to 4 of 4

Thread: Commodore PC 20 III "Bad ram at 8075D = 00h, expected = 00h"

  1. #1

    Default Commodore PC 20 III "Bad ram at 8075D = 00h, expected = 00h"

    Error during startup:

    "Bad RAM at 8075D = 00h, expected = 00h."

    Which is the ram memory? Where to look for the cause?

    badram.jpg

  2. #2
    Join Date
    May 2006
    Location
    Melbourne, Australia
    Posts
    7,035

    Default

    First, be aware that I am unfamiliar with the Commodore PC 20-III.
    Maybe someone who is will 'chime in'.

    From information on the Internet, I see:
    * The PC 20-III is simply a PC 10-III with a hard drive added. ASSUMPTION: From that, the two motherboards are the same.
    * 640 KB of RAM

    Quote Originally Posted by dippaolo View Post
    "Bad RAM at 8075D = ...
    "Bad RAM at 8075D" equates to address 526173, approximately 514 K.

    That fits with the displayed "512 Kbytes OK"; that is, at least 512 KB of the fitted 640 KB is okay.

    From what I make of the technical information online:

    RAM bank 0
    ------------
    256 KB sized, addressed 0 to 256K.
    Eight 41256 type RAM chips, each providing 1 bit.
    No parity bit.
    U312 = bit 7, U311 = bit 6, U310 = bit 5, U309 = bit 4, U308 = bit 3, U307 = bit 2, U306 = bit 1, U305 = bit 0

    RAM bank 1
    ------------
    256 KB sized, addressed 256K to 512K.
    Eight 41256 type RAM chips, each providing 1 bit.
    No parity bit.
    U320 = bit 7, U319 = bit 6, U318 = bit 5, U317 = bit 4, U316 = bit 3, U315 = bit 2, U314 = bit 1, U313 = bit 0

    RAM bank 2
    ------------
    64 KB sized, addressed 512K to 576K.
    Two 41464 type RAM chips, each providing 4 bits.
    No parity bit.
    U321 = bits 7/6/5/4, U322 = bits 3/2/1/0

    RAM bank 3
    ------------
    64 KB sized, addressed 576K to 640K.
    Two 41464 type RAM chips, each providing 4 bits.
    No parity bit.
    U323 = bits 7/6/5/4, U324 = bits 3/2/1/0

    Quote Originally Posted by dippaolo View Post
    ... = 00h, expected = 00h."
    When I first saw that, I thought to myself, "That will be just like the POST in the IBM PC; if no data bits are reported as faulty, it means that it is the parity bit that has failed."

    But when I looked at the circuit diagams for the PC10-III/PC20-III motherboard, I could not find RAM for parity. The motherboard's FE2010 chip supports motherboard parity, but I see that the FE2010's PTYIN pin (pin 32)(FE2010 datasheet: "Parity bit from parity RAM") is grounded. The FE2010's PTYOUT pin (pin 33)(FE2010 datasheet: "Parity bit to parity RAM") is routed through driver U108, but I cannot see it after that. So, because the FE2010's PTYIN pin is grounded, the POST in the PC10-III/PC20-III motherboard must be disabling parity checking by the FE2010, via bit 4 of the FE2010's control register.

    With no RAM parity bits, the "= 00h, expected = 00h" puzzles me. If the POST wrote 00h to address 526173 and then read back 00h, what mechanism informed the POST that there was a "Bad RAM" problem at that address?

  3. #3

    Default

    FINE.

    I ordered the MSM41464-10 RAM (+socket dil1 to start.

    I will replace the RAM. I will write what the test showed.

  4. #4

    Default

    Ram replaced. Bad ram is still there.

    I read the diagram again. I checked the connection for 2 hours.

    I found one more error.

    U325 -> U109

    NDACK0.jpg

    I fixed the connection. Works correctly. I have 640Kb RAM ... YUPI

    Thank you all

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •