Image Map Image Map
Page 2 of 11 FirstFirst 123456 ... LastLast
Results 11 to 20 of 101

Thread: PET 3032 garbage screen problem

  1. #11
    Join Date
    Feb 2009
    Location
    Southern California, USA
    Posts
    2,662

    Default

    Quote Originally Posted by MikeS View Post
    Not sure why you need two sockets; bending out the pins disconnects the data lines from the board although you might want to put a piece of paper under the socket to avoid inadvertent contact with the bent pins.
    Yes, if one is careful, one socket will do.

  2. #12

    Default

    Quote Originally Posted by iz8dwf View Post
    If RAM and ROMs are ok (but I doubt they can be safely tested from a daughterboard, having made my own daughterboard too), then this looks like a data (or even address/decode) conflict with some other parts. It could be an I/O chip or maybe a data bus transceiver (74LS244) or it might be an address bus buffer failing or any part of address decoding logic faiiling. Do you have an oscilloscope or logic probe?
    Frank IZ8DWF
    Commodore PET ROM / RAM replacement board https://d3s5r33r268y59.cloudfront.ne...er%20Guide.pdf
    This board has 9 DIP switches:
    1-2 - RAM replacement options
    3-5 - ROM replacement options
    6-9 - ROM selection

    used switches
    1-2 ON (pcb Ram replaced by Ram board)
    3- ON 4-5 OFF (pcb Roms replace by Roms board)
    7- ON 6-8 OFF (BASIC 2)

    In this scenario the pet uses the ram and roms of the replacement board....

    NOP generator made with the MikeS scheme

    20190826_213904.jpg

    Quote Originally Posted by dave_m View Post
    A0 will have the highest frequency of about 500 KHZ, A1 half of that, and A2 half of A1. If you spot something not in that pattern, you have found the problem. Note however that the RAM address inputs are multiplexed with the dynamic RAM refresh logic so it gets a little messy there. Hopefully with this method you will spot a bad buffer or whatever.
    The frequency at A0 (6502 pin9) is 250Khz, is it correct?

    SDS00001.jpg

  3. #13

    Default

    Quote Originally Posted by Fran View Post
    Commodore PET ROM / RAM replacement board https://d3s5r33r268y59.cloudfront.ne...er%20Guide.pdf
    This board has 9 DIP switches:
    1-2 - RAM replacement options
    3-5 - ROM replacement options
    6-9 - ROM selection

    used switches
    1-2 ON (pcb Ram replaced by Ram board)
    3- ON 4-5 OFF (pcb Roms replace by Roms board)
    7- ON 6-8 OFF (BASIC 2)

    In this scenario the pet uses the ram and roms of the replacement board....
    I used this configuration to verify that the garbage error was not caused by the ram or roms.

    With the NOP generator I am using the ram and roms of the pet.

  4. #14
    Join Date
    Feb 2009
    Location
    Southern California, USA
    Posts
    2,662

    Default

    Quote Originally Posted by Fran View Post

    The frequency at A0 (6502 pin9) is 250Khz, is it correct?

    SDS00001.jpg
    That looks right. I thought the NOP would execute faster but it must be a two cycle instruction. Shows how much I know about the 6502.

    You are doing good and will get to bottom of this problem.
    -Dave

  5. #15

    Default

    Quote Originally Posted by dave_m View Post
    That looks right. I thought the NOP would execute faster but it must be a two cycle instruction. Shows how much I know about the 6502.

    You are doing good and will get to bottom of this problem.
    -Dave
    Yes, NOP takes two cycles (one fetch from memory, one idle cycle). A0 toggles each two cycles, so A0's edges happen at 500 KHz rate, but its actual frequency as a square wave is 250 KHz.

    Frank

  6. #16
    Join Date
    Feb 2009
    Location
    Southern California, USA
    Posts
    2,662

    Default

    Quote Originally Posted by iz8dwf View Post
    Yes, NOP takes two cycles (one fetch from memory, one idle cycle). A0 toggles each two cycles, so A0's edges happen at 500 KHz rate, but its actual frequency as a square wave is 250 KHz.

    Frank
    Frank,
    Good explanation, thanks.
    -Dave

  7. #17

    Default

    Quote Originally Posted by iz8dwf View Post
    With a nop generator under the 6502, I'd first check each address going through UC3 and UB3:

    Input and output from the buffers must be the same for all addresses. Since the garbage pattern is exactly 16 bytes wide, I'd pay special attention to A4 (AB4 on that schematic) and its path.
    A4 path to the RAMs goes through UE5, that's the second place to look for problems:
    Finally, should it be only a video RAM issue (I don't believe so however), there's UF5 that passes BA4 to them:

    HTH
    Frank IZ8DWF
    6502 Output Khz (AB0 to AB7) are the same as the input and output UC3 (BA0 to BA7)
    6502 Output Khz (AB8 to AB15) are the same as the input and output UB3 (BA8 to BA15)

    BA4 output UC3 (15,63 Khz) is the same as the input pin4 UE5
    BA4 output UC3 (15,63 Khz) is the same as the input pin13 UF5

    What other pins and values must be checked in UE5 and UF5?

  8. #18

    Default

    Quote Originally Posted by dave_m View Post
    Yes, if one is careful, one socket will do.
    How many times do you bend the pins on a 40 year old part?
    I vote for two sockets.
    Dwight

  9. #19
    Join Date
    Feb 2009
    Location
    Southern California, USA
    Posts
    2,662

    Default

    Quote Originally Posted by Dwight Elvey View Post
    How many times do you bend the pins on a 40 year old part?
    I vote for two sockets.
    Dwight
    I have broken many an IC pin. When that happened, I would throw the chip on the lab floor and stomp on it as if it were the chip's fault!

  10. #20
    Join Date
    Dec 2005
    Location
    Toronto ON Canada
    Posts
    7,176

    Default

    Quote Originally Posted by Dwight Elvey View Post
    How many times do you bend the pins on a 40 year old part?
    I vote for two sockets.
    Dwight
    Not sure what you mean; the relevant pins of the socket are bent out and up and connected appropriately (with a piece of paper underneath to prevent the bent out pins from touching the system socket), and the 6502 is just normally inserted into the socket.

    But sure, if you have two sockets that works as well and no piece of paper needed.

    If I could find mine I'd post a picture

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
  •