Posts by plazma

    I can share the schematic if anyone is interested.

    Thank you, please share the schematics. : emojiSmiley-106:

    Here is a simplified schematic. It is missing 4 configuration resistors which select if neatSRAM is installed to a 2 or 8 DRAM chip motherboard.

    CAS also got its own latch. RAS latch is latched when RAS signal goes down but it is released when both RAS and CAS are high (AND-gate U5).
    SRAM output is controlled by CAS as with original DRAM.
    I enabled pin swap to latch inputs and SRAM address and data pins. You can scramble address and data pins and the memory works the same. This made layout easier.

    Interesting. So you are collecting DIY HW prototypes?
    I have quite a few projects and prototypes.
    Here is one in progress. Device called HEXcart. Supports all cart images in 8k and 16k. Selectable with a hex swtich hence the name.
    Logic for different type of carts in diferent memory banks is handled by same kind of flash chip as which stores the cart ROM images. The parts are very cheap. Using flash as logic is cheaper than logic IC:s. And it's configurable for different king of cart images.

    Here is a small overvoltage protector which fits inside the DIN power connector.

    Plus many more.
    Currently I have some for sale at Amibay and Facebook. See this sales post for a list. http://www.amibay.com/showthre…k-port-ESD-protector-v1-1

    I sell the neatSRAM for € 15 assembled and € 10 as a DIY kit. DIY kit requires soldering of QFN and TSSOP parts.


    I can check if I have prototypes left over from different projects. Some 1st proto milled PCBs have been disposed.

    I made a new version of my SRAM board called neatSRAM. The old one had some problems with six cyclone 466 and VIC-II R5.

    Both CAS and RAS have their own latches. SRAM output is enabled by CAS only as with original DRAM.

    The schematic was developed with an other Finnsih designer. I can share the schematic if anyone is interested.
    All machines which had problems are now working.

    I also made it easy to interface the board with older 8 DRAM chip motherboards.

    Cartridge PCB made right. Hard gold and beveling on edge connector.

    One going for free to kim_jorgensen as gift for the effort.

    Waiting for the last components to arrive. Also got a stencil for soldering.



    One guy bought an SRAM board which was a clone of jamarju's design. It did not work in a 466. Random characters on screen and multiple failures with diagnostics cart. He bough my board and it runs just fine. He just sent me a picture of zero faults after running VSP lab for over six hours. Both my and jamarju's design have the or-gate.

    I have a 16 channel logic analyzer and two 4 channel scopes. I'll check my breadbins for Fairchild or Fujitsu chips and move them to the 466 board. I'll take some measurements. It would be good to find the root cause

    First you need a C64 board which shows an error with the mentioned RAM test. Then you can try to measure.

    I have a 16 channel logic analyzer and two 4 channel scopes. I'll check my breadbins for Fairchild or Fujitsu chips and move them to the 466 board. I'll take some measurements. It would be good to find the root cause

    First you need a C64 board which shows an error with the mentioned RAM test. Then you can try to measure.

    I have only one 466 motherboard. I changed the mux chips to Fairchild brand. I took the chips from an older breadbin model. As I wrote above the unit runs fine.

    Do you have any known failing mux chips you could share? It would be good to measure this properly. The circuits with OR-gate or inverter are equivalent in logic. But other got problems and other does not.

    Did you route CAS or RAS through the inverter?


    I measured the voltage levels:

    466 469
    CAS 3,3V 4,9V
    RAS 3,7V 3,4V

    This should not be a problem since the logics are TTL level as is the ISSI brand SRAM chip.

    I guess I'll give free 74HCT257 chips for those who order my SRAM boards for 250466 with Fairchild or Fujitsu chips. And change the OR gate for the next batch .


    I have a 16 channel logic analyzer and two 4 channel scopes. I'll check my breadbins for Fairchild or Fujitsu chips and move them to the 466 board. I'll take some measurements. It would be good to find the root cause

    As Gerrit said. You can freely mix the address pins.


    The board got only one capasitor near the end of the 5V line. It is wrong value in the schematic. Just used some with the same case. I'll fix that. There are two capasitors for the original DRAM chips on the C64 motherboard. 5V line to SRAM 5V pin is very short.


    My 250466 MB got a trianle logo on the 74LS257 chips. Like your chips on top of a row. What are they brand?
    Convert C64 to SRAM

    Notice the signal level of RAS and CAS. One of them was quite low (RAS IIRC) so HC series OR gate may cause trouble. I selected AHCT because of speed and TTL levels. The SRAM got TTL level inputs.


    I'll test my 466 board again if I can find any instability with an OR gate. Maybe even swapping the 74LS257 chips. I only tested with VSP Lab.


    Notice you can freely mix SRAM address pins together and data pins together for an easier layout. Same for latch inputs. I enabled pin swap for those signals when doing layout. Mixed signals can bee seen in the schematic.

    My PCB:

    My schematic:

    I got a link to this thread at a FB group. I made an SRAM board for 466 & 469. It is based on gerrits schematic.

    I also had a plan to make one for older models. Luckily the DRAM chips are in the same grid in all 8 chip models.