glitch
Veteran Member
Full writeup here: http://www.glitchwrks.com/2022/06/22/6507-sbc
BillO on the 6502 forums traded me a blank board for his then-current OMS-02 6507 SBC back in 2011. He was interested in building one of my 8085 SBCs and I just happened to have a spare board for the 8085 SBC rev 2 on hand, at that time. He provided the CPU, ROM, and an ACIA with the bare board. Here's what it looked like when I originally built it up, in 2011:
I recently decided to do some rework on it to fix up a few things about the original build I wasn't satisfied with. Here's its current state:
Most notably, the huge orange Sprague electrolytic has been replaced with something shorter
I also had to change the 0.1uF caps around the MAX232: the one I used in 2011 was fine with 0.1uF, but I stole that one for another project at some point, and the replacement didn't like caps that small. I stuck in some 0.33uF radial ceramics I had on hand, which seem to be fine. I also installed a shrouded expansion header with latches (displaced a bypass cap, which is on the back now), and socketed a 74LS138 used for memory decode. The socket is for future hacking. Added a C&K pushbutton with 0.1" lead spacing for reset, just happened to have it in the junk drawer.
I ported GWMON-65 to the 6507 SBC and couldn't get it going. It wouldn't sign on, and debugging was getting me nowhere. I'd sometimes get a few garbage bytes out of the serial port. I was ready to shelve it for a little bit but decided to test some solder-pull Rockwell R6551s I'd scavenged. To my surprise, GWMON-65 came up immediately! I thought about it a while and figured maybe the reset pulse from the unbuffered cap-and-resistor reset circuit was holding the CMOS RCA CDP65C51A in reset longer than the NMOS R6551. Sure enough, that was the problem, something about the speed or shape of the reset pulse caused it to hang in reset longer than the CPU. I hacked in a DS1233 EconoReset:
That fixed it up, and now GWMON-65 works just fine with the CDP65C51A! I'm planning on doing some hacking/expansion on this SBC in the near future, but figured I'd post about it before I started modifying anything.
BillO on the 6502 forums traded me a blank board for his then-current OMS-02 6507 SBC back in 2011. He was interested in building one of my 8085 SBCs and I just happened to have a spare board for the 8085 SBC rev 2 on hand, at that time. He provided the CPU, ROM, and an ACIA with the bare board. Here's what it looked like when I originally built it up, in 2011:
I recently decided to do some rework on it to fix up a few things about the original build I wasn't satisfied with. Here's its current state:
Most notably, the huge orange Sprague electrolytic has been replaced with something shorter
I also had to change the 0.1uF caps around the MAX232: the one I used in 2011 was fine with 0.1uF, but I stole that one for another project at some point, and the replacement didn't like caps that small. I stuck in some 0.33uF radial ceramics I had on hand, which seem to be fine. I also installed a shrouded expansion header with latches (displaced a bypass cap, which is on the back now), and socketed a 74LS138 used for memory decode. The socket is for future hacking. Added a C&K pushbutton with 0.1" lead spacing for reset, just happened to have it in the junk drawer.I ported GWMON-65 to the 6507 SBC and couldn't get it going. It wouldn't sign on, and debugging was getting me nowhere. I'd sometimes get a few garbage bytes out of the serial port. I was ready to shelve it for a little bit but decided to test some solder-pull Rockwell R6551s I'd scavenged. To my surprise, GWMON-65 came up immediately! I thought about it a while and figured maybe the reset pulse from the unbuffered cap-and-resistor reset circuit was holding the CMOS RCA CDP65C51A in reset longer than the NMOS R6551. Sure enough, that was the problem, something about the speed or shape of the reset pulse caused it to hang in reset longer than the CPU. I hacked in a DS1233 EconoReset:
That fixed it up, and now GWMON-65 works just fine with the CDP65C51A! I'm planning on doing some hacking/expansion on this SBC in the near future, but figured I'd post about it before I started modifying anything.