View Full Version : Cromemco Z-2 Update

February 26th, 2010, 07:48 PM
My replacement transistors came in to rebuild the active terminator section of my Wunderbuss motherboard, so today after work I got a chance to test my ZPU board a little further. The first thing I discovered was a complete lack of clock on pin 49 of the S100 bus (the 2 MHz clock line)...I found the ZPU schematic in the Harte archive, and printed it off on a tabloid sheet (we've got a very big laser printer at work, specifically for making large schematics).

After discovering two dead 7474 D-type flip-flops and a 7404 inverter with a bad section, I got the clock generator working, including the 2/4 MHz select switch. The address bus seemed to be stepping fine, and the control signals typical of a free-running Z80 were found on the S100 bus. My copy of Dave Bursky's "S-100 Bus Handbook" came in today, so I decided to wire up a board for testing the ZPU's functionality a little further. I recovered a S100 prototyping board made by Vector from our pile of old protoboards at work (it had to be stripped of its old components first). Rather than going right into a RAM/ROM board, I wired up a 2N2222 transistor and LED from the SHALT line (pin 48), and a DIP switch between ground and 8 pull-up resistors to the DATA IN lines. Pictures below...


Setting the DIP switch to 0x76 lights the halt LED on reset! Setting the DIP switch to something other than HALT, resetting, and then re-keying HALT causes the LED to light when HALT is finally set. It seems the Z80 is capable of pulling data from the DATA IN bus! Now, it's time to add an EPROM to the board.

February 26th, 2010, 10:38 PM
Address and Data In buffers added: three 74ALS245 octal buffers. I also added a 74ALS04 for decoding SMEMR and SOUT for talking to a 2716 EPROM and a DL2416 ASCII character display. I used an EPROM I'd burned for my 8085 single board project to test the board, which starts at 0x0000 and writes "HELO" character by character to output ports 0-3. The DL2416 display is addressed backwards from the PDSP-1881 in my 8085 project, so "HELO" came out as "OLEH"...still, it shows that both data buses work, the address bus works, and several of the status lines work. It seems chances are good that I've got a fully working ZPU!


The test program terminates with a HALT, so the halt LED lights as well.