Run, Forrest, run!

With power, clock and reset, we have everthing in place to let a Z80 run!

SMT first, then sockets, then ICs

I really recommend socketing parts wherever possible, and with the cost of components, a few cents more for quality sockets should be in the budget, even if "low-cost" sockets are more authentic, because that's what Coleco used. So, let's start with a few pullups and other components the CPU needs for running:

Then solder

• R4, R5, R6, R18, R31 3k3 resistor

• R7 1k

• U11 74LS04D (SO14)

• U12 74LS138D (SO16)

• U14 74LS02D (SO14)

• C89 22uF/16V tantalum (thru-hole)

Note: U14 is oriented upside down!

Set these jumpers:

• SJ3 - this enables the BIOS since there is no ADAM memory config register (yet)

• SJ4 - this maps RAM only to the top 8K from $6000-$7FFF

• SJ7 - this maps BIOS only to the bottom 8K from $0-$1FFF

• Set SJ8 to 1-2 (short to VCC) - this mirrors the 8K RAM as 8x1K for CV compatibility

• SJ9 - needed as long as U41 is not populated (IO-address decoder)

• SJ10 - needed as long as U45/47 are not populated (IO-address decoder)

• SJ17 - needed if the 64K RAM circuitry is not populated

Note: except for SJ8 (which is near the power switch) all jumpers can be seen in the picture above!

To RUN - you need ROM

Now burn an EPROM or EEPROM with the CV boot ROM you are going to use. If the EEPROM is larger than 8K, program the boot ROM image to the first 8K of the EPROM and leave the rest at $FF.

• If using a 27C64 EPROM or 28C64 EEPROM: set jumpers SJ1 and SJ2 to 8K

• If using a 27C256 EPROM: set jumpers SJ1 and SJ2 to 32K

Now power up unit. Verify reset on pin U1 pin 26 and 3.57MHz clock on U1 pin 6. Verify 5V on U1 pin 11. Verify 5V on U3 pin 28. Turn unit off.

It's time to solder the first sockets

Note: If the board is supposed to go into an original ColecoVision case, the CPU socket pins 19/20 (and possibly pin 1) must be cut flush with the circuit board before soldering to prevent interference with two plastic support pins (see white circles on bottom of circuit board)!

Now, finally, solder the 40 pin socket for U1 and a 28 pin socket for U2 and insert the Z80 CPU and BIOS.

As always: testing!

To be able to test, we need to temporarily install the 10k pullup resistor for R65. This is the NMI pullup that's built into the VDP, and since that's not on board yet, we need to pull this one high.

Again, power up unit. Using a scope, verify activity on U1 data and lower address bus lines as well as MREQ and RD, WR. (Note: without RAM, the Z80 runs only garbage code, so don't worry!).

With a (modern) Z80C400 and CMOS 27C256 my setup now draws about 30mA @ 12V supply.

Finally: RAM!

Before you can install ram, you need to install

• U48 74LS32D (SO14)

because this one sits below the RAM chip. You also need to decide whether or not you'd like to install 64K later on, then install a 32 pin socket. Otherwise a 28 pin socket will suffice. If your socket interferes with U48, remove the "stabiliser"! Now install the RAM in the socket:

• U2 62256P DIL28-6

and repeat tests (activity on data, lower address bus and MREQ, RD, WR), then power down. Note: with a modern CMOS RAM my circuit now draws under 35mA @12V.