An experimental attempt of building something like a 6521\6821
with 74XX TTL chips.
The design might have some issues,
try reading the schematics at your own risk...
I\O and bus connectors are supposed to be compatible
to Daryl Rictor's SBC-2.
Test setup:
Bottom: my DRC2 SBC, which is supposed to be compatible to the SBC-2.
2B22 bus interface from the X22 project (6522) is plugged into DRC2.
X21 backplane is plugged into the 2B22 bus interface.
Plugged into the X21 backplane, we have:
1* 1PA22, Port A module
1* 1PB22, Port B module
2* H21, handshake module
X21 backplane:
Jumper\switch in the upper left area is for enabling the blinkenlights.
Jumpers in the lower left area are for connecting the Port A, Port B
related interrupt outputs to either NMI or IRQ.
Jumpers in the lower middle area are for generating handshake signals
which the original 6521\6821 is not supposed to have,
the default setting for those two jumpers is OPEN.
2B22 bus interface
The jumper field in the lower right area is for the chip select.
The bus interface is from the X22 project (6522),
what means that it decodes A0..A3.
Unlike the 6522, the 6521\6821 doesn't have 16 registers,
so for better compatibility it's possible to deactivate
the unused address lines with the three jumpers
in the lower left area of the PCB.
1PA22 Port A module
Port A has open collector outputs.
There is a pullup resistor network for the port lines
in the schematic and layout of the port lines.
I suggest to solder a single row precision socket into the PCB,
to make sure that you are able to try different resistor values,
or to go without pullup resistors if necessary.
1PB22 Port B module
Port B has three state outputs.
There is a pullup resistor network for the port lines
in the schematic and layout of the port lines.
I suggest to solder a single row precision socket into the PCB,
to make sure that you are able to try different resistor values,
or to go without pullup resistors if necessary.
BTW: Port A module and Port B module have identical pinouts.
H21 handshake module
Two identical handshake modules required.
One handles CA1 and CA2, the other handles CB1 and CB2.
(c) Dieter Mueller 2015