SBC-2 Construction

First and foremost, I offer this board "AS-IS", with no
guarantee that it is without defects or flaws. So, use
it at your own risk! Having said that, I plan to buy at
least two myself, and have worked very hard to ensure
everything is covered. I will help with whatever
"problems" arise, but do not promise anything!

First, see the parts placement picture for component locations and values.

The power busses are at least .030 wide and are rated for 1 amp (according to ExpressPCB), be careful not to overload them or you may melt traces.   Be especially careful to avoid shorts or excessive loads from your devices plugged into the IO ports.

If you look closely at the board layout, you'll see several extra pads, especially around the 65C02.   I have tried to make the board as versatile as possible.   Note especially that if the 65C02 Pin 1 is to be grounded, you'll need to install the strap to the left of the TTL Osc.   This allows for use of the W65SC02S, which has a VPB output on pin 1. Also, there are pads for MLB, RDY, SYNC, BE, & SOB.   Hopefully these will allow for special uses.

I have made the IO completely strap-selective.   You can select/deselect both 6522s and the 6551.   You can connect their IRQ outputs to NMI, IRQ, or leave them open.   I've added pads for diodes in the IRQ's of all three IO chips. If you don't need to use a diode, you can simply solder a strap across the pads.   The diodes would be needed for totem-pole IRQ's, which cannot be wire-OR'ed together.

For the 6551, you can choose between the DTR/DSR, RTS/CTS, & DCD handshake connections to the MAX-232.   You can also select the appropriate pins from the MAX232 to the 10 pin RS-232 connector.   See the "options.gif" file for stapping examples.   Use a 22pF capacitor next to the 1.8432MHz crystal.

The four 6522 Ports are wired for power, gnd, 8 bits + 2 handshake.   I was unable to re-wire the power as to prevent damage by reverse connection, but I did add one pad to the right of the +5 pins, so if you have room, you can use a 16 pin socket and key the hole to prevent reversals.

RP-2 is optional and is only needed if you want to be able to leave a 6522 plugged in but want to disable it by removing the CS strap.   RP-2 would provide the necessary pull-up for the CS pins.

I've also added straps to support both 28256 EEPROM and 27256 EPROM.   See "options2.gif" for strapping examples.

Here is the parts list:

C1	10uF
C2	4.7uF
C3	4.7uF
C4	10uF
C5	10uF
C6	10uF
C7	22pF
C8	.1uF
C9	.1uF
C10	.1uF
C11	.1uF
C12	10uF
C13	.1uF
C14	.1uF
C15	.1uF
C16	.1uF
IC1	W65C02S
IC2	28256 EEPROM
IC3	62256 Static RAM
IC4	74LS30
IC5	74LS00
IC6	74LS138
IC7	65C51
IC8	MAX232
IC9	65C22
IC10	65C22
IC11	DS1813 or equivelant
J1	ROM OPTS - 2x3 pin header
J2	SER EN - 1x2 pin header
J3	D3 (Switching Diode or strap)
J4	INT SEL3 - 1x3 pin header
J5	VIA1 EN - 1x2 pin header
J6	D1 (Switching Diode or strap)
J7	INT SEL1 - 1x3 pin header
J8	VIA2 EN - 1x2 pin header
J9	D2 (Switching Diode or strap)
J10	INT SEL2 - 1x3 pin header
J11	POWER IN (power inputs - user preference for part)
J12	Expansion Port - 2x25 pin header
JP1	Serial Port - 2x5 pin header
OPT1	2x5 pin header	
OPT2	(2x3 & 1x3 headers)
OSC-1	1.0 MHz TTL Osc
P-1	2x7 pin header
P-2	2x7 pin header
P-3	2x7 pin header
P-4	2x7 pin header
RP-1	3.3k Ohm (5 res SIP with common)
RP-2	3.3k Ohm (3 res SIP with common)
SW1	RESET switch inputs
T1	LM78L05
XTAL	1.8432MHz crystal

All info provided "as-is" and is Copyright 2020.