SBC6120 RBC Edition

SBC6120 RBC Edition

In the late 90s, Robert Armstrong of Spare Time Gizmos designed a single board computer using the Harris HD-6120 CMOS “PDP-8 on a chip”.

Bob no longer offers new SBC6120 kits or boards, be he has made the design documentation, source code, and design files for the SBC6120 available for others to use at the Spare Time Gizmos SBC6120 Page with the following license:

All SBC6120 files are Copyright (C) 2001-2003 by Spare Time Gizmos.


The SBC6120 Model 2 is a conventional single board computer with the typical complement of EPROM, RAM, a RS232 serial port, an IDE disk interface, and an optional non-volatile RAM disk memory card. What makes it unique is that the CPU is the Harris HD-6120 PDP-8 on a chip. The 6120 is the second generation of single chip PDP-8 compatible microprocessors and was used in Digital’s DECmate-I, II, III and III+ “personal” computers.



The SBC6120 can run all standard DEC paper tape software, such as FOCAL-69, with no changes. Simply use the ROM firmware on the SBC6120 to download FOCAL69.BIN from a PC connected to the console port (or use a real ASR-33 and read the real FOCAL-69 paper tape, if you’re so inclined!), start at 2008, and you’re running.

OS/278, OS/78 and, yes – OS/8 V3D or V3S – can all be booted on the SBC6120 using either RAM disk or IDE disk as mass storage devices. Since the console interface in the SBC6120 is KL8E compatible and does not use a HD-6121, there is no particular need to use OS/278 and real OS/8 V3D runs perfectly well.




The features of the HD-6120 CPU include:

A completely PDP-8/E compatible instruction set
Built in KM8E compatible memory management
A separate 32KW control panel memory for a bootstrap/monitor
To this the SBC6120 adds:

64K twelve bit words of RAM – 32KW for panel memory and 32KW for conventional memory.
32KW of EPROM which contains the BTS6120 firmware.
An optional RAM disk card containing up to 2Mb of eight bit of battery backed up, non-volatile SRAM.
A memory management system that controls the mapping of RAM, EPROM and RAM disk into the panel memory space.
A real, straight-8 compatible console terminal interface. The logic for this interface is implemented in a GAL – no 6121 is used and no software emulation is required.
An IDE/ATA disk interface.
A four LED display used to show POST error codes.


Harris HD-6120 CMOS “PDP-8 on a chip

Tested and working great, Boot screen came up first time.

Playing Adventure in OS/8

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