MASSBUS disk emulator (Was: Unibus controller for MFM disks)

Rich Alderson RichA at
Tue Oct 18 13:26:06 CDT 2016

From: Noel Chiappa
Sent: Monday, October 17, 2016 7:21 PM

>> From: Ian S. King

>>> Does anyone know anything about the status of the plans to open-source
>>> it?

>> I've made some inquries, stay tuned....

> OK, thanks!

>>> Can you briefly describe what it was? 

> I'm still curious about what it was: was it a stand-alone board with a
> separate power supply, or did it plug into some sort of backplane? Did it
> use something like SD cards for storage? And what was the MASSBUS
> connection like: a set of 3 Berg headers into which one plugged the flat
> cables, or was there some oddball connector that wound up connected to a
> standard MASSBUS connector?

There have been 2 generations of Massbus Disk Emulator (MDE) at LCM.  The
one of which people have seen pictures was the first generation, created
when there were only 2 people working on the collection which became the
museum years later.

This was a breadboarded prototype which supported 4 emulated RP06 disks,
represented by directories of track files, each representing 20 sectors of
128 36-bit words.  These mimicked the physical layout of the data on disk,
where each 18 bits of data has an associated parity bit; this was stored as
3 8-bit bytes (3rd byte being 2 data + overall parity).  The MDE itself was
made up of a Rabbit and 3 PICs; the driver/receiver portion of the board
terminated the Massbus.  Standard Massbus cables connected it to the 2065
running Tops-10.  Data was transferred via FTP over a 100baseT crossover
cable connected to a Slackware server; the Rabbit was able to keep up with
4 drives at this speed, but a 5th drive (MDE could in theory represent an
entire 8-drive string) would cause data dropouts on all units.

Various improvements were attempted on this (work with Unibus on a 2020, for
example), but it was never quite stable enough.

MDE 2.0 was created by a different engineer with lots of resources thrown at
him.  He replaced the Rabbit and PICs with a Mesa 5i22 Anything I/O card
(includes a Xilinx Spartan-III FPGA) that plugs directly into the PCI bus in
a server-class X86-64 box, and used a revision of a separate driver/receiver
card designed for MDE 1.0 to connect to the Massbus, still via standard
cables.  An outside contractor wrote a control program for the PC side which
runs under Windows 2008/2012 Server.

MDE 2.0 supports SimH-format RP07 and RP06 disk drive images.  There is also
a variant for tape drive emulation called the MTE, which reads and writes to
SimH-format tape files.

We currently run the DECSYSTEM-2065 (public Tops-10 v7.04), DECsystem-1070
(public Tops-10 v6.03A), PDP-11/70 (public Unix v7), and DECSYSTEM-1095
(under development: WAITS operating system from Stanford Artificial
Intelligence Laboratory) using MDEs.  The two KL-10 systems also have an MTE
apiece; we haven't needed tape service on the KI-10 or the 11/70 so far.


Rich Alderson
Vintage Computing Sr. Systems Engineer
Living Computer Museum
2245 1st Avenue S
Seattle, WA 98134

mailto:RichA at

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