Minimal CP-M SBC design

Allison ajp166 at
Sun May 11 06:24:28 CDT 2008

>Subject: RE: Minimal CP-M SBC design
>   From: "Andrew Lynch" <lynchaj at>
>   Date: Sat, 10 May 2008 14:43:26 -0400
>     To: <cctalk at>
>> Date: Sat, 10 May 2008 12:29:24 -0400
>> From: Dave McGuire 
>>    I dunno Chuck...the only reason more CP/M systems weren't ROM- 
>> resident back in the day was due to convention, not technical  
>> restrictions.  I (personally) don't think there's anything  
>> non-"period" about ROM-ing CP/M.
>It's not the ROM-ing of CP/M that disturbs me, but rather the 
>"disklessness" of the thing.  Wasn't the whole idea of CP/M 
>originally to give you something to manage files on your floppy 
>drives?  I mean, that's what the bulk of the code in CP/M is for--
>heaven knows, the support for other I/O is nothing to write home 
>If one wants to enjoy a "vintage" experience, what sense is there in 
>being diskless?  At any rate, even something as simple as a WD1770-
>type controller added to the design would give that capability with a 
>minimum of support "glue".  
>Alternatively, one could stay diskless and add a sound-effects module 
>to emulate the "chunk" and "grrr" of a head-load and seek--and the 
>"thunk-click" of a drive door being opened and a floppy inserted.  
>I still don't have the hang of this "vintage" thing yet, probably 
>because I'm vintage myself.  Please forgive my density...
>Hi Chuck,
>I hear what you are saying and agree there is something just very
>disconcerting about diskless CP/M computers.  However, CP/M in the CBIOS is
>really just about block devices and the OS really could care less whether
>you attach a 8" SSSD floppy, a CF drive or a ROM.  It is all the same to the
>My goal here is to *eventually* allow expansion to include IDE and floppy
>drives.  As a matter of fact, the CBIOS does support IDE hard disks already
>but requires the interface IO card and the ECB backplane to attach it to the
>SBC.  I have an IDE hard disk with CP/M format and some programs on it.
>My goal with the SBC using ROM/RAM drives was to allow something minimal to
>operate as a SBC and have some functionality with the option to expand to as
>desired.  I am trying for a modular, low cost approach with easy to build
>Assuming I get this SBC respun and into manufacturing my next project is to
>redo my ECB backplane as a PCB.  After that will be the disk IO board and
>bus debuggers which are also made from prototype boards.
>My Test Prototype home brew computer was built entirely with prototype
>boards and point to point wiring.  It supported IDE drives and even had a
>NEC765 FDC circuit built in.  I wrote some software but never got around to
>test the FDC part since the machine started experiencing reliability
>problems which I think trace back to poor grounding and power distribution
>issues.  The new PCB SBC version seems much more solid than the prototype

You used it as it's one of the few you can still buy.

I happen to use that chip as I have them and was even supporting them back
years ago.  But you know adding that chip with it's support nearly doubles
the chip count of a minimal CP/M engine.

A few areas I watch for.  Sockets do not help reliability.  Ground is never 
ground enough. Bypass everything.


>The SBC is something which works but gives only limited functionality.  If
>that is enough, people can stop there.  If they want more they can plug it
>into the ECB backplane and add peripheral cards.  So far only two peripheral
>cards exist; the disk IO card and the bus debugger. Hopefully more in the
>future.  I have some ideas kicking around in my head but am concentrating on
>the SBC for now.
>Thanks and have a nice weekend!
>Andrew Lynch

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