Date   : Sat, 11 Jul 2009 08:49:24 +0100
From   : mike@... (Mike Tomlinson)
Subject: Master Compact Drive Cable

In article <4A5779FB.1040701@...>, Pete Turnbull
<pete@...> writes

>One odd thing about the Compact interface is the presence of a jumper 
>called PL3 (normally not fitted) between the DS2 Drive Select line 
>(which isn't normally used, only DS0 and DS1 are) and ground.  I can't 
>remember what that's for!

On the Compact, pins 2 and 3 of the 25-way connector go respectively to
pins 10 and 12 on the disk drive, which are DS0 and DS1.  

pin 2 of the 25-way disk connector is a NAND of the Motor On line and
the FDC select circuitry (IC1). PL3 forces it to 0v, so jumpering PL3
would force a single-drive system, as DS0 would be permanently asserted.
Why would Acorn want to do this though?  To cripple the Compact's
capabilities so it didn't compete with the M128?

pin 3 of the FDC connector is a similar circuit from a different gate on
IC3.  The Master 128 cct is the same.  There's also circuitry for Side
Select on pin 32.  It looks to me as if 1770/2 filesystems assert drive
ID and side selection via memory addressing.

-- 
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