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From: mattw@phred.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: Why 4.77MHz on clones?
Message-ID: <1023@phred.UUCP>
Date: Thu, 29-Jan-87 10:40:46 EST
Article-I.D.: phred.1023
Posted: Thu Jan 29 10:40:46 1987
Date-Received: Sat, 31-Jan-87 07:22:14 EST
References: <1292@ho95e.ATT.COM> <5426@amdahl.UUCP>
Reply-To: mattw@phred.UUCP (matt)
Distribution: na
Organization: Physio Control Corp., Seattle, WA
Lines: 16

Some of the high speed xt clones 7.16, 8.00 Mhz will not format or
'diskcopy' at the high speed. This is due to many factors including
the ram speed and DMA channel circuit topology. Some of the clones
change the speed of the clock to the DMA circuit during high speed
clock operation ie: if the speed of the CLK88 line is 8Mhz the DMA
clock is lowered to 4Mhz. This will allow the clone to format diskettes
at the high speed. Another problem with some of the 'Turbo' clones is
that the high speed design does not preserve the 14.x Mhz clock for
the proper operation of the CGA video system (monochrome will continue
to work fine) and the interrupt for the real time clock, tone, and
keyboard scan circuits. The result of this is that the CGA card will
not work and the real time clock will not keep time at the high speed.
There is a plethora of xt 'TURBO' clones on the market. Some are
correctly implemented others are not. I would not buy another without
throughly analysing the high speed operation. Good luck on your 
purchase. <8^)