IBM PC printer adapter schematic

Chuck Guzis cclist at
Sat Apr 7 00:00:06 CDT 2007

On 6 Apr 2007 at 17:13, Ethan Dicks wrote:

> That's handy to know - every once in a while, I long for a machine
> with 2 or 3 parallel adapters, for things like old Connectix cameras,
> LCD displays and the like.  I've never assembled an ISA machine with
> more than one printer adapter.  I know that it really doesn't matter
> to your own code what the I/O address is, but I wasn't sure how
> flexible the underlying hardware might be to being assigned to
> alternate addresses.

2P+2S ISA adapters aren't particularly difficult to find.  Some of 
them will even support EPP and ECP modes.  I seem to recall that 
there were 3 "stock" port addresses (3BCh, 378h, 278h), but others 
were possible, because, in a moment of sanity, IBM decided to 
dedicate some memory at 40:8 et seq. to actually record the base port 
addresses of the parallel port adapters one had. 

 So, rather than compiling the possible port addresses into one's 
application, it was only necessary to look at low memory.  For 
example, if one had a parallel port adapter set for 378h and an MDA 
at 3BCh, DOS and the BIOS would normally assign LPT1:  to 3BCh and 
LPT2: to 378h.  However, if an application exchanged the two words at 
40:0008 and 40:000A, the reverse mapping would hold.


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