[sci.electronics] Dual monitors

jim@rwsys.lonestar.org (James Wyatt KA5VJL) (02/09/91)

In article <26762@uflorida.cis.ufl.EDU> jdb@reef.cis.ufl.edu (Brian K. W. Hook) writes:
>
>Someone recently posted that if I have a 16-bit VGA card and an 8-bit
>monochrome card in my system that my VGA will automatically be throttled
>down to 8-bit...this would become an unacceptable bottleneck in my
>IO (I have a 386-33).  If so, is there a way around it?

We use SuperVGA boards for running Windows and add a modified MDA board for
running CodeView (ToadView!) to debug. An unmodified MDA really slows video
access even when you aren't debugging, so we added a switch on the back. If
we had Turbo switches on the front I'd use that and a relay/gate.

The switch disables memory and IO reads to the MDA board, so the BIOS won't
detect it during the POST. Just flip the switch and reset (NOT Ctl-Alt-Del
on most machines) to choose speed or debugging.

To add the enable switch, you'll need:
	1ea, DPDT or DPST (rare) switch (small!) (Radio Shack 275-626)
	2ea, 10K Ohm 1/4W resistors (2/5 of Radio Shack 271-1339)
	2ft, 30ga wire-wrap wire (Radio Shack 278-503 or -501)

1. Hardware work: Remove the mounting bracket from the board. Drill a hole
	near the bottom for the switch (make sure it clears anything near the
	edge of the board) and install the switch in the bracket. Reattach the
	mounting bracket to the MDA board.

2. Cut the traces going to the B12 and B14 contacts on the 62-pin bus
	connector. Wire the switch and two resistors like so:

	        cut trace-\
	                   v
	B12 >-----------\  X /---------*----> to -SMEMR uses
	                 |   |         |   10K
	                 O   O   O     \--/\/\/---*---> +5V
	 [add switch]->                           |
	                 O   O   O     /--\/\/\---/
	                 |   |         |   10K
	B14 >------------/ X  \--------*----> to -IOR uses
	                   ^
	        cut trace-/

3. Install the board and test for proper operation. The resistors are to
	pull-up the memory and IO read lines if %sMOS is used while not
	loading the bus lines too much.

Hope this helps someone else as much as much as it has us. - jim
----
James Wyatt (KA5VJL) - Standard disclaimer applies...          (H)817-595-0571
{letni.lonestar.org,merch.tandy.com}!rwsys.lonestar.org!jim    (W)817-390-2864