SUMMARY: ab switch problem.

Mark Andrew Nassy (
Wed, 1 Oct 1997 20:42:13 -0400

hardware and software:

manual ab switch connected to the only printer port of an ibm clone running
windows 95. connectix quickcam and a cannon color printer connected to the
ab switch.

the problem:

error message when cornell cuseeme tries to access quickcam. quickcam and
the printer work fine if individually connected directly to the printer

the cause:

since the printer and the camera work fine with the port this suggests the
problem lies with the ab switch. in order for quickcam or the printer to
work it is necessary for the ab switch to switch all 25 wires and not just
those necessary for the printer to work. The reason for this is because the
newer printers with bidirectional drivers must read and write to the
printer port to get the port status. should not all the wires get switched
windows95 will have problems scanning the parallel port.

the solution:

if you have a free isa slot in your computer purchase:

(1) ecp/epp/bidirectional parallel port that uses IRQ 5 or 7, for
$20.00+us, or

(2) ecp/epp/bidirectional parallel port with a 83 byte buffer that uses
IRQ's 5, 7, and 9 through 15 for $40.00+us, or

(3) i/o port combo. card for $25.00us and install any of these in a free
isa slot.

the printer should then b connected to lpt2 and quickcam to lpt1. (im not
sure y it is recommended to connect quickcam to lpt1 and not lpt2 however.)

if you dont have a free isa slot in your computer purchase

(1) an electronic ab switch. the cost of an electronic ab switch is
approximately $40.00+us.


when installing the new card in a free isa slot there is the potential for
an irq conflict. that is, u may disable the already installed lpt port or
disable (one of) the port(s) on the new card. the solution to this is to
buy a more expensive card with multiple printer port addresses.

there is the potential for damaging printers connected to manual ab
switches by shorting out something if all the pins in the ab switch dont
engage or disengage simultaneously. manual ab switches void the warantee on
some printers.

electronic ab switches dont have any moving parts so they cant short out
any device (in this case a printer and camera) connected to the switch.

Thanks to all those who replied.

[ source:

Randy Smothers <>,
"Bill Woodland (Squeek)" <>,
"Tom Prugh",
Rolf Hemmerling <>,
"John W.Osborne" <>,
Jason Williams <>,
"Kevin McCluskey",
"Mark L. Bardenwerper, Sr." <>. ]

Mark Andrew Nassy <>