NOTE:  On the schematics published in Speaker Builder and downloadable from this site, one connection is missing.  There should be a connection from J5 pin32 to U14 pin 5 (the signal "GO").


A common complaint from IMP owners, usually after they have upgraded their computers, is that the IMP module no longer seems to work.  Listed below are some common causes for this.

1)  (MOST COMMON CAUSE).  The printer port is marginal or incompatible with IMP's 5V CMOS interface.  Parallel printer ports of modern computers (particularly laptop computers) often use lower voltage circuitry, reducing the noise margins and varying the trigger thresholds.  Sometimes this can be cured by installing an add-in parallel printer port card (make sure to configure it for one of the port addresses which IMP supports). Often it can be cured by using a different (usually shorter) printer cable to connect the IMP to the computer's printer port.

2)  Operating system no longer supports DOS programs or direct hardware access. It is not possible to run IMP from within Windows NT, Windows2000 nor WindowsXP.  Not even from within a "DOS" box!

3)  Older IMP software version crashes with faster processors.  This can usually be fixed by installing the final IMP software version 2.05.

4)  The display adaptor of the new computer does not support the "EGA" display mode used by IMP.  There is usually no cure for this except to change to a different video card in the computer, if that is possible

5) A bad or incomplete printer cable.  That happened often!  Just because a printer can print with a given cable doesn't mean that all the wires are present or connected in it -- printers don't usually need all the pins that IMP needs.   
6) A defective printer port -- lines on these seem to fail rather often.

7) A printer port that is not at a standard MSDOS port address (278 or 378 --Hexadecimal-- are the standard ones).  IMP is an old program and only looks for ports that are at these addresses and one other (I forget the third address it can use--sorry).  So if you are trying to run a new computer, set its printer port address to 278h or 378h (usually from BIOS, or in Windows Device Manager).

8) An unusable printer port type.  Type "SPP" (Standard) is best.  But usually "EPP" will work.  "ECP" will almost never work.  This is a very common cause of problems.  You should be able to set your motherboard's printer port type from the BIOS settings at bootup. 

9) Too long a printer cable to control the IMP, causing crosstalk between input and output lines and erratic operation.   Try several different cables before you assume the problem is your IMP or computer.  The price of the cable has almost no positive relationship to usability with IMP (the expensive ones usually work worse because of crosstalk!).