For the most part, no, you should not need those cards. If you look carefully at the module utilization chart, you should see little notations indicating what options these cards are for - look right under the module designation on the sheet. Some pointers. First, CHECK THE LAMP. Lamps are *highly* failure prone. Swap with another. Cards that offhand might affect memory address register bit 5: MA 05 / MB 05: AB5 (M220) Input Bus 05: A32 (M516) AC 05: D29 (M906) You might consider swapping with cards immediately next to these to see if you can make the problem move. Diagnosing PDP-8/L's with an oscilloscope is relatively straightforward, for the most part. Enjoy! A15 (M162) is for Memory Parity option B15 (M119) ditto A16 (M162) ditto B16 (M162) ditto B34 (M903) is for bus extension to an expansion chassis B35 (M903) ditto B36 (M903) ditto D10 (M216) is for data break option D11 (M113) ditto C16 (M111) is for memory parity option D34 (M903) is for bus extension to an expansion chassis C35 (M903) ditto D35 (M903) ditto C36 (M903) ditto D36 (M903) is also for bus expansion BUT: The documentation I have indicates that if you have no expansion box, you should have an M902YB terminator in slot D36. (Since the signals there are only for I/O, it should not be causing the problem you are seeing). At 11:43 AM 11/29/98 -0500, you wrote: --- Jay R. Jaeger The Computer Collection Jay.Jaeger(a)msn.fullfeed.com visit http://www.msn.fullfeed.com/~cube