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I tried installing the 53C80-40 into the machine and again and the same thing. The machine just shows checkerboard. so the 53C80-40 is not compatible with the 53C80 that came in the classic. Im at a loss unless I can get a working 53C80.

Makes sense from what I am seeing. I would say its compatible. So I dont know where to find a working 53C80 to fix my classic at this point and I am already out the cost of the 53C80-40 and no better off :(

So Going by your image I have traced continuity from pins 8 to11 and 17 to 19. Only one doesnt have continuity Pin 18 of the scsi chip should be going to pin 8 of the 680000 cpu. I am getting NO coninuity. Could you please check that your get it on your board? I am getting continuity from pin...

Might not be compatible after all: https://68kmla.org/bb/index.php?threads/macintosh-classic-stalls-booting-when-anything-is-plugged-into-the-scsi-bus.39144/#post-423125

I figured the same on the voltages. Not low enough to be a problem. I opened another thread asking about the SCSI chip: https://68kmla.org/bb/index.php?threads/looking-to-replace-my-onboard-scsi-chip-and-have-a-question.39143/ I ordered the -40 variant and it came last night. I put it in today...

So I am getting 4.87 without a load and 4.83 with a load on the 5v rail and 11.66 without a load and 11.61 with a load on the 12v rail.

Just curious if there are any other components besides the SCSI chip which could be involved in this?

ok ill clean the contacts with some deoxit. i have a new scsi chip on order. the board was already clean when it was recspped but i can try cleabing the scsi connector better.

Just to be thorough here are all the voltages without and with a scsi device:

Ok Ill check the rest as well. Does it matter if I have a device connected or not?

Well I dont know what it could possibly be. Like I said this thing will just wait forever and thats not normal behavior

Ok here are the voltages. First I took them with no SCSI devices in the machines. PIN1 /REQ gave 0.11 VDC Pin 5 /ACK gave 0.18 VCD and pin 25 TRMPWR gave 4.5 VDC Next I tried an internal drive connected to the internal SCSI connector. With pin 1 /REQ I got 2.19 VDC with pin 5 /ACK I got 2.2...

Yes I am aware about the termination and there was termination set on the drives. I will post back with the pin voltages.

Thanks YMK I will test it tomorrow and report back what I find.

Two working hard drives, 6 working blue SCSI devices, and two working external SCSI2SD devices that all work fine on other system. Its definitely not the devices or the cables because they work on other machines. And I guess the more information the better right. This system was already...

Thank you guys. I was pretty confident they were compatible I just wanted to hear from someone else.

I have a MAC classic board which will boot and work fine from a floppy drive but the second I connect a hard drive via internal ribbon or external connector the system just sits forever without progressing to even the floppy drive. The mouse cursor still works, its just perpetually waiting it...

I am looking to replace my onboard SCSI chip on my Macintosh Classic board. The chip it came with is this one NCR 53C80 But when I look online all I can seem to find are NCR 53C80-40. What does the -40 specify? They both appear to be 44 pin IC's.

I think the black thing at the top is just a cover that snaps on. Most IDC connectors dont come with those. Anyway I never had any luck making my own cables with IDC ends. The wires just dont stay put. That is why I assume the right tool for the job is needed.

IDC connectors?