The two-drive mod.

[jmetal88]

I'm trying to add a second floppy drive to my PCjr, following the documentation in the PCjr Club Archive, and I'm having limited success. The controller does seem to be sending the correct select signals to each drive (i.e. when I type A:, the drive A LED lights up and when I type B:, the drive B LED lights up), but when I type B:, the drive motors do not spin up (I understand that with this mod, both drive motors are supposed to spin when either drive is accessed, correct?). When I type A:, both drive motors spin up as they should.

Looking at the datasheets for the various chips and the description of the connections to be made, the only part of the modification I don't understand is the connection of the 74ls175 pin 5 to pin 7 of the FDC chip. Can someone explain to me why a flip-flop would need to be connected to bit 2 of the FDC's data bus? Might this be a mistake in the write up? If it is a mistake, where should pin 5 on the on the 74ls175 be connected? If it's not a mistake, then I'm not sure where my mod is going wrong. I've triple-checked everything, and since part of the mod works, I don't think my chips are bad.

[jmetal88]

Alright, the pin 7 thing is right, I guess.

I found the problem. I didn't have a clean separation on pin 13 of the 74ls04. Unfortunately, while trying to complete the separation, I broke off the pin entirely.

I figured out a way to get around it though - I utilized the third portion of the 7410 as an inverter by tying together all the inputs, and I moved the motor enable from the 74ls04 to the 7410. I now have two drives!

[Chromedome45]

Where could I find the link for the 2 drive mod?

[Brutman]

There are a few of them out there. The original modification is known as "The Crumley" mod, and I think it has the unfortunately side-effect of spinning both drive motors at the same time no matter which drive is in use. There was a good modification written up in Home Computer Magazine. I think I have scans of that, but I'm not sure.

Does anybody else have the HCM article available? I'll post it if we get a clean copy. (There seems to be more interest lately.)

[jmetal88]

I must've done the Crumley mod, as I definitely have both drive motors spinning.

It doesn't bother me too much, though.

EDIT: The modification I did was described in a text file from the PCjr Club Archive. I'll attach it.

Actually, yeah, Crumley is credited in the text file.

[Chromedome45]

I was actually looking for the HCM mod. I saw it somewhere but forgot to bookmark it.

[southernbob]

This is the only thing I can find that I think pertains to this topic, but isn't much.
I think maybe "Chuck" may have put the schematic he developed on the old
FTP site that Brian/Mike started. If I recall correctly (not a sure thing) that
Brian also had something about it on the old Forum.

Well it won't let me upload the file as I get this message:


"The upload was rejected because the uploaded file was identified as a possible attack vector."

I don't know what that means so no go... Sorry, but it wasn't much anyway.
Bob

[jmetal88]

This is the only thing I can find that I think pertains to this topic, but isn't much.
I think maybe "Chuck" may have put the schematic he developed on the old
FTP site that Brian/Mike started. If I recall correctly (not a sure thing) that
Brian also had something about it on the old Forum.

Well it won't let me upload the file as I get this message:


"The upload was rejected because the uploaded file was identified as a possible attack vector."

I don't know what that means so no go... Sorry, but it wasn't much anyway.
Bob

Maybe ZIP it first?

[Brutman]

Try this:

http://brutman.com/PCjr/docs/Home_Compu ... py_Mod.pdf