Mike's PCjr Page

I was wondering why the QumeTrak drives I have seen on the PCjr dont have a T-Res. I'm guessing it because of the floppy controller that came with the Jr and that stock Jrs only supported one drive.

The following FDC hardware functions are not implemented or supported by the IBM PCjr Diskette Drive Adapter.
• DMA data transfer
• FDC interrupt
• Drive polling and overlapped seek
• data incoding
• select status bits

I'm you do the mod to the Jr's floppy controller for more drives do you then need to put a T-Res on the appropriate drive like I need to with my XT?

All floppy drives of that era have a place to put a terminating resistor.

Check out the manual for the QumeTrak 142 ... page 20 discusses it.

http://www.primrosebank.net/computers/m ... floppy.pdf

The rules are the same - the last drive in a chain gets the terminating resistor.

Yes all my PCjrs that came with Qumetrak drives didnt have a T-res but one I got 2nd hand came with the T-res. I thought maybe that drive came out of a 5155 and thats why it had the T-res.

I just found it strange that none of the Jrs I have seen dont have a T-res on them when they are the only drive in the machine where as the XTs I have when they only have 1 drive they still seem to require a T-res on that drive.

At least according to http://www.minuszerodegrees.net they should have them

I just spot checked a few of mine .. no terminating resistor in RN1. Now I'm confused.

The tech ref doesn't mention anything about this. But I did find an internal IBM document that provided this explanation:

"On the PCjr diskette adapter, the chips that drive the 34 wire interface are *not* open collector devices. The adapter drives the lines to correct logic high values. (In effect, the pullups are built into the chips.) Therefore, putting a "terminator" on any drive attached to a PCjr diskette adapter is redundant and may cause operational problems by loading the drivers on the adapter excessively. (With the extra pullup, they may not be able to pull the level down to the low TTL level.) Therefore, terminating resistor packs should *never* be used on an internal or external drive connected to the PCjr diskette adapter."

I am totally amazed that I did not catch onto the lack of terminating resistors on PCjr systems all of these years.