• Please review our updated Terms and Rules here

Enabling double sided operation on a model 4

A

ac7kh

Member
Joined
Aug 4, 2018
Messages
49
Location
Junction City, OR
OK I give up and am crying uncle.

How do I enable double sided ops on a model 4. I have replaced the drives with Mitsubishi M4851s (double sided drives) and it doesn't enable the operation. Do I need to replace the cable or is it something else???

Thanks

Robert
 
If you are trying to use the stock floppy cable you'll need to replace it as the stock cable has missing connectors.
 
Are your M4851s both jumpered as drive 0? Is there termination on either drive?

Remove any termination, jumper one drive a drive 0 and one as drive 1. The RS FDC can address two internal and two external dives, for a total of four, but the internal and external are on separate channels. The internal channel can only address two drives, and it addresses drive 0 on pin 10 and drive 1 on pin 12 of the cable.

A PC usually uses both drives jumpered as 0 and uses a cable with a twist in it to allow one to be differentiated from the other (the controller thinks one is drive 2, both drives think they are drive 0). The RS cable has no twist; instead it removes teeth from the connector to pin 12 in one position (for drive 0) and one in pin 10 position (for drive 1).

So put drive 0 at the bottom, drive 1 at the top, make sure you have the cable connected properly and make sure there is no termination on either drive.

That's the way I understand it, but (disclaimer) I've never tried it. If anyone knows better, please let them add to the discussion.

-CH-

M4Pins.jpg
 
The stock cable will be missing the pin for the second side and you can't add it back manually. It's pin 32. I don't know why Tandy left it out- there must have been a reason.

You need to replace it with a full straight through floppy cable (with no twist) and set the ID on the drives correctly (since the new cable won't do this for you - TRS-80 drives are jumpered for and respond to all 4 drive ID's by default and the pins are removed in the cable to identify them instead).
 
rittwage said:
The stock cable will be missing the pin for the second side and you can't add it back manually. It's pin 32. I don't know why Tandy left it out- there must have been a reason.
Click to expand...

Pin 32 was defined as the drive select for the fourth drive on the original TRS-80 Model I wiring standard. This was later moved to the standard Pin 6, but an older Tandy-hacked "I respond to all the signals" single-side drive mechanism interprets pin 32 as a drive select so they kept pulling that pin on all their cables for as long as they kept using single-sided drives.

(This is why there's this obscure limitation where if you attach double-sided drives to a Model I you can only have three drives, it has to use the drive 3 DS as the side select and there's nothing wired from the controller to pin 6.)
 
Internal Drives (0 & 1) have the PULLUP (Terminator Resistors) on the FDC, so remove the
Terminator's from the Internal Floppy Drives.

External Drives (also strapped for 0 & 1) need the Terminator installed on the Drive that is
connected on the End of the external Ribbon Cable (the Floppy on the middle connector, if
used does not have the Terminator Installed.)

Larry
 
ac7kh said:
OK I give up and am crying uncle. How do I enable double sided ops on a model 4. I have replaced the drives with Mitsubishi M4851s (double sided drives) and it doesn't enable the operation. Do I need to replace the cable or is it something else??? Thanks Robert
Click to expand...

Tandy made their cable for 180K SSDD drives and were lazy about drive identification, you will not access your DSDD drive, set one drive to SD0 the other to SD1, Get some 3x 24 pin connectors about a foot of ribbon and put it together, its straight line nothing special about it. takes 3 mins to make one. If your in the USA I can probably make you one for cost of parts (3 x 2.50 + 2.00 for cable) lmk
 
You must log in or register to reply here.
Back
Top