[Coco] FDD 26pin to 34pin

[Chester A Patterson]

I think now I've got it gentlemen.
Passing this info for your far more knowledgeable review and advice.

Coco FDD interface (Disto)	PC FDD interface		FDD
26pin (Citizen OSDC)
------------------------------------------------------------------------
------
1	Gnd				Gnd
+5V
2	N/C				Density Select		INDEX
3	Gnd				Gnd
+5V
4	N/C				N/C
DRIVE SELECT 0
5	Gnd				Gnd
+5V
6	N/C				N/C
DRIVE SELECT 1
7	Gnd				Gnd
+5V
8	INDEX				INDEX
READY
9	Gnd				Gnd
+5V
10	DRIVE SELECT 0		MOTOR ON DRIVE 0		MOTOR ON
11	Gnd				Gnd
N/C
12	DRIVE SELECT 1		DRIVE SELECT 1		DIRECTION SELECT
13	Gnd				Gnd
Gnd
14	DRIVE SELECT 2		DRIVE SELECT 0		HEAD STEP
15	Gnd				Gnd
Gnd
16	MOTOR ON			MOTOR ON DRIVE 1
WRITE DATA
17	Gnd				Gnd
Gnd
18	DIRECTION SELECT		DIRECTION SELECT
WRITE GATE
19	Gnd				Gnd
Gnd
20	HEAD STEP			HEAD STEP
TRACK 00
21	Gnd				Gnd
Gnd
22	WRITE DATA			WRITE DATA
WRITE PROTECT
23	Gnd				Gnd
Gnd
24	WRITE GATE			WRITE GATE
READ DATA
25	Gnd				Gnd
Gnd
26	TRACK 00			TRACK 00
SIDE 1 SELECT (HS)
27	Gnd				Gnd
28	WRITE PROTECT		WRITE PROTECT
29	Gnd				Gnd
30	READ DATA			READ DATA
31	Gnd				Gnd
32	DRIVE SELECT 3		HEAD SELECT
33	Gnd				Gnd
34 	N/C				DISK CHANGE

The two FDD 26pin I have are from a TANDY 1400LT, they have the 0123
switch onboard but the manual, from which I took the above info,
mentions not the switches that I could find (they were set to 0 and 1
when I yanked the drives) But the 1400 has a provision to add an
external FDD and at the flip of a switch (on the side of the 1400) to
make the external FDD either A or C drive.

So, I guess my lingering question is: where/how do I connect the Drive
Select lines to the 26pin FDDs?

Thanks.

/Chester
-----Original Message-----
From: Chester A Patterson 
Sent: Monday, August 07, 2006 2:42 PM
To: CoCoList for Color Computer Enthusiasts
Subject: RE: [Coco] FDD 26pin to 34pin

I see. Way back when, I had two 5.25" 360k and one 3.5" 720k running in
different configs and it was great (but as I think about it, never with
the 3.5" as FDD0)

I see what you mean about current FDD PC drive ID scheme, I bought two
brand new ALPS 1.44, however I was able to get at the solder pads and
change one FDD to drive 0. I was thinking of putting in some pins and a
jumper but it's not worth the bother.

Returning to my project... While I've got plenty of info on PC FDD
connections, cables, twists and what not, I guess what I REALLY need to
know is what signals are at the FDD end of a typical Coco FDD
controller. In my particular case a Disto Mini controller (and a SCI as
backup)

Thanks.

-----Original Message-----
From: Mike Pepe [mailto:lamune at doki-doki.net]
Sent: Monday, August 07, 2006 9:00 AM
To: CoCoList for Color Computer Enthusiasts
Subject: Re: [Coco] FDD 26pin to 34pin

Chester A Patterson wrote:

I'm not sophisticated digitally/softwarewise but do enjoy soldering and
tinkering very much. I assume the 0/1 drive select lines are two bits,
00=FDD0, 01=FDD1, 10=FDD2 and 11=FDD3. These two 26pin FDDs have the
0123 switch. Some
connections are fairly obvious however I'm stumped at the 0/1 items...
(Head select, Density select, Side select) Where to they connect on the
controller? STOP (pin14) perhaps a typo and should it be STEP? My plan
is to end up having two 5.25 360k (/D0, /D1) and these two 3.5" 720 (D2,
/D3)

>>>>> <<<<<

No, they're not. They are separate drive select signals. The original
spec has 4 individual drive select lines, but PCs only supported two, so
the other two "vanished". Many 3.5" drives are physically hard-coded to
drive select 1 due to the "twisted" cable select used on the PC. You'd
have to make your own twisted cable to use a PC floppy as drive 0.

You really can't do what you want due to a glitch in the radio shack
controller design. What the CoCo calls drive select 3, the rest of the
world calls side select. In a nutshell, you can only use 3 double sided
drives. If you want to use 4, you'd have to hack something yourself and
patch it.

I don't think there's much point though. Given the choice I'd move over
to the 720k media exclusively. One 360k drive and two 720k drives would
be sufficient to transfer the old disks and give you the opportunity to
read and write them when the need arises.

> There's been much here in the last week about FDDs, controllers and 
> High Density. I find I greatly enjoy this discussion. Does anyone have

> the schematics for the FD500, FD501 and FD502 controllers?

I should make a web page with the schematics I have.

Incidentally there is some verbage on an Intel floppy controller
datasheet I have that warns about mixing 5.25 and 3.5" drives on the
same cable. This is due to the differences in termination and the bus
driver difference that's used between the two types of drives. That
being said, I've never seen a problem with using both on either a PC or
the CoCo.

-Mike