External floppy cabinet - Termination?

[per]

Ok, so I am working on putting together an external floppy disk drive cabinet, which is designed to hook directly onto the cable from the internal drive in a computer. The computer is meant to work fine on it's own without the external drives, but it's also meant to work with the external drives too.

So how do I set up the termination resistors? two 150 Ohm paths to 5V would mean 67mA sink current, a little on the high side for the 74S240-chips on the controller card driving the floppy interface. One option is of course to get termination resistor-packs of higher resistance, but then on the other side, the total cable-length isn't terribly long, only half a meter or so.. It could work just having the resistor packs on the internal drive I guess?

Any thoughts on this?

 

[MiaM]

You could have drivers for each resistor in the terminators, to be able to switch the ones in the computer on/off.

Also, do you need 150 ohm? I think that 3.5" drives tend to have 1k and each drive has terminators always enabled. (Take this as kind of hearsay though).

In particular there were never any termination on/off for the Amiga, with it's support for up to 4 drives, with 2 or 3 being external (depending on if the case had space for one or two internal drives). I remember having to DIY a terminator pack using a component adapter and separate resistors at IIRC 1k as the 150 ohm terminator in 5.25" drives consumed too much current (and of course no terminator at all didn't work either). IIRC i had about a meter or so of cable in this case.

 

[hmb]

Ok, so I am working on putting together an external floppy disk drive cabinet, which is designed to hook directly onto the cable from the internal drive in a computer. The computer is meant to work fine on it's own without the external drives, but it's also meant to work with the external drives too.

So how do I set up the termination resistors? two 150 Ohm paths to 5V would mean 67mA sink current, a little on the high side for the 74S240-chips on the controller card driving the floppy interface. One option is of course to get termination resistor-packs of higher resistance, but then on the other side, the total cable-length isn't terribly long, only half a meter or so.. It could work just having the resistor packs on the internal drive I guess?

Any thoughts on this?

The resistor pack ideally installed in the floppy drive at end of a floppy drive cable really function as "pull up" resistors vs "termination" resistors. If your internal floppy drive is a newer 3.5" drive that has built-in pullup resistors (typically 1K or more), then install a traditional 150 ohm resistor pack on the last drive of your external drive enclosure. If your internal floppy drive is an older drive with a traditional 150 ohm resistor pack, your external drive enclosure will likely work just fine with no additional pullup resistor packs installed in its drives.

 

[jlang]

With short cable length (<3ft.) you usually can get away with 1K pull ups. If you are going with a long cable length you will be better off with 220/330 terminator networks.
220 pull up/330 pull down. It matches the line impedance and requires less current from the driver.

 

[per]

The resistor pack ideally installed in the floppy drive at end of a floppy drive cable really function as "pull up" resistors vs "termination" resistors. If your internal floppy drive is a newer 3.5" drive that has built-in pullup resistors (typically 1K or more), then install a traditional 150 ohm resistor pack on the last drive of your external drive enclosure. If your internal floppy drive is an older drive with a traditional 150 ohm resistor pack, your external drive enclosure will likely work just fine with no additional pullup resistor packs installed in its drives.

The internal drive is a SA850 8" drive (bi-compliant type). This is in of itself a bit interesting, since it first of all does not add resistors to unused drive-select lines, and also some inputs have soldered-in 150-ohm pull-up resistors that can't be easily removed.

Since the controller itself uses 74S240 chips to drive the lines (an unusual non-open-collector choice), rated to drive 133 ohm at minimum, I decided to go with 2.2K in the external unit. This way, the combined impedance should not get below 140 ohm. Using 1.5K would have been a little better and still within spec, but I don't have the right type of resistor packs of that value. It seems to work for now with the 2.2K one, including writing.

 

[MiaM]

Btw, worth checking is what logic chips is used to drive the signals.
Different families of the 74xx chips have different drive capacity.

as you mention 8" drives, you can probably be fairly certain that this isn't an issue in your particular case, but still.

 

[per]

Btw, worth checking is what logic chips is used to drive the signals.
Different families of the 74xx chips have different drive capacity.

as you mention 8" drives, you can probably be fairly certain that this isn't an issue in your particular case, but still.

As mentioned, 74S240 (just S, not LS). Those are good for 60mA sinking per line, and datasheets usually quote at minimum 133 ohm line impedance for reliable operation (I guess for some margin, and to keep levels in the clear of the undetermined zone)