SCSI DB25 External to 50-Pin Internal

[JimS]

Xamiche wrote:
Although Radio Shack, or Tandy as it's called here, stop selling such things a decade ago. They're pretty much and electrical appliance store now. Electronics stores are scarce these day. Electronics is not as popular a hobby as it once was. :-(

Most of the radio shacks here in the US seem to be cell phone and satellite TV stores. ;-/ Although you can still find components tucked away in a disused lavatory behind the "beware of the leopard" sign. ;-) I'm with you on the decline of the electronics hobby. Maybe it has to do with so many of the interesting components being SMT and difficult for the hobbyist to use. It used to be that there were over half a dozen hobbyist magazines here... now there's one general and maybe a couple ham radio mags left...

Hell, at one time there were more Amiga magazine titles than the number of users left here in 2008. ;-)

[kamiga]

Jim,

That really is a shame.  My radio shacks around here (Pittsburgh) are all cell phone stores.  Ask about a replacement cell phone battery or cordless headset, and you'll probably be taken care of.

Ask for a 555 timer IC and they'll look at you like you are on crack.

It's really a shame because I'm just starting to get a little hot and heavy into electronics the last couple years, and sort of know enough now to be dangerous.  I liked RS in the past because they almost always had the most common components on-hand.  It wasn't unheard of to walk in with a list of popular ICs, parts, connectors, and them to have everything on hand and in stock.

I've played around with SMT components, and as long as the pitch isn't too bad on them, I can hand solder them fine.... I don't have a bench magnifier, but need one.

There are also those schmartboards, which although I've never used them, look really great for prototyping SMT stuff.

It seems the past few years there's been an increase in the number of ready made modules for tasks.  Like wireless modules for xmitting data, or LCD modules with simple interfaces.  A lot of them come out to .1 on center for breadboading.... sparkfun has a decent selection of them.

I also like the trend of these IC manufacturers to eliminate external component requirements to make them work in standard ways.  No need for external caps etc.

Keith

[RiP]

I tried to connect 50pin external HDD/CD-RW drives to GVP's external 25pin with this cable but couldn't:
https://www.bhphotovideo.com/c/product/476681-REG/Adaptec_1816200R_DB_25_Male_to_HD.html

[BLTCON0]

I tried to connect 50pin external HDD/CD-RW drives to GVP's external 25pin with this cable but couldn't:
https://www.bhphotovideo.com/c/product/476681-REG/Adaptec_1816200R_DB_25_Male_to_HD.html

Assuming the cable is OK in the first place, the next thing to be painstaking and pedantic about is termination of the bus.

Speculation: GVP's card doesn't provide termination and/or termination power (it's a 5 MB/s asynchronous-only card under SCSI-1 specs, and only passive termination was prescribed at that revision level).

Fact: Active terminators (either stand-alone or found on SCSI devices) *do need* termination power to work properly

Fact: A SCSI bus must be terminated at both ends (it must be stressed that it's the bus that gets terminated at the ends, not the devices at the end. Whether, for convenience reasons, a device may also provide a terminator is a different story).


Let's begin with the external segment:
You're obviously using a SCSI box with a high-density 50pin connector, with a CD-RW and an HDD in.
Take the time to read the possible settings for the CD-RW and the HDD.
At least one of them should be able to provide both termination AND termination power.
Enable both on that device, and place it last on the box's internal cable.
Disable both on the other device and place it first on the box's internal cable.
Do not use an externally pluggable terminator on the 2nd (free) 50-pin connector.

[If you do have such an external terminator, then enable terminator power on one of the devices and disable termination for both. Cable order doesn't matter in this case.]

The other end of the bus is the internal 50pin IDC connector on the GVP card. You must either connect a device there (typically an HDD) even if it's just to enable termination on it, or use a suitable standalone IDC-plug terminator (you'll still need the short 50-pin cable for that, can't plug the terminator directly on the IDC connector as it also has a male connector).

To sum up:

1. if your setup includes devices at both ends of the bus, enable termination on them (and only them). Termination power can be sufficiently enabled on one device, although enabling it on both ends won't generally hurt anything either.

2. If your setup has no device at one end of the bus, you must use a dedicated terminator there. As long as the other end is terminated by a device with termination power enabled, it won't matter if the standalone terminator is passive or active.

3. If both ends of the bus are to be enabled by standalone terminators, then no device should have termination enabled. If the two terminators are passive, termination power is irrelevant. If however at least one of them is active, then at least one device must be configured to provide termination power (but not termination).


Of course the general SCSI guidelines still fully apply, i.e. maximum 3 meters bus length, unique SCSI IDs on the devices, parity disable if not supported by the host etc.