SCSI DB25 External to 50-Pin Internal

[Xamiche]

Okay, I've create a new project for myself. Towering an Amiga 500. Why? Just for fun. I have a cool little PD device called a SCRAM 500. It has RAM on board as well as an external DB25 SCSI connector. My question is, not being very familiar with SCSI devices, can I create an adaptor to go from a DB25 to a 50pin SCSI device?



Would such an adaptor need any interface circuitry, or would it just be a matter of aligning the correct pins?

I know I could use an external case to house the HD and then just use a DB25 SCSI cable but I'd like to be able to have the HD inside the tower I put the Amiga into. Also, can I have more than one SCSI device on such an interface. Say a HD and a CD-ROM drive?

[mingle]

Hi,

I recently purchased just such an adaptor on eBay.

It cost me just over $AU17 shipped from the US to Melbourne.

Very neat, excellent quality and much better than trying to hack one together yourself.

http://item.express.ebay.com/New-SCSI-I-O-Adapter-INTERNAL-50Pin-To-EXTERNAL-25Pin_W0QQitemZ120024099379QQihZ002QQcmdZExpressItem

Cheers,

Mike.

[Xamiche]

Thanks for the link. :-)

It's not quite what I'm after though. I need a DB25 Male -> 50 Pin IDC Male adaptor. The adaptor in the auction is DB25 Female -> 50 Pin IDC Male. It's cool though. The item in the auction tells me one thing, any such adaptor can be made as a straight up connector converter without the need to include an interface circuit.

Maybe that item and a gender changer could work out. :idea:

[kamiga]

Hi there.

Yeah, I think it's just a matter of pinning.

DB25 for SCSI was used for SCSI 1.  It's the old school SCSI, and many old external drives and SCSI cards had a DB25 on them.  I had a scanner with a DB25 on it once.

50-pin IDC? Is this an internal drive?

I'm pretty sure the DB25 is the external version of SCSI 1, and 50-pin IDC is the internal version of SCSI 1.

Most newer SCSI devices are backwards compatible with old cards, so even if its a newer device you should be ok.

I'll look around and if I happen to see an adapter that works I'll report back.

HTH

Keith

[Xamiche]

50-pin IDC? Is this an internal drive?

Yeah, it's the connector on an internal SCSI HD I have. I also have a CD-ROM drive with the same 50-pin connector.

It's good to know it's just a pinning thing. Should be much easier to set it all up.

[kamiga]

Two questions also come up:

1> power.  How are you planning on powering those drives?  I think the power is normally on the IDC50 IIRC. (although I could be wrong )

2> termination.  You know you have to terminate the SCSI bus with a terminator, right?  So daisy chain your devices and then have a terminator at the end of the chain.

which brings up,

yes, you can have multiple devices.  I think the limit is 7 devices.  They all have to have different SCSI id's which is normally set by jumpers or a little push-button wheel thingy that lets you set it.

You know, you could probably rip the guts out of an external scsi case to get the right connections.......

Keith

[Xamiche]

1. I'm going to power the whole thing with a ATX PSU. I have a modified ATX PSU I'm using to power my Amiga CD32s at the moment. I'll just rig up another one for the A500 tower project. The SCSI HD and CD-ROM drive have the usual molex power connectors so there's no drama there.

2. Yeah. Though I'm not sure if I need to terminate if all the connectors on the ribbon are being used by a device. If I use a SCSI cable with 3 connectors; one going to the SCRAM 500, one to a HD and the last one to a CD-ROM, would that be a complete and correctly terminated set up?

[Ilwrath]

Yeah. Though I'm not sure if I need to terminate if all the connectors on the ribbon are being used by a device. If I use a SCSI cable with 3 connectors; one going to the SCRAM 500, one to a HD and the last one to a CD-ROM, would that be a complete and correctly terminated set up?

The thing you have to remember is that in ALL CASES on the SCSI bus, both ends of the chain MUST be terminated.  

If there is only one connector on the SCRAM, it should have an on-board termination on it's end, so that will take care of one end.  

The other could be taken care of by setting a termination jumper on the SCSI CD-ROM drive unit.

The logical SCSI chain would thus look like:
------------

This would be a correct configuration as it would have termination on both ends of the chain.

[Xamiche]

Ilwrath wrote:

Yeah. Though I'm not sure if I need to terminate if all the connectors on the ribbon are being used by a device. If I use a SCSI cable with 3 connectors; one going to the SCRAM 500, one to a HD and the last one to a CD-ROM, would that be a complete and correctly terminated set up?

The thing you have to remember is that in ALL CASES on the SCSI bus, both ends of the chain MUST be terminated.  

If there is only one connector on the SCRAM, it should have an on-board termination on it's end, so that will take care of one end.  

The other could be taken care of by setting a termination jumper on the SCSI CD-ROM drive unit.

The logical SCSI chain would thus look like:
------------

This would be a correct configuration as it would have termination on both ends of the chain.

Excellent. Thanks for that info. :-)

[rkauer]

 Almost right!

kamiga wrote:
Two questions also come up:

1> power.  How are you planning on powering those drives?  I think the power is normally on the IDC50 IIRC. (although I could be wrong )

 You are. Only mini-SCSI (SCSI for laptops) have power to feed a device.

2> termination.  You know you have to terminate the SCSI bus with a terminator, right?  So daisy chain your devices and then have a terminator at the end of the chain.

 Almost right. You need to terminate both ends of the SCSI chain with the same type of terminators.

 Some SCSI controllers have an internal terminator, but don't count on it.

 There are two types of terminators: passive and active.

 Passive ones are cheap and don't require the "term_pwr" line active (+5V on the middlest wire of the cable).
 
 Active terminators are more expensive, but they work greatly and speed the bus, since you'll never get a data pack loss (a lot of less bus noise).

which brings up,

yes, you can have multiple devices.  I think the limit is 7 devices.  They all have to have different SCSI id's which is normally set by jumpers or a little push-button wheel thingy that lets you set it.
You know, you could probably rip the guts out of an external scsi case to get the right connections.......
Keith

 That's true for SCSI type "1". SCSI 2 and 3 can use up to 15 devices at the same time.

[JimS]

I used to make these back when i was an Amiga dealer. We used them to convert pc minitowers into external SCSI boxes. You could get the stuff at Radio Shack for the most part, although it may no longer be true. They made a small circuit board that was a match for one of their prototyping boards.
radio shack
If you look at that board, you see two common strips down the middle of the board. All of the pins on one side of the idc50 are ground except one. so if you isolate that pin on the board, you can solder idc50 there. Then it gives easy access to all the pins on the other side. Then you get a short DB25 cable - making sure it has all 25 pins... cut it in half and wire it to the proper pins of the IDC50. Use a standard internal scsi ribbon cable to connect to the drive & you're done.

easier to do than to describe. ;-)

[kamiga]

rkauer wrote:
 
 Almost right. You need to terminate both ends of the SCSI chain with the same type of terminators.

Thanks for an almost verification of what I already knew to be true.

 Some SCSI controllers have an internal terminator, but don't count on it.

MOST scsi controllers are terminated internally.  Because the SCSI hosts are usually the starting point of most SCSI chains, they are usually terminated.

Keith

[doctorq]

Maybe that item and a gender changer could work out. :idea:

Yes, it will work. I used to convert the DB25 socket on a Blizzard SCSI setup with this exact method (removing the bracket from the adapter though).

[Xamiche]

Thanks for the excellent advice from all of the above. :-)

I think I'm going to go the route JimS advised. I enjoy building stuff and I wont have to wait weeks for an adaptor to arrive. 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. :-(

[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. ;-)