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Amiga computer related discussion => Amiga Hardware Issues and discussion => Topic started by: Xamiche on April 21, 2008, 12:29:48 PM
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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?
(http://i107.photobucket.com/albums/m301/aaronlittle/scsi.gif)
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?
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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.
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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:
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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
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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.
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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
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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?
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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.
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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. :-)
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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.
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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 (http://www.radioshack.com/product/index.jsp?productId=2102845&cp=2032058.2032230.2032265&parentPage=family)
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. ;-)
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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
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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).
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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. :-(
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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. ;-)
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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
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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
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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.