Also, I'm curious how it works if I have a device connected to the 68 pin internal cable, a device connected to the 50 pin internal cable, and a device connected to an external cable. Then the layout no longer models a bus with 2 endpoints--there would be 3 endpoints. All the SCSI tutorials I've looked at simply gloss over this possible scenario for some reason (not surprising, since they all gloss over my scenario where I'm using two different internal cables).

Hello, I have some experience with SCSI 50 pin installation.. But 68 pin wont be that much different. Normally all the devices you wanting to connect should have options to Terminate them. These Terminating options could be different as different type of termination.. The older devices needed to install an resistor network device in the pre-installed holes on the PCB side like a harddisk drive. There are also types that just setting a jumper (mostly on CD-ROM and other optical drives) These terminators on optical drivers are pre-installed on the PCB of the drive.. So other terminators are just switch close to the interface connector..

ABout terminating:

You only have to make sure the last device needs always be terminated. And other devices before the last device, needs the terminators being removed or disabled.

I dont know how it would work with two seperate interface cables connected to the same SCSI controller.

If you have one chain (without external devices you also need to terminate the SCSI controller.. If you using one internal chain and one external chain.. The last drive on the internal chain needs to get terminated.
Also the last device on the external device needs to have termination, but your SCSI controller the termination needs to get removed.

Do you have one or two SCSI buses? Your card is auto-terminated or has some kind of resistor packs to terminate the bus?

In some (cheaper) cards the internal and the external connectors are in the same bus. In this case, one end is terminated on the device or the cable (your HDD) and the other end is terminated either in the card (resistor packs) or the device. If your card has only one bus (check it in your BIOS), the terminators should be placed on the CD and the cable, and the card should not terminate the bus.

Other cards have two or more buses. In this case, every bus should be terminated in both ends. If this is your case, the terminators should be placed at the card and the cable (on the HDD bus) and in the card and the external device (on the external bus).

It seems that your card has TWO buses. One comprises the external connector and the left 68 pin connector, the other comprises both 50 and 68 pin right connectors. It is important where are connected your devices.

Adaptec manuals and other documentation for this card is still available on Adaptec website, so it's best to start from there.

I owned a CD-Burner which has a Jumper for SCSI Terminator.

Looks like the Jumper on newer Drives for selecting Master/Slave.

If removed=no termination

Take Jepaels advice in his previous post above,
the specific document to refer to at that link is;
'Install Guide for the AHA-2940U2W (PDF 3 MB)'

Doesn't 68-pin and 50-pin SCSI always run separate buses?

LunarG Ref; 'aha2940u2w.pdf' again from that cited webpage above;
In the case of his 2940u2w, there are two electronically isolated SCSI
bus segments on the card - there is an External and an Internal 68 pin
connector both on a (maximum speed) Ultra2 bus, whilst the remaining
68 pin connector is on a Ultra bus capable of 40 MByte/sec in 16-bit mode
(aka Wide Ultra) and the 50 pin connector is also on that Ultra bus but only
capable of 20 MByte/sec in 8-bit mode (aka Ultra Narrow). All four
connectors on the 2940u2w card can be used at the same time.

Zup, Maybe its simpler to quote from 'aha2940u2w_ur.pdf';
"To ensure reliable communication on the SCSI bus, the peripheral at
the end of each cable, or the end of the cable itself, must have a
terminator installed (or enabled). The peripherals between the ends
of each cable must have the terminator removed (or disabled)."
"Termination on internal SCSI peripherals usually is controlled
by manually setting a jumper or a switch on the peripheral, or
by physically removing or installing one or more resistor modules
on the peripheral.
Termination on external SCSI peripherals usually is controlled
by installing or removing a SCSI terminator. On some external
peripherals, termination is controlled by setting a switch on the
back of the drive.
By default, termination on the SCSI Card 2940U2W itself is
automatic (the preferred method).
Internal Ultra2 peripherals are set at the factory with termination
disabled and cannot be changed. Proper termination for
internal Ultra2 peripherals is provided by the built-in terminator
at the end of the Ultra2 internal SCSI cable.
Most non Ultra2 SCSI peripherals come from the factory with
termination enabled."

Thanks for the replies. I have read through the Adaptec manuals, although they somewhat gloss over the part about which connector is on which bus, or the fact that there are multiple buses on the cards. Zup's post clears things up on the 2940U2W. I also have a 2940UW, though, and it has a 68 pin external connector, a 68 pin internal connector, and a 50 pin internal connector. The manual is not very clear about which bus each connector is on. I would guess the 68 pin external and internal connectors are on the same bus as the installation guide mentions manual options on page 4 for setting termination, and shows Hi and Low termination to be set to OFF in that case. But if the 50 pin connector and either of the 68 pin connectors are used, it lists Low OFF and High ON. That makes me think that all three connectors share the same bus, and it's a single bus card. The manual does not specify what to do or what would happen if all three connectors are used simultaneously.

In any case, my main focus for this thread is just to better understand in more detail the specifics of SCSI termination. As far as my personal systems go, they're setup and working fine. But I'd still like to understand more.

http://ps-2.kev009.com/ohlandl/SCSI/Mixing-narrow-wide.html

Great link. I think this all makes sense now.

tayyare, I assume you didn't quote the current relevant adaptec webpage
because it is missing all its pictures. There is an older findable copy of the
adaptec webpage with its pictures intact, its located here;
http://web.archive.org/web/20071130225645/htt … bit_devices.htm

AllUrBaseRBelong2Us;"The manual does not specify what to do or what would
happen if all three connectors are used simultaneously."
From adaptec webpage above;
"Caution: Do not connect SCSI devices to all three connectors
of an AHA-2940W or AHA-2940UW."
Sounds ominous...probably best not to try that.

buyerninety wrote:

tayyare, I assume you didn't quote the current relevant adaptec webpage because it is missing all its pictures. There is an olde […]
Show full quote

tayyare, I assume you didn't quote the current relevant adaptec webpage
because it is missing all its pictures. There is an older findable copy of the
adaptec webpage with its pictures intact, its located here;
http://web.archive.org/web/20071130225645/htt … bit_devices.htm

AllUrBaseRBelong2Us;"The manual does not specify what to do or what would
happen if all three connectors are used simultaneously."
From adaptec webpage above;
"Caution: Do not connect SCSI devices to all three connectors
of an AHA-2940W or AHA-2940UW."
Sounds ominous...probably best not to try that.

You are good, I didn't post it, because of the exact reason you mentioned. 🤣

In the past, I managed to use two internal and the external connectors of an Adaptec card successfully. It was a 29160. It has two 64 pin and one 50 pin internal connectors, and a 64 pin external connector. All three 64 pin connectors was in use (External: SCSI DVD RAM, Internal A: 4 x SCSI HDD, Internal B: 1 x SCSI HDD and a SCSI Backup). the machine is still in working condition as far as I know, but haven't fired it up for at least a year now.

Yup 😀 Adptec 29160 is an even later generation card than the 2940u2w...
Ref; '29160_install_guide.pdf' and '29160_users_reference.pdf'
In the case of your 29160, there are two electronically isolated SCSI
bus segments on the card - there is an External and an Internal 68 pin
connector both on a (maximum speed) U160 bus, whilst the remaining
68 pin connector is on a Ultra bus capable of 40 MByte/sec in 16-bit mode
(aka Wide Ultra) and the 50 pin connector is also on that Ultra bus but only
capable of 20 MByte/sec in 8-bit mode (aka Ultra Narrow). All four connectors
on the 29160 card can be used simultaneously (according to pic in the 2nd ref).

AllUrBaseRBelong2Us I guess the way to think of termination is like 'waves
coming up onto a shoreline' - if there is no termination its like the wave goes
up onto the beach and hits rocks, reflecting it back into the waves coming behind
it, resulting in collisions and damaging the waveforms. Whilst with termination its
like the wave goes up onto the beach and then drops into a trench, being drained
away so it then doesn't interfere with the waves coming behind it... 😀

This SCSI stuff is so complicated 🙁

I bought two 9GB Cheetah drives with SCA-2 interface. Also I have SCA-2 -> IDC50 adapter from AliExpress but I don't see any type of termination resistors on it - looks like plain converter. AFAIK SCA-2 drives never have terminators inside.

One of the drives is detected but media check or formatting is impossible. Another one is detected and can be formatted and verified. I remember that when I had 4.3 drives with same SCA-2 inteface I had to use SCSI streamer on the end of the cable with included terminator, though both drives were bad anyway. One of them did not even start and another one did but was detected only when the streamer was terminating the line.

Is it possible that drive works without terminator? It's the only device (except for controller) in the line. Can it be that second drive is actually working but has problems due to absence of termination? They are both of the same model.

Sometimes SCSI just works without termination, or works but unreliably.

Long before I knew what termination even was, I had a SCSI scanner that would only work when the weather was humid. I guess the moisture in the air was acting as a terminator.

rasteri wrote on 2022-08-31, 13:19:

Sometimes SCSI just works without termination

Seems like my case. Or I have proper humidity level, 🤣.

rkurbatov wrote on 2022-08-31, 14:50:

Seems like my case. Or I have proper humidity level, 🤣.

Yeah my recommendation to anyone struggling with SCSI issues is to buy one of those room humidifiers