The yellow LED on Paralan's Converters, RegeneratoR™s (these are all SCSI "expanders") illuminates whenever any data line on the SCSI bus is asserted ("de-asserted" is technically more correct because SCSI signals are "inverted". But I will use "asserted" here because de-asserted is kind of an awkward word). That means that any time there is any activity on the bus, the yellow LED will be blinking. In some cases, it blinks so rapidly that it appears to be on solid, rather than blinking. This LED can be used for some simple, but very effective troubleshooting of the SCSI subsystem. Note that similar LEDs on competitor's units do not necessarily work the same way as the yellow LED on Paralan products.
Suppose you have a simple bus with one host (initiator) and a couple of peripherals (targets). Let's further suppose that the host is single-ended (SE) and all the peripherals are differential (HVD) so we have a Paralan SE to HVD SCSI Converter between them to make the signal conversion. Everything is connected together and powered up. By the way, SCSI peripherals and the Converter should be powered up before, or at the same time as, the host. Also, any time the Converter, RegeneratoR™ is turned on, the green LED should be on steady.
When the host adapter initializes the SCSI bus, it will ask each SCSI address (ID) if there is anything there. When it does this, there is activity on the bus and the yellow LED will blink briefly. If the yellow LED does not blink, there is a problem with the SCSI driver, the host adapter, terminations or cables between the host and the Converter because it is not getting any signals from the host.
If the yellow LED blinks briefly then goes out, but the host does not "see" the peripheral(s), suspect the driver, the terminations or cables anywhere in the system, or one of the peripherals.
If the yellow LED comes on and then stays on, we can determine which side of the expander the problem is on, or if it is the expander itself. First, leave everything powered up and unplug the SCSI cable from the host to the expander. Unplug it at the expander, not at the host. If the yellow LED goes out, the problem is between the host and the expander, probably terminators or cabling. It could also be the host adapter.
If the yellow LED stays on, disconnect the cable from the expander to the peripherals. Again, unplug it at the expander. If the yellow LED goes out, it indicates that the problem is between the expander and the peripheral(s) and is probably terminators or cabling. Of course, it could be a failure in one of the peripherals.
If the yellow LED still says on with both SCSI cables disconnected, the expander has a failure. Be sure to check that you have the single-ended cabling connected to the single-ended side of the expander and the HVD cabling connected to the HVD side of the expander. Believe me, a failure in one of Paralan's expanders is extremely rare.
Here are some additional troubleshooting hints to use once you have determined which side of the expander the problem is on. Although these tips can be used to find a problem on either side of the expander, for the purposes of this memo, let's assume that the problem is on the peripherals side of the expander. This is a procedure to follow even if the yellow LED did not indicate a problem on one side or the other.
- Make sure that every device on the SCSI bus has a different SCSI ID. The host adapter is almost always ID 07 and it makes no difference if it is a Narrow (8 IDs) or a Wide (16 IDs) bus, the host will be ID 07. It does not make any difference how the other device IDs are assigned. They do not have to be in the same sequence that the devices are located in the SCSI chain. Just be sure that no two devices are assigned the same ID. Also, if this is a Wide SCSI bus and you have both Wide and Narrow devices on it, be careful of how you assign the IDs. A Narrow device cannot communicate with a Wide device having an ID in the range of 08 to 15 because the Narrow device cannot recognize a device with an ID outside the range of 00 to 07.
- Make sure that there are two terminators and only two terminators (not one, not three, but two) for each SCSI bus segment.
Here is a definition for you: A SCSI "domain" consists of all the peripherals, cabling and terminators connected to a SCSI host. Or, you can think of it as all the SCSI devices (initiators and targets) that can communicate with each other directly over one SCSI bus. One SCSI expander such as a Paralan Converter, RegeneratoR™s divides the SCSI domain into two SCSI bus segments. Two expanders would, of course, divide the bus into three SCSI bus segments. A segment is defined as all the devices and cabling on a SCSI bus between terminators. In other words, a SCSI segment is defined by two terminators.
- Be sure that the cables are not too long for the type of SCSI you are using. That means a maximum total length of 1.5 m (5 feet) for single-ended Ultra SCSI, 25 meters (82 feet) for HVD SCSI and 12 m (40 feet) for LVD SCSI. Refer to Q64, Q65 and Q66 on Paralan's SCSI FAQ site for more detailed info on SCSI cable lengths.
- Make sure that the SCSI cabling has no "stubs". A stub is an unterminated segment of transmission line (and at the frequency that SCSI operates, the bus is, indeed, a transmission line). There are two analogies I can think of for stubs:
- A stub can look like a "tee" or a "wye" in the SCSI cabling. A definite no-no.
- A stub is a piece of cabling that extends beyond the terminator. This is usually not found on external, round, shielded SCSI cabling and is more commonly found with internal ribbon cables. It is easily cured once it is found.
The authors of the SCSI specifications recognize that a "tee" type of stub is going to exist with virtually every device connected to the bus so they limit the maximum length of these stubs to 0.1 meter (about 4 inches) on the single-ended and LVD SCSI buses and 0.2 meter (about 8 inches) on the HVD SCSI bus. The short cable from the connector on the outside of a SCSI enclosure to the peripheral itself plus the short conductors on the PCB inside the peripheral constitute an unavoidable stub. LONGER STUBS KILL SCSI! Their effects also accumulate, so SCSI devices should be separated from each other by 0.5 meters (about 18 inches) to minimize this effect.
- If steps 1, 2 and 3 above do not find a problem, remove all peripherals but one from the problem side of the expander. Make sure terminators are correct and there are no stubs. Reset the SCSI bus (usually by re-booting the computer) and observe the yellow LED while the system is booting. If the bus still hangs, replace the peripheral with another one to see if that cleans up the problem. If it does not clean up the problem, again review terminations. Make sure that you have a HVD terminator if the bus is HVD, an active SE terminator (don't use passive SE terminators) if it is single-ended or an LVD (or LVD/MSE) terminator if it is LVD. Some people do not realize that SE and differential terminators are different.
- If all seems well with the terminators, the problem is probably the cable. If it has 50- or 68-pin high density connectors, look carefully at all the pins in the connectors as they are very easy to bend over so that they no longer make contact. Although less common, the sockets in the connectors can also bend. Test the cable on a Paralan SCSI Cable Tester or try it on another SCSI system. Note that a cable that works on a SE SCSI system may not work at all on a differential SCSI system, so if you are using it on a differential system, you should test it on a differential system. The best bet is to buy SCSI cables from Paralan to make sure they are correctly made.
- If placing one peripheral on the problem side of the expander (step 4) works okay, add a second peripheral, again making sure the terminators are correct and there are no stubs. If that does not work, go back to step 5. If it does work, add another peripheral until you locate the problem and the system works with all peripherals connected. By then, you will know if the problem was a terminator, a cable, or a peripheral.