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UMAX Astra 600s & 1200s Termination Problems.

For some strange reason UMAX is persisting in not providing terminator power from it's scanners. In the Astra 600s and the 1200s, just like in most earlier models, this is again the case. We have notified UMAX of this fact and that we see it as an important problem. Their response is that they see no problem.

Read on to see why there is a problem and what you can do to solve it. Even if you don't have a UMAX read on to see that a SCSI bus is not so mysterious after all.

Scanners concerned: S6, S12, Astra 600s, Astra 1200s ans probably a few more.

What is a 'Terminator' anyway?

At both ends of a SCSI chain there has to be a terminator. This is an electronic component that dampens reflections in the signal. Without a terminator the signal would reflect right off the end of the cable and 'bounce' back. Somewhere along the cable the signal could not be read properly because one sees not only the signal itself but also a weaker and slightly delayed reflection.

You can see the mechanical equivalent in real life quite easily. Tie a piece of washing line a few metres long with one end to a metal hook firmly secured in the wall. Hold the other end and pull the line tight. Now 'code' a message by shaking the end you are holding up and down. The waves you make will travel down the line nicely. But as they reach the metal hook in the wall they will bounce right back. It will be hard or even impossible for someone to 'read' your message because the data you are sending is polluted by this reflection. Now we put a big sponge around the hook where the line attaches to it. You will see that the waves are absorbed by the sponge and there are no more reflections. And, surprisingly perhaps, although we are damping the signal it is actually easier to 'read' because there are no more reflection to confuse us.

In electronic form a terminator looks like this. One resistor pulls the signal up towards the + 5 volts. The other resistor pulls the signal down towards the ground. These two forces combined pull the signal toward the middle and damp the signal. If the signal is high the lower resistor will 'pull' extra hard, and if the signal is low the upper resistor will do its best to raise the signal. For each and every signal line there is one of these constructions.

So what is the big deal with this Terminator Power?

As you can see in the above discussion it is important that this terminator is connected to the Ground and to a +5 volt supply. If this is not the case the terminator can not work properly and in fact if it is only connected to one of the two it will be worse than having no terminator at all. As a terminator is an external device with no power supply of it's own it is entirely dependent on the device it is plugged into to supply this +5 volt supply. This is called the terminator power because it powers the terminator. A ground is always available on each and every SCSI connector or cable so there are no problems there. The issue concerns the +5 volt which is not supplied by every device although it should be.

The case of the UMAX Astra 600

This diagram shows the situation in the UMAX Astra 600. Other scanners have a similar set up. Note that the Terminator Power of the two SCSI connectors are connected together. Also note that the diode D1 and the fuse F1 are present. The fuse would protect the scanner against too much power being drawn from the Terminator Power line. The diode prevents problems occurring in case more than one device is putting +5 volt on the Terminator Power line. The way the UMAX scanners ship the jumper is in the 'off' position and no terminator power is supplied by the scanner.

So why is all this a problem?

If a terminator is plugged in the back of a UMAX scanner then you are relying on three things:

  • Some other device in the SCSI chain is better behaved and is supplying this terminator power for you.
  • This other device is actually turned on.
  • All the SCSI cables between this device and the scanner have a wire for the Terminator Power.

This will usually be OK but sometimes there will be trouble.

Some cable manufacturers omit the wire for the Terminator Power from their cables in order to save money. Some of the cheaper (thinner) cables are guilty of this.

Not all devices supply Terminator Power. In the case of modern devices that have an internal terminator that can be turned on and off with a little switch on the back there is no need for them to supply this power and many on them don't.

If one of these two things goes wrong then the poor little terminator on the back of the scanner is not getting it's Terminator Power. The normal user then mutters some nasty things about SCSI bus 'black magic', he or she will swap a few cables and devices around, and with a bit of good luck a device that does supply the Termination Power moves closer to the scanner or a cable that does not have the extra wire moves further away from the scanner and the problem will go away. This then only confirms the poor user's belief that getting a SCSI bus to work is indeed 'black magic' as all he did is swap a few 'identical' things around and suddenly everything was OK again.

So how do I fix this mess?

Now that you (so I hope at least) understand that you want you scanner to be a good boy and supply some Terminator Power for that poor little terminator you will want to know how to fix the problem.

This is easy! Surprisingly enough UMAX has almost taken care of this for you. They made sure that there is a small jumper hidden away inside your scanner and all that you have to do is set this to 'on'.

Step 1:

Remove the circuit board from the scanner. This involve removing a few screws. If you are careful it is not necessary to remove the board completely. If you do need too loosen the connectors around the back keep an eye on how they are located so that you can put them back in the same way.

Step 2

Locate the jumper close to the one SCSI connector as marked on the photo.

Step 3

Move the jumper from the 'off' position to the 'on' position. It should slide over the left two pins as seen below.

Step 4

Put the scanner back together again making sure that the big ferrite beads around the cables don't obstruct the circuit board.

If you have any questions or remarks please email us at termination@chipmunk.nl

All the information presented here is our opinion. It might not be correct. Till this day we have had no response from UMAX as to why the jumper should be in the 'off' position. They do however say it should be in the 'off' position. We are not against UMAX. In fact they make good scanners and we sell lots of them, but we do feel that this is an issue that has to be resolved. We openly invite comments from people who feel that the presented information is not correct.

Comments that we got earlier.





Copyright Chipmunk 1997