sane-project-website/old-archive/1998-05/0195.html

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<title>sane-devel: Re: SCSI card advice (again)</title>
<h1>Re: SCSI card advice (again)</h1>
<a href="mailto:becka@rz.uni-duesseldorf.de"><i>becka@rz.uni-duesseldorf.de</i></a><br>
<i>Sat, 16 May 1998 11:56:04 +0200 (MET DST)</i>
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Hi !<br>
<p>
<i>&gt; Thanks to those who responded to my earlier message. I decided to buy the </i><br>
<i>&gt; ASUS PCI SCSI card (since I got a good deal on it). However, it has a </i><br>
<i>&gt; Symbios Logic 53c810a chip which caused my 2.0.30 kernel to hang with a </i><br>
<i>&gt; "scsi bus busy" error during the SCSI host probe. Fortunately, after </i><br>
<i>&gt; patching to 2.0.33, the kernel successfully recognised the card, but </i><br>
<i>&gt; I got a (hopefully, minor) warning that the chipset revision was greater </i><br>
<i>&gt; than 2 (I've included the text of this warning at the end of my message). </i><br>
<p>
Yeah - looks harmless. The driver just tells you, that it doesn't know that<br>
revision, and there might be things that have changed ...<br>
<p>
<i>&gt; Is this anything to be concerned about (maybe I should use one of the </i><br>
<i>&gt; 2.1.x kernels which I understand have a 53c810a driver) ?</i><br>
<p>
Don't think so.<br>
<p>
<i>&gt; On another note, does the scsi card become associated with any particular </i><br>
<i>&gt; /dev file </i><br>
<p>
No. Eventually with something in /proc/scsi/* to allow for setting parameters,<br>
but not with a /dev.<br>
<p>
<i>&gt; or are the disks, scanners, CD-Rs, etc. on the SCSI chain the ones </i><br>
<i>&gt; that receive a device file (I know the 1st disk would be /dev/sda) ? </i><br>
<p>
Yes.<br>
<p>
<i>&gt; I understand that I have to do a 'ln -s /dev/sga /dev/scanner', </i><br>
<p>
Wait - this depends on the SCSI-ID of the scanner and the other devices.<br>
<p>
Linux assigns SCSI device names in the order in which devices are found.<br>
<p>
/dev/sga is the device with the lowest ID on the Bus. sgb the next one<br>
etc.<br>
<p>
The same goes for disks and cdrom, where only devices of that type count.<br>
<p>
<i>&gt; but what is sga associated with (I'm assuming it's the card itself) ? </i><br>
<p>
No. It is something like a "raw data channel" to the devicxe with the<br>
lowest ID number.<br>
<p>
<i>&gt; I'm also a bit fuzzy regarding SCSI termination: The manual states that </i><br>
<i>&gt; termination is required on the end device, </i><br>
<p>
This is correct. Let me explain what termination is about:<br>
<p>
Electrical waves that travel through a cable will suffer a reflection,<br>
when they hit the open end of a cable. Thus a reflected pulse will go<br>
back the line and interfere with the data on the cable.<br>
<p>
If you put a "terminator", which is a resistor with a special value on the<br>
end, the electrical wave will be kind of absorbed in that resistor, thus<br>
avoiding the reflection problem.<br>
<p>
This should illustrate where terminators need to be put :<br>
<p>
At the ends of the device chains.<br>
<p>
Putting them earlier will cut the signal from the later devices, leaving <br>
them out will cause reflection.<br>
<p>
Thus you should turn on termination on the last devices on the chain<br>
on either side. If the device has no internal termination, put an<br>
external terminator on the outgoing SCSI plug of the device or on<br>
the next connector of a flat cable.<br>
<p>
<i>&gt; but I was told that I don't have to worry about it (auto-termination). </i><br>
<p>
This only applies to the SCSI host (the adaptor), if it happens to be on <br>
one end of the chain. It will sense the termination resistors, see if it<br>
is in the middle or at the end of the cable and thus terminate itself <br>
correctly.<br>
<p>
However this only works on the assumption, that alll other devices are<br>
set up correctly.<br>
<p>
<i>&gt; Does the scanner have to be switched on before I boot-up for association </i><br>
<i>&gt; to /dev/sga to take place </i><br>
<p>
This would be better for the beginning. Linux can add devices later, but<br>
this can cause confusion and should be avoided.<br>
<p>
<i>&gt; Finally (for the sake of argument) if I put 2 scanners on the SCSI chain </i><br>
<i>&gt; how could I associate both with sga ?</i><br>
<p>
You don't. Both scanners have different SCSI-IDs (or it won't work). Thus they<br>
get different /dev/sg? entries.<br>
<p>
CU,ANdy<br>
<p>
<pre>
-- 
= Andreas Beck                    |  Email :  &lt;<a href="mailto:andreas.beck@ggi-project.org">andreas.beck@ggi-project.org</a>&gt; =
========  GGI - The Right Thing To Do : <a href="http://www.ggi-project.org/">http://www.ggi-project.org/</a>   ========
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