A+ Practice Exam Question 33

se12endipi7y

The question asks what the switches are set to. I learned in class that 6 being the ID would be 5, or 101--on off on, because 0 would be 000, or off off off.
Below from left to right: binary digit, switches, SCSI device

0-off off off-1
1-off off on-2
2-off on off-3
3-off on on-4
4-on off off-5
5-on off on-6
6-on on off-7
7-on on on-8

and so on.

Well, just wondering. am I right or am I wrong?

Plantwiz

The test asks for configuration of SCSI 6.

That answer appears correct. Your binary is on the three switches is all zero's 0,0,0 = 0. The switches will either be 1 (on) or 0 (off) as you indicate above. Switch six would be 1 (on or 4), 1 (on or 2), 0 (off or 0). 4+2+0 = 6. Seem right. I'm not clear on what your question is.

The settings by the manufacturer can be left-handed or right-handed and while configuring a tower of SCSI CD-ROm's years back, I ran into this with three drives one brand and three another. Finally figured out that the ID's were switched

So it can look like this Left most bit being most significant:

0 = 0-0-0
1 = 0-0-1
2 = 0-1-0
3 = 0-1-1
4 = 1-0-0
5 = 1-0-1
6 = 1-1-0
7 = 1-1-1

And the reverse for a right most bit being most significant:

0 = 0-0-0
1 = 1-0-0
2 = 0-1-0
3 = 1-1-0
4 = 0-0-1
5 = 1-0-1
6 = 0-1-1
7 = 1-1-1

So, I am not clear what your question is about the test? Do you feel it is incorrect?

se12endipi7y

hmm...lemme try to simplify it. it's like having two rows of numbers, 0-10 and 1-11.

0 1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 10 11

the first row is the binary digit, and the second row is the SCSI ID. now am i right that the first device would be the host adapter being 000 as a binary digit or off off off as in switches, or zero in plain decimal?

i don't know if it's correct or not lol... either my teacher or the test?

caustik

hmm not sure what your question is either but this may help:


scsi id's = decimal #'s 0-7 (0-7 just for example)

to set the id on the device you must set the jumper to the binary equivalent

this involves a conversion

decimal numbers are base 10 so each column of numbers represents a multiple of ten

256

hundreds | tens | ones
2 | 5 | 6

10^10 | 10^1 | 10^0

multiply the column by the value in it and add the columns to get the number

10^10=100 * 2 = 200 + 10^1=10 * 5 = 50 + 10^0 =1 * 6 =6 = 256


Binary is base 2 so each column is a multiple of 2

0011

0 | 0 | 1 | 1

2^4 2^2 2^1 2^0


2^4=16 *0=0 + 2^2=8*0=0 + 2^1=2*1=2 + 2^0=1*1=1 = 3

since its only 3 rows this may be easier

0 0 0
4 2 1

multiply the the column by the value in it and add the columns, since it base 2 just remember left to right 1 2 4


0 0 1
4 2 1
4*0 + 2*0 + 1*1 = 1

1 1 1
4 2 1
4*1 + 2*2 + 1*1 = 7

1 0 0
4 2 1
4*1 + 2*0 + 1*0 = 4

see a pattern?

Plantwiz

JBoy990 wrote:

hmm...lemme try to simplify it. it's like having two rows of numbers, 0-10 and 1-11.

0 1 2 3 4 5 6 7 8 9 10
1 2 3 4 5 6 7 8 9 10 11

the first row is the binary digit, and the second row is the SCSI ID. now am i right that the first device would be the host adapter being 000 as a binary digit or off off off as in switches, or zero in plain decimal?

i don't know if it's correct or not lol... either my teacher or the test?

Ok. I think I get what you are talking about, but I've never heard SCSI referred to like this. Not saying it isn't so, just not familiar with that method.

Basically, your instructor is saying that where the SCSI ID = 0 this is the first device on the chain? When The SCSI ID =1 this is your second device.

I think this though process is trying to mix tangible devices with computeese. Whereby in computers or logical math counting of whole numbers will begin with Zero and in tangible counting (or school children counting) we begin with 1.

As I mentioned, not saying that isn't a good way to mention it - just seems more difficult to me as I've not heard of it before - and not with SCSI, though if it helps you remember it....that's fine.

Logically, a good plan, but not always true.


BTW- Caustik - Nice explanation

caustik

having re-read your question i think i get it...

i think the class was just pointing out that computers start counting at 0

so even though there are 8 devices they are counted 0-7 which is why the numbers seem off-set.

That first device, device 1 as you say, set to zero, should actually be referred to as device zero and so gets the ID 0

se12endipi7y

oh. haha lol i get ya now... well thanks for clearing up the confusion caustik. i agree, nice explanation.