[ LUGOS ] SCSI
Bostjan Lemut
Bostjan.Lemut na guest.arnes.si
Čet Jan 14 02:07:42 CET 1999
Marko Cuk wrote:
>
> SCSI ...
>
> Jah... vem, da je bilo ze dosti pisanega, opisanega, ...pa me vseeno
> zanima, ce mi zna kdo povedati, kje naj zacnem iskati problem.
>
> Skratka, kontroler je Adaptec UW 2940 .
> Diska sta UW, Seagateja. Razlicna.
>
> Ko kopiram podatke iz katerega koli diska, lahko je IDE, na prvi, manjsi
>
> disk, je vse ok. Ta disk je tudi terminiran.
>
> Ob tem naj napisem, da bi bil zelo vesel, ce bi lahko kdo, ki ima res
> veliko pojma o samem SCSIju, povedal, kaj je to termination from drive,
> from bus ali pa kar from bus and drive. Prav tako tudi to, kaj pocne
> aktivna ali pasivna terminacija. Kaksne so upornosti kablov ( 132 ohm ??
> ).
>
terminacija je ena zadeva, ki dela ljudem sive lase, pa ne samo
terminacija, tudi dolzina kablov oz vodila.
prvo kot prvo moras sprejeti za dejstvo naslednje:
SCSI vodilo mora biti na obeh koncih terminirano (zakljuceno) in sicer
proti termpower (mislim da je 5V) 220ohmov in proti groundu 330ohmov ni
vazno za kaksne tip scsi vodila gre (narrow, wide, itd). Ce je aktivno
terminirano pomeni samo, da ima termpower signal napetostni regulator na
doloceno napetost (sm ze pozabu katero, ker mi je zadoscal pasivni
terminator). No scsi vodilo lahko terminirat na koncu kablov ali pa
pustis, da to opravijo naprave same ( ce jim reces)
Vecina scsi naprav dandanes podpira avtoterminacijo, kar pomeni, da
pogruntajo, ce so na tistem koncu kabla same, ga bodo tudi terminirale,
ce niso, ga ne bodo. Za scsi naprave se steje VSE kar je na vodilo
priklopljeno, ja, tudi tvoj scsi kontroler je le ena izmed scsi naprav s
svojim scsi IDjem (ponavadi zadenj, kaksna stevilka je odvisno od sirine
vodila - narrow ponavadi 7, wide pa 15)
Ker je torej kontroler le ena izmed scsi naprav med vodili (v bistvu ga
POVSOD opisujejo kot SCSI to ISA Host adapter ali ap SCSI to PCI Host
adapter - kar pomeni, da je le pretvornik med scsi in isa oz pci
vodilom), mora biti tudi on terminiran, saj je ponavadi na koncu izmed
enega od kablo (zunanjega ali notranjega). PAZI, SCSI vodilo skupaj
tvorita OBA kabla, zunanji in notranji, ce imas oba, se vodilo na eni
strani zakljucuje na zadnji napravi na notranjem kablu na eni strani in
na zadnji napravi na zunanjem kablu na drugi strani vodila - torej
zunanji in notranji kabel tvorita celoto!!! Tu moras potem paziti na
dolzino kabla, ki jo deklarirajo na 9 cevljev oz malo manj kot 3m, pri
cemer moras paziti se na raznorazne dodatke kot so prikljucni konektorji
in adapteri (pri scannerjih, wide to narrov izvedbah, itd), ker vsaka
malenkost prispeva k dolzini (ne bos verjel mene je zajebaval en disk
prevec na vodilu pri dolzini). No da se vrnem h kontrolerjem.. ker je le
ena izmed naprav, mora biti torej terminiran ce se zakljucuje vodilo pri
njem. Obstajajo razlicne izvedbe kontrolerjev, take za samo notranje
povezave, take za samo zunanje in take ki imajo oboje, za namecek pa se
wide in narrow prikljucke (tu moras paziti, da ne izvedes Y vezave -
treh krakov vodila - ce imas oba konektorja na kontrolerju, se ne
pomeni, da smes kar tako vse gor prikljucevati!)
aja, pri terminaciji pazi, da ne pozabis kaki napravi omogociti
terminacije dandanasnji scsi diski imajo namrec sposobnost avto
terminacije, scannerje je pa ponavadi treba rocno terminirati, stari
scsi adapterji kot so aha154x imajo par uporovnih letvic na sebi, ki jih
po potrebi pobiramo dol, novejsi pa ze imajo avtoterminacijo - recimo
aha1510 itd
ok, malo premora, me ze prsti bolijo od tipkanja...
dalje..
obdelali smo ze terminacijo, dolzino in obliko vodila...
aja, standardi..
Standardi so fajn zadeve, zaradi katerih si lahko dosti pomagas sam
(recimo izdelava kablov, terminatorjev, adapterjev itd)
nastel ti bom recimo tri, stiri standarde (splosne), ki povedo, za
kaksen tip vodila gre in kaksen konektor uporabljajo
- narrow oz scsi-2 lahko tudi recemo
8 bitno vodilo
konektorji 50 pinski flat, 50 pinski mini-micro, 50 pinski centronics
- wide oz scsi-3
16 bitno vodilo
konektorji 68 pinski flat, 68 pinski mini-micro
- ibmjev (pustimo pri miru...)
- macintoshev oz future domain dsub 25 pinski konektor
v bistvu gre za scsi-2 z malo popravki, zdruzuje namrec nekatere
signale, uporablja se pri scannerjih
hmmm, kaj iams se za vedet...
no, se nekej sem ti hotel se napisat, samo sem ze pozabil... ze ni bilo
tako vazno
aja, vecino informacij sem dobil v news grupi comp.periph.scsi , tam je
par sijajnih fantov, ki potrpezljivo odgovarjajo na vec ali manj enaka
vprasanja v stilu, zakaj mi scsis ne dela?? (ponavadi je povsod
terminacija in dolzina kablov problem)
za nameceh ti prilagam se dve datoteki, v katerih so malo bolje opisani
standardi, pinlay-outi, konektorji in na koncu tudi terminatorji
(slednje sem moral sam spisati, ker nikjer nisem nasel sheme za 25pinski
terminator)
upam, da ti je kaj razcistilo pojme..
scsi je fajn zadeva, ko jo enkrat dojames..
p.s.
pazi na motnje, vecuina zunanjih kablov ima gor dusilke, da se znebijo
raznoraznih motenj - primer na neki lokaciji je delal sistem
aha1510+umax astra 1200s na dvometrskem zunanjem kablu super, ko se je
preselilo zraven nadometnega (kovinskega!) kanala je zadeva prenehala
delovati, pomagal je krajsi kabel
ajt
Bostjan
QUESTION: What are the pinouts for SCSI connectors?
ANSWER From: snively na scsi.Eng.Sun.COM (Bob Snively)
[ Edited and expanded by Gary Field (gfield na grcelect.ultranet.com) ]
====
Originally dated May 23, 1990
The connector families described by the drawings have standard
pin numberings which are described the same way by all vendors
that I have encountered. The SCSI-2 specification identifies the
standard numbering, using that convention. It happened to be
documented by AMP, but all the vendors use the same convention.
The following diagrams have the outline drawings of connector
sockets at the bottom. This is really for reference only, because
the connector sockets and plugs are both specified as to their
numbering and usually are labeled.
There are some minor problems in naming the microconnector conductor
pairs, which I have corrected in the enclosed diagram. All the conductor
pairs of the Mini-Micro (High Density) connector are in fact passed
through on the cables. SCSI-2 defines the RSR (Reserved) lines as
maybe ground or maybe open, but they are still passed through the cable.
Most present standard SCSI devices will ground those lines.
-------------------- microSCSI to SCSI Diagram ---------------------------
SCSI Connector Pinouts (single-ended)
_____________________________________ _____________________________________
| SCSI | | MINI | | | SCSI | | MINI | |
| SIGNAL| DD-50P | MICRO | DD-50SA | | SIGNAL| DD-50P | MICRO | DD-50SA |
------------------------------------ -------------------------------------
| -DB(0)| 2 | 26 | 34 | | GND | 1 | 1 | 1 |
| -DB(1)| 4 | 27 | 2 | | GND | 3 | 2 | 18 |
| -DB(2)| 6 | 28 | 19 | | GND | 5 | 3 | 35 |
| -DB(3)| 8 | 29 | 36 | | GND | 7 | 4 | 3 |
| -DB(4)| 10 | 30 | 4 | | GND | 9 | 5 | 20 |
| -DB(5)| 12 | 31 | 21 | | GND | 11 | 6 | 37 |
| -DB(6)| 14 | 32 | 38 | | GND | 13 | 7 | 5 |
| -DB(7)| 16 | 33 | 6 | | GND | 15 | 8 | 22 |
| -DB(P)| 18 | 34 | 23 | | GND | 17 | 9 | 39 |
| GND | 20 | 35 | 40 | | GND | 19 | 10 | 7 |
| GND | 22 | 36 | 8 | | GND | 21 | 11 | 24 |
| RSR | 24 | 37 | 25 | | RSR | 23 | 12 | 41 |
|TERMPWR| 26 | 38 | 42 | | OPEN | 25 | 13 | 9 |
| RSR | 28 | 39 | 10 | | RSR | 27 | 14 | 26 |
| GND | 30 | 40 | 27 | | GND | 29 | 15 | 43 |
| -ATN | 32 | 41 | 44 | | GND | 31 | 16 | 11 |
| GND | 34 | 42 | 12 | | GND | 33 | 17 | 28 |
| BSY | 36 | 43 | 29 | | GND | 35 | 18 | 45 |
| -ACK | 38 | 44 | 46 | | GND | 37 | 19 | 13 |
| -RST | 40 | 45 | 14 | | GND | 39 | 20 | 30 |
| -MSG | 42 | 46 | 31 | | GND | 41 | 21 | 47 |
| -SEL | 44 | 47 | 48 | | GND | 43 | 22 | 15 |
| -C/D | 46 | 48 | 16 | | GND | 45 | 23 | 32 |
| -REQ | 48 | 49 | 33 | | GND | 47 | 24 | 49 |
| -I/O | 50 | 50 | 50 | | GND | 49 | 25 | 17 |
----------------------------------------------------------------------------
* NC = NOT CONNECTED
CONNECTOR TYPES:
DD-50SA
________________________ MINI-MICRO
DD-50P | ------------------- | ______________________
______ ______ |17 \. . . . . . . . ./1 | | _________________ |
49| . . . . . . |1 | 33 \. . . . . . . ./18 | | 1\ - - - - - - - /25 |
50| . . . . . . |2 | 50 \. . . . . . ./ 34 | | 26\- - - - - - -/50 |
------------- | ------------- | | ------------- |
-------------------------- ----------------------
ribbon cable Old style Sun SCSI "SCSI-2"
male male
____________________
( 1 25 )
\ ++++++++++++++ /
\ 26 50 /
--------------
"Centronics" 50 male (use pin numbers for MINI-MICRO)
(VIEWED FROM FACE OF CONNECTOR - USE VENDOR NUMBERING SYSTEM AS SPECIFIED)
16 bit Wide SCSI-3 "P" (Primary) Connector pinout (single-ended)
_____________________ ___________________
| SCSI | HIGH DEN | | SCSI | HIGH DEN |
| SIGNAL | 68 PIN | | SIGNAL | 68 PIN |
-------------------- --------------------
| GND | 1 | | -DB(12)| 35 |
| GND | 2 | | -DB(13)| 36 |
| GND | 3 | | -DB(14)| 37 |
| GND | 4 | | -DB(15)| 38 |
| GND | 5 | | -DB(P1)| 39 |
| GND | 6 | | -DB(0) | 40 |
| GND | 7 | | -DB(1) | 41 |
| GND | 8 | | -DB(2) | 42 |
| GND | 9 | | -DB(3) | 43 |
| GND | 10 | | -DB(4) | 44 |
| GND | 11 | | -DB(5) | 45 |
| GND | 12 | | -DB(6) | 46 |
| GND | 13 | | -DB(7) | 47 |
| GND | 14 | | -DB(P) | 48 |
| GND | 15 | | GND | 49 |
| GND | 16 | | GND | 50 |
|TERMPWR | 17 | |TERMPWR | 51 |
|TERMPWR | 18 | |TERMPWR | 52 |
| RSRVD | 19 | | RSRVD | 53 |
| GND | 20 | | GND | 54 |
| GND | 21 | | -ATN | 55 |
| GND | 22 | | GND | 56 |
| GND | 23 | | BSY | 57 |
| GND | 24 | | -ACK | 58 |
| GND | 25 | | -RST | 59 |
| GND | 26 | | -MSG | 60 |
| GND | 27 | | -SEL | 61 |
| GND | 28 | | -C/D | 62 |
| GND | 29 | | -REQ | 63 |
| GND | 30 | | -I/O | 64 |
| GND | 31 | | -DB(8) | 65 |
| GND | 32 | | -DB(9) | 66 |
| GND | 33 | | -DB(10)| 67 |
| GND | 34 | | -DB(11)| 68 |
----------------- -----------------
____________________________
| _______________________ |
| 1\ - - - - - - - - - - /34 |
| 35\- - - - - - - - - -/68 |
| ------------------- |
----------------------------
"WIDE SCSI-3 P"
male
---------------------------------------------------------------------------
IBM's "Not really SCSI" connectors: [Editor(GF)]
Note that this connector is NON-COMPLIANT WITH ANY SCSI STANDARD!
60 pin Burndy connector as used on IBM RS/6000 systems:
Pin Signal Pin Signal
--------- ----------
1 Gnd 31 Gnd
2 -DB(0) 32 -ATN
3 Gnd 33 Gnd
4 -DB(1) 34 Gnd
5 Gnd 35 Gnd
6 -DB(2) 36 -BSY
7 Gnd 37 Gnd
8 -DB(3) 38 -ACK
9 Gnd 39 Gnd
10 -DB(4) 40 -RST
11 Gnd 41 Gnd
12 -DB(5) 42 -MSG
13 Gnd 43 Gnd
14 -DB(6) 44 -SEL
15 Gnd 45 Gnd
16 -DB(7) 46 -C/D
17 Gnd 47 Gnd
18 -DB(P) 48 -REQ
19 Gnd 49 Gnd
20 Gnd 50 -I/O
21 Gnd 51 Gnd
22 Gnd 52 Reserved
23 Gnd 53 Reserved
24 Gnd 54 Reserved
25 N/C 55 Reserved
26 TERMPWR 56 Reserved
27 Gnd 57 Reserved
28 Gnd 58 Reserved
29 Gnd 59 Reserved
30 Gnd 60 Reserved
====
ANSWER From: Gary Field (gfield na grcelect.ultranet.com)
Macintosh Plus SCSI Connector Pinouts
Note that this connector is NON COMPLIANT WITH ANY SCSI STANDARD!
The grounding is insufficient and does not allow for proper twisted-pair
transmission line implementation. It is recommended that a short adapter cable
be used to convert to the more common Centronics style 50 pin connection
rather than extend the 25 pin connection any further than necessary.
The Macintosh Plus used a NCR 5380 SCSI chip controlled by the MC68000
processor.
___________________
| SCSI | |
| SIGNAL| DB-25S |
+-----------------+ DB-25S (female)
| -DB(0)| 8 | _____________________________
| -DB(1)| 21 | 13\ o o o o o o o o o o o o o /1
| -DB(2)| 22 | 25\ o o o o o o o o o o o o /14
| -DB(3)| 10 | ------------------------
| -DB(4)| 23 | View from rear of computer.
| -DB(5)| 11 |
| -DB(6)| 12 |
| -DB(7)| 13 |
| -DB(P)| 20 |
| GND | 7,9,14 |
| GND |16,18,24 |
| -ATN | 17 |
| BSY | 6 |
| -ACK | 5 |
| -RST | 4 |
| -MSG | 2 |
| -SEL | 19 |
| -C/D | 15 |
| -REQ | 1 |
| -I/O | 3 |
+-----------------+
Pin 25 is NOT CONNECTED in the Mac Plus implementation. Newer Macs
connect TERMPWR to pin 25, but are otherwise the same.
Future Domain 25 pin connector pinout
Used on TMC-830/845 and TMC-850/860/885.
Note:
Use the Macintosh pinout above for TMC-850M, TMC-1610M, TMC-1650/1670 or MCS-600
___________________
| SCSI | |
| SIGNAL| DB-25S |
+-----------------+ DB-25S (female)
| -DB(0)| 14 | _____________________________
| -DB(1)| 2 | 13\ o o o o o o o o o o o o o /1
| -DB(2)| 15 | 25\ o o o o o o o o o o o o /14
| -DB(3)| 3 | ------------------------
| -DB(4)| 16 | View from rear of computer.
| -DB(5)| 4 |
| -DB(6)| 17 |
| -DB(7)| 5 |
| -DB(P)| 18 |
| GND |1,6,8,13 |
| GND |13,19,25 |
| -ATN | 20 |
| BSY | 23 |
| -ACK | 22 |
| -RST | 10 |
| -MSG | 21 |
| -SEL | 7 |
| -C/D | 11 |
| -REQ | 24 |
| -I/O | 12 |
+-----------------+
Pin 9 is NOT CONNECTED
QUESTION: SCSI terminators should measure 136 ohms?
ANSWER From: stevel na coos.dartmouth.edu (Steve Ligett)
====
Yes, that is what you should measure. Let's see how that is so. The
terminator contains 18 220-ohm resistors from signals to termpower, and
18 330-ohm resistors from those signals to ground. I've drawn that
below:
termpower--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| | | | | | | | | | | | | | | | | |
220 ohms-> R R R R R R R R R R R R R R R R R R
| | | | | | | | | | | | | | | | | |
signals -> o o o o o o o o o o o o o o o o o o
| | | | | | | | | | | | | | | | | |
330 ohms-> R R R R R R R R R R R R R R R R R R
| | | | | | | | | | | | | | | | | |
ground --+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
When you measure from any one signal to termpower, you aren't measuring
that resistor in isolation, you are measuring that resistor IN PARALLEL
with the combination of the corresponding 330 ohm resistor plus 17
220+330 ohm resistor pairs in series. I've redrawn the schematic to
make this easier to see:
termpower--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| | | | | | | | | | | | | | | | | |
| R R R R R R R R R R R R R R R R R <- 220 ohms
| | | | | | | | | | | | | | | | | |
| o o o o o o o o o o o o o o o o o
| | | | | | | | | | | | | | | | | |
| R R R R R R R R R R R R R R R R R <- 330 ohms
220 ohms R | | | | | | | | | | | | | | | | |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+-- ground
| |
| R <-- 330 ohms
| /
signal -> o-/
<our resistor><--------- other stuff that's in parallel ---------->
We're trying to measure that one resistor from a signal to termpower,
but there's a ton of other stuff in parallel. The resistance of that
"stuff" is 330 + 550/17 ohms (the 330 ohm resistor, in series with a
parallel combination of 17 550 ohm resistors). The general formula for
the equivalent of two resistances in parallel is r1*r2/(r1+r2).
Whipping out my trusty spreadsheet, I find that the "stuff" has a
resistance of about 362 ohms, and that in parallel with 220 ohms is
about 137 ohms.
====
QUESTION: Can someone explain to me the difference between 'normal' scsi
and differential scsi?
ANSWER From: ralf na wpi.WPI.EDU (Ralph Valentino)
====
"Normal" SCSI is also called "Single-ended" SCSI. For each signal
that needs to be sent across the bus, there exists a wire to carry it.
With differential SCSI, for each signal that needs to be sent across
the bus, there exists a pair of wires to carry it. The first in this
pair carries the same type of signal the single-ended SCSI carries.
The second in this pair, however, carries its logical inversion. The
receiver takes the difference of the pair (thus the name
differential), which makes it less susceptible to noise and allows for
greater cable length.
====
QUESTION: What are the pinouts for differential SCSI?
ANSWER From: ralf na wpi.WPI.EDU (Ralph Valentino)
====
Differential SCSI Connector Pinouts
_____________________________________ _____________________________________
| SCSI | | MINI | | | SCSI | | MINI | |
| SIGNAL| DD-50P | MICRO | DD-50SA | | SIGNAL| DD-50P | MICRO | DD-50SA |
------------------------------------- -------------------------------------
| -GND | 2 | 26 | 34 | | (open)| 1 | 1 | 1 |
| -DB(0)| 4 | 27 | 2 | | +DB(0)| 3 | 2 | 18 |
| -DB(1)| 6 | 28 | 19 | | +DB(1)| 5 | 3 | 35 |
| -DB(2)| 8 | 29 | 36 | | +DB(2)| 7 | 4 | 3 |
| -DB(3)| 10 | 30 | 4 | | +DB(3)| 9 | 5 | 20 |
| -DB(4)| 12 | 31 | 21 | | +DB(4)| 11 | 6 | 37 |
| -DB(5)| 14 | 32 | 38 | | +DB(5)| 13 | 7 | 5 |
| -DB(6)| 16 | 33 | 6 | | +DB(6)| 15 | 8 | 22 |
| -DB(7)| 18 | 34 | 23 | | +DB(7)| 17 | 9 | 39 |
| -DB(P)| 20 | 35 | 40 | | +DB(P)| 19 | 10 | 7 |
| GND | 22 | 36 | 8 | |DIFSENS| 21 | 11 | 24 |
| GND | 24 | 37 | 25 | | GND | 23 | 12 | 41 |
|TERMPWR| 26 | 38 | 42 | |TERMPWR| 25 | 13 | 9 |
| GND | 28 | 39 | 10 | | GND | 27 | 14 | 26 |
| -ATN | 30 | 40 | 27 | | +ATN | 29 | 15 | 43 |
| GND | 32 | 41 | 44 | | GND | 31 | 16 | 11 |
| -BSY | 34 | 42 | 12 | | +BSY | 33 | 17 | 28 |
| -ACK | 36 | 43 | 29 | | +ACK | 35 | 18 | 45 |
| -RST | 38 | 44 | 46 | | +RST | 37 | 19 | 13 |
| -MSG | 40 | 45 | 14 | | +MSG | 39 | 20 | 30 |
| -SEL | 42 | 46 | 31 | | +SEL | 41 | 21 | 47 |
| -C/D | 44 | 47 | 48 | | +C/D | 43 | 22 | 15 |
| -REQ | 46 | 48 | 16 | | +REQ | 45 | 23 | 32 |
| -I/O | 48 | 49 | 33 | | +I/O | 47 | 24 | 49 |
| GND | 50 | 50 | 50 | | GND | 49 | 25 | 17 |
----------------------------------------------------------------------------
Please note that I can only verify the DD-50P connector. The Mini
Micro and DD-50SA pinout above is a pin for pin mapping from the SCSI
pinout in the FAQ.
How to tell if you have a single ended or differential drive:
- Use an ohm meter to check the resistance between pins 21 & 22.
On a single ended system, they should both be tied together
and tied to GND. On the differential drive, they should
be open or have a significant resistance between them. Note
that most drives today are single ended so you usually only
have to worry about this with old drives scavenged from
other systems.
[ Editor(GF): The preceeding comment about differential drives being old
is not valid. Differential drives are less common than single-ended ones,
because they are mainly used only where longer cable runs are necessary,
and they are not generally used in PCs, but state of the art drives are
available with differential interfaces. Generally only the higher
performance drives have a differential option because of the added cost.]
SCSI-1 DB25 Terminator
--------------------------------------------------------------------------
| SCSI | |
| SIGNAL| DB-25S |
+-----------------+ DB-25S (male)
| -DB(0)| 8 | _____________________________
| -DB(1)| 21 | 1\ . . . . . . . . . . . . . /13
| -DB(2)| 22 | 14\ . . . . . . . . . . . . /25
| -DB(3)| 10 | ------------------------
| -DB(4)| 23 | View from face of terminator.
| -DB(5)| 11 |
| -DB(6)| 12 |
| -DB(7)| 13 |
| -DB(P)| 20 |
| GND | 7,9,14 |
| GND |16,18,24 |
| -ATN | 17 |
| BSY | 6 |
| -ACK | 5 |
| -RST | 4 |
| -MSG | 2 |
| -SEL | 19 |
| -C/D | 15 |
| -REQ | 1 |
| -I/O | 3 |
|TERMPWR| 25 |
+-----------------+
RESISTOR SCHEME
We have 18 signals, 6 GNDs and 1 TERMPOWER, We should connect all 6 GNDs togoether.
termpower--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| | | | | | | | | | | | | | | | | |
220 ohms-> R R R R R R R R R R R R R R R R R R
| | | | | | | | | | | | | | | | | |
signals -> o o o o o o o o o o o o o o o o o o
| | | | | | | | | | | | | | | | | |
330 ohms-> R R R R R R R R R R R R R R R R R R
| | | | | | | | | | | | | | | | | |
ground --+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
Use 9+1 SILs if possible (3 of each resistance for a single terminator).
For GND termination (we use 330ohm SILs):
You have to chop 3 resistors (pins) off from the end of the both SILs used on pins 1-13.
We put connect common junctions of SILs to pin 7 and chop off SIL pin at DB pin 9.
For pins 14-25, we connect the common junction SIL pin to DB pin 24, and chop off SIL
pins at DB pins 16 and 18.
For TERMPOWER termination (use 220ohm SILs):
For DB pins 1-13 we chop off one resistor from the end of one SIL and
5 resistors from the end of second upper SIL. We connect common junctions of
both upper SILs togoether over DB pin 9, but NOT connect to it.
Instead bend it back and connect it to DB pin 25 with a piece of wire.
For DB pins 14-25 just stretch the SIL that the common junction fits
DB pin 25 and chop off SIL pins over DB pins 16 and 18. The SIL pin above
DB pin 24 is stretched to DB pin 25 (the common junction SIL pin)..
This should work.
Let's do some measurment: (schemes at the end)
(*)Between signal pins and termpower pin, the resistance should be 136 ohms.
Between signal pins and GND pins, the resistance is 142 ohms.
Between GND pins and TERMPOWER pin, the resistance is 30 ohms (18x550 ohms parallel).
Between one signal pin and another signal pin, the resistance is arround 262 ohms.
(*) Explanation
When you measure from any one signal to termpower, you aren't measuring
that resistor in isolation, you are measuring that resistor IN PARALLEL
with the combination of the corresponding 330 ohm resistor plus 17
220+330 ohm resistor pairs in series. I've redrawn the schematic to
make this easier to see:
termpower--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
| | | | | | | | | | | | | | | | | |
| R R R R R R R R R R R R R R R R R <- 220 ohms
| | | | | | | | | | | | | | | | | |
| o o o o o o o o o o o o o o o o o
| | | | | | | | | | | | | | | | | |
| R R R R R R R R R R R R R R R R R <- 330 ohms
220 ohms R | | | | | | | | | | | | | | | | |
| +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+-- ground
| |
| R <-- 330 ohms
| /
signal -> o-/
<our resistor><--------- other stuff that's in parallel ---------->
We're trying to measure that one resistor from a signal to termpower,
but there's a ton of other stuff in parallel. The resistance of that
"stuff" is 330 + 550/17 ohms (the 330 ohm resistor, in series with a
parallel combination of 17 550 ohm resistors). The general formula for
the equivalent of two resistances in parallel is r1*r2/(r1+r2).
Whipping out my trusty spreadsheet, I find that the "stuff" has a
resistance of about 362 ohms, and that in parallel with 220 ohms is
about 137 ohms.
The same goes for resistor between gnd and a signal. 330 parallel with (220+550/17).
It comes about 142 ohms.
1 2 3 4 5 6 7 8 9 10 11 12 13 <- pins
136 136 136 136 136 136 30 136 30 136 136 136 136 <- ohms
_______________________________________________________________
30 136 30 136 30 136 136 136 136 136 30 | <- ohms
14 15 16 17 18 19 20 21 22 23 24 25 <- pins
middle line is just a common junction to a pin with | instead of resistance
1 2 3 4 5 6 7 8 9 10 11 12 13 <-pins
142 142 142 142 142 142 | 142 | 142 142 142 142 <-ohms
_______________________________________________________________
| 142 | 142 | 142 142 142 142 142 | 30 <-ohms
14 15 16 17 18 19 20 21 22 23 24 25 <-pins
1 2 3 4 5 6 7 8 9 10 11 12 13 <-pins
262 262 262 262 262 262 142 262 142 262 262 262 262 <-ohms
_______________________________________________________________
142 262 142 262 142 262 262 262 262 | 142 136 <-ohms
14 15 16 17 18 19 20 21 22 23 24 25 <-pins
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