SCSI, Small Computer System Interface
| Speed: | Slow SCSI max 5Mbyte/s |
| | Fast SCSI min 5Mbyte/s |
| Length: | Single-ended SCSI max 6m |
| | Differential SCSI max 25m |
| Word length: | Narrow SCSI has 8-bit data transfer |
| | Wide SCSI has 16- or 32-bit data transfers |
An overview
| SCSI |
Speed (MBps) |
Bus Width |
Cable |
Pins |
ID's |
Connector |
|---|
| SCSI-1 |
1 to 5 |
8 |
A |
25 or 50 |
8 |
Sub-D25, Amphenol 50, Sub-D50 |
| SCSI-2 |
5 to 10 |
8 |
A |
50 |
8 |
Micro-D50 |
| SCSI-2 fast/wide |
up to 40 |
16 |
A&B |
50 |
8 |
Micro-D50 |
| SCSI-3 |
|
16 |
P |
68 |
32 |
Micro-D68 |
| SCSI-3 fast/wide |
|
32 |
P&Q |
68 |
32 |
Micro-D68 |
| SCSI-4? |
|
32 |
L |
110 |
|
|
General
SCSI stands for Small Computer System Interface. The name is misleading, because SCSI is primirally used in the larger machines, (486, pentium, mini's and mainframes). The SCSI interface is a parallel interface, which means the data is not send one bit after another, but several bits over several lines at the same time. The communication between devices is done by initiators and targets. An initiator is a device which requests something from a target. The initiator is most commonly a host-adapter in a computer.
The target is the one that takes the job and carries it out. Because of the definition of SCSI that a job is given by a initiator and then carried out by a target without the initiator knowing how the target is doing it and even not knowing when the job is done the roles of initiator and target may switch. As soon as the target is done with it's job it initiats the host, which will become target.
The purpose of all this is that as soon as the target knows what the initiator wants the bus will become free for other jobs to be send to other targets. The bus is used more economically. Instead of a computer waiting for data coming from a scanner, the bus can be used by the computer to read data from the harddisk.
SCSI is not like other parallel buses like centronics. SCSI has the capability to connect more than two devices to a bus. These devices may be targets or initiators. So it is possible for two hosts to share one tapestreamer, but it is also possible for one host to have acces to several harddisks. The identification of the devices is done by an ID. SCSI-1 and SCSI-2 have a maximum of 8 ID's and SCSI-3 has even 32 possible ID's.
There is no such thing as plain SCSI. There is SCSI-1, -2 and -3 and together with this there is Differential and Single-ended, and for the termination there is passive and active. Single-ended means that there is a ground and a signal wire. Much like in RS232. Differential on the other hand has no ground wire, but al signals have two wires, a positive and a negative one and the voltage difference between them carries the information (1 or 0). Much like RS422.
To make everything more complex the SCSI bus must be terminated to work properly, like in LAN networks. But LANs only use passive termination, which means a resistor is placed at the end and at the beginning of the cable, that's all. In SCSI there is also active termination, which means the termination is done by a voltage regulator and some resitors. This is for the Single-Ended interface. With differential SCSI live is easier. There is only passive termination. But it's not the same termination as for passive single-ended SCSI. And finally there is the difference between SCSI-3 and SCSI-2 wide. Both have 16 bytes transmissions, but SCSI-2 has only 50 wires and SCSI-3 has 68, so why take a, more expensive cable?
Due to the fact that SCSI-2 has a 50 wire cable and only 8 data lines there will be a low and high byte transmission. Each 16 bit word is split in a low and a high byte. These are transmitted one after the other and thus taking twice as long as 16 bit SCSI-3. This makes SCSI-3 faster and more economic.
Pinnings
 |  |
| Sub-D25 Male | Sub-D25 Female |
| Pin | Signal | Abbr. | Source
|
|---|
| 1 | | REQ
|
| 2 | | MSG
|
| 3 | | I/O
|
| 4 | | AST
|
| 5 | | ACK
|
| 6 | | BSY
|
| 7 | | GND
|
| 8 | | DB0
|
| 9 | | GND
|
| 10 | | DB3
|
| 11 | | DB5
|
| 12 | | DB6
|
| 13 | | DB7
|
| 14 | | GND
|
| 15 | | C/D
|
| 16 | | GND
|
| 17 | | ATN
|
| 18 | | GND
|
| 19 | Select | SEL
|
| 20 | | DBP
|
| 21 | | DB1
|
| 22 | | DB2
|
| 23 | | DB4
|
| 24 | | GND
|
| 25 | | N/C
|
Found on Mac Plus/SE/II.
Termination power (TERMPWR) is not provided by the Macintosh. Special precaution is necessary when considering a SCSI device for this port. Check with the SCSI device manufacturer to make sure the device provides termination power.
 |  |
| Sub-D50 Male | Sub-D50 Female |
 |  |
| Centronics 50 Male | Centronics 50 Female |
| Pin | Singel-Ended | Differential
|
|---|
| 1 | GND | GND
|
| 2 | GND | D0+
|
| 3 | GND | D1+
|
| 4 | GND | D2+
|
| 5 | GND | D3+
|
| 6 | GND | D4+
|
| 7 | GND | D5+
|
| 8 | GND | D6+
|
| 9 | GND | D7+
|
| 10 | GND | DPAR+
|
| 11 | GND | DIFFSENS
|
| 12 | Reserved | Reserved
|
| 13 | Open | TERMPWR
|
| 14 | Reserved | Reserved
|
| 15 | GND | ATN+
|
| 16 | GND | GND
|
| 17 | GND | BSY+
|
| 18 | GND | ACK+
|
| 19 | GND | RST+
|
| 20 | GND | MSG+
|
| 21 | GND | SEL+
|
| 22 | GND | C/D+
|
| 23 | GND | REQ+
|
| 24 | GND | I/O+
|
| 25 | GND | GND
|
| 26 | D0- | GND
|
| 27 | D1- | D0-
|
| 28 | D2- | D1-
|
| 29 | D3- | D2-
|
| 30 | D4- | D3-
|
| 31 | D5- | D4-
|
| 32 | D6- | D5-
|
| 33 | D7- | D6-
|
| 34 | DPAR- | D7-
|
| 35 | GND | DPAR-
|
| 36 | GND | GND
|
| 37 | Reserved | Reserved
|
| 38 | TRMPWR | TRMPWR
|
| 39 | Reserved | Reserved
|
| 40 | GND | GND
|
| 41 | ATN- | ATN-
|
| 42 | GND | GND
|
| 43 | BSY- | BSY-
|
| 44 | ACK- | ACK-
|
| 45 | RST- | RST-
|
| 46 | MSG- | MSG-
|
| 47 | SEL- | SEL-
|
| 48 | C/D- | C/D-
|
| 49 | REQ- | REQ-
|
| 50 | I/O- | I/O-
|
 |  |
| Micro-D50 Male | Micro-D50 Female |
| Pin | Singel-Ended | Differential
|
|---|
| 1 | GND | GND
|
| 2 | GND | D0+
|
| 3 | GND | D1+
|
| 4 | GND | D2+
|
| 5 | GND | D3+
|
| 6 | GND | D4+
|
| 7 | GND | D5+
|
| 8 | GND | D6+
|
| 9 | GND | D7+
|
| 10 | GND | DPAR+
|
| 11 | GND | DIFFSENS
|
| 12 | Reserved | Reserved
|
| 13 | Open | TERMPWR
|
| 14 | Reserved | Reserved
|
| 15 | GND | ATN+
|
| 16 | GND | GND
|
| 17 | GND | BSY+
|
| 18 | GND | ACK+
|
| 19 | GND | RST+
|
| 20 | GND | MSG+
|
| 21 | GND | SEL+
|
| 22 | GND | C/D+
|
| 23 | GND | REQ+
|
| 24 | GND | I/O+
|
| 25 | GND | GND
|
| 26 | D0- | GND
|
| 27 | D1- | D0-
|
| 28 | D2- | D1-
|
| 29 | D3- | D2-
|
| 30 | D4- | D3-
|
| 31 | D5- | D4-
|
| 32 | D6- | D5-
|
| 33 | D7- | D6-
|
| 34 | DPAR- | D7-
|
| 35 | GND | DPAR-
|
| 36 | GND | GND
|
| 37 | Reserved | Reserved
|
| 38 | TRMPWR | TRMPWR
|
| 39 | Reserved | Reserved
|
| 40 | GND | GND
|
| 41 | ATN- | ATN-
|
| 42 | GND | GND
|
| 43 | BSY- | BSY-
|
| 44 | ACK- | ACK-
|
| 45 | RST- | RST-
|
| 46 | MSG- | MSG-
|
| 47 | SEL- | SEL-
|
| 48 | C/D- | C/D-
|
| 49 | REQ- | REQ-
|
| 50 | I/O- | I/O-
|
 |  |
| Micro-D68 Male | Micro-D68 Female |
Single-Ended
| Pin | Signal | Pin | Signal | Pin | Signal
|
|---|
| 1 | GND | 24 | GND | 47 | D7-
|
| 2 | GND | 25 | GND | 48 | DPAR-
|
| 3 | GND | 26 | GND | 49 | GND
|
| 4 | GND | 27 | GND | 50 | GND
|
| 5 | GND | 28 | GND | 51 | TRMPWR
|
| 6 | GND | 29 | GND | 52 | TRMPWR
|
| 7 | GND | 30 | GND | 53 | Reserved
|
| 8 | GND | 31 | GND | 54 | GND
|
| 9 | GND | 32 | GND | 55 | ATN-
|
| 10 | GND | 33 | GND | 56 | GND
|
| 11 | GND | 34 | GND | 57 | BSY-
|
| 12 | GND | 35 | D12- | 58 | ACK-
|
| 13 | GND | 36 | D13- | 59 | RST-
|
| 14 | GND | 37 | D14- | 60 | MSG-
|
| 15 | GND | 38 | D15- | 61 | SEL-
|
| 16 | GND | 39 | DPAR- | 62 | C/D-
|
| 17 | TRMPWR | 40 | D0- | 63 | REQ-
|
| 18 | TRMPWR | 41 | D1- | 64 | I/O-
|
| 19 | Reserved | 42 | D2- | 65 | D8-
|
| 20 | GND | 43 | D3- | 66 | D9-
|
| 21 | GND | 44 | D4- | 67 | D10-
|
| 22 | GND | 45 | D5- | 68 | D11-
|
| 23 | GND | 46 | D6-
|
Differential
| Pin | Signal | Pin | Signal | Pin | Signal
| | 1 | D12+ | 24 | RST+ | 47 | D6-
|
| 2 | D13+ | 25 | MSG+ | 48 | D7-
|
| 3 | D14+ | 26 | SEL+ | 49 | DPAR-
|
| 4 | D15+ | 27 | C/D+ | 50 | GND
|
| 5 | DPARH+ | 28 | REQ+ | 51 | TRMPWR
|
| 6 | GND | 29 | I/O+ | 52 | TRMPWR
|
| 7 | D0+ | 30 | GND | 53 | Reserved
|
| 8 | D1+ | 31 | D8+ | 54 | ATN-
|
| 9 | D2+ | 32 | D9+ | 55 | GND
|
| 10 | D3+ | 33 | D10+ | 56 | BSY-
|
| 11 | D4+ | 34 | D11+ | 57 | ACK-
|
| 12 | D5+ | 35 | D12+ | 58 | RST-
|
| 13 | D6+ | 36 | D13+ | 59 | MSG-
|
| 14 | D7+ | 37 | D14+ | 60 | SEL-
|
| 15 | DPAR+ | 38 | D15+ | 61 | C/D-
|
| 16 | DIFFSENS | 39 | DPARH+ | 62 | REQ-
|
| 17 | TRMPWR | 40 | GND | 63 | I/O-
|
| 18 | TRMPWR | 41 | D0- | 64 | GND-
|
| 19 | Reserved | 42 | D1- | 65 | D8-
|
| 20 | ATN+ | 43 | D2- | 66 | D9-
|
| 21 | GND | 44 | D3- | 67 | D10-
|
| 22 | BSY+ | 45 | D4- | 68 | D11-
|
| 23 | ACK+ | 46 | D5-
|
Resources:
