SCSI Basics - Termination

The place to visit to learn tips on how to troubleshoot your machine.
User avatar
Posts: 6842
Joined: Thu Aug 17, 2017 7:40 pm
Location: Essex

SCSI Basics - Termination

Post by rubber_jonnie »

Small Computer System Interface, or SCSI for short, is a very versatile interface and can commonly be found on Atari ST era computers and PCs.

It can be used for hard drives, optical media, scanners, tape drives etc.

SCSI 1 usually allows for up to 8 devices, with SCSI IDs ranging from 0-7.

SCSI is an excellent interface, but has some rules that must be followed in order to run reliably and consistently.

The basic rules are as follows:

1. The Host Bus Adapter, or HBA, should be SCSI ID 7.
2. The drive you wish to boot from should be SCSI ID 0.
3. Any other devices attached to the bus may have an ID of anything between 1 and 6, and no two devices may have the same ID.
4. The ends of the SCSI bus must be terminated.

Rule 4 is probably the most important rule, because in my experience of working with SCSI as a field service engineer, nearly every problem I encountered was related to incorrect termination.

In typical configurations there are two main types in use:

1. A chain with the HBA at one end and a hard drive at the other.
2. A chain with a hard drive at one end, the HBA is in the middle, and other devices are at the other end of the chain, possibly external to the computer, and may be any type of SCSI compatible device.

In both types of bus, the rules for termination are the same. The device at the end of each chain MUST be terminated for correct operation. Note that terminators can be directly connected to the cables and the devices left unterminated as long as both ends of the bus are terminated.

Here is an example of the chain described in item 1:

Screenshot 2022-01-03 at 12.18.11.png
Screenshot 2022-01-03 at 12.18.11.png (1.72 MiB) Viewed 2175 times

It is a single chain with the HBA at one end and a hard drive at the other. Both the HBA and tape drive are terminated, the hard drive is not

Here is an example of the chain described in item 2, also known as a 'Y' or 'T' chain:

Screenshot 2022-01-03 at 12.16.21.png
Screenshot 2022-01-03 at 12.16.21.png (448.06 KiB) Viewed 2175 times

In this second example, we can see the HBA is in the middle, and it is unterminated. There are only two devices terminated, the internal drive at one end of the bus, and the scanner at the other end of the bus. All other devices MUST be unterminated for all the devices to work correctly.

Termination can be either passive, where a terminator is a simple resistor pack, and it uses a 5v supply usually derived from the HBA, or active, where the terminators have a voltage regulator to ensure the voltage to the resistor packs is always a consistent 5v and does not waver, even if the terminator power does.

Active termination is typically required where bus speeds are higher and improved termination is necessary to ensure correct operation at higher speeds.

Note, there are many different types os SCSI connectors in use, so it is up to you to identify the cables and terminators you require. Typical connectors are 50 pin IDC, Centronics type and high density 50 pin connectors, so it is important you understand the specific type you have.

Terminators can either plug into cables or devices, or may be a resistor pack that plugs into the device.

Some devices, HDDs in particular may have built in resistor packs and terminator power that are enabled by setting a jumper or switch, so you must be very careful when adding new devices in the middle of the SCSI bus to ensure that termination is not enabled.

In summary, many SCSI issues are caused by under/over termination. Follow the simple rule of terminating only the devices at the ends of the bus and you should have a trouble free time with SCSI.
Collector of many retro things!
800XL and 65XE both with Ultimate1MB,VBXL/XE & PokeyMax, SIDE3, SDrive Max, 2x 1010 cassette, 2x 1050 one with Happy mod, 3x 2600 Jr, 7800 and Lynx II
Approx 20 STs, including a 520 STM, 520 STFMs, 3x Mega ST, MSTE & 2x 32 Mhz boosted STEs
Plus the rest, totalling around 50 machines including a QL, 3x BBC Model B, Electron, Spectrums, ZX81 etc...