SCSI termination is a property of the complete bus, not a feature you choose for one cable in isolation. The rule is simple but often applied incorrectly: terminate the two physical ends of the bus and do not terminate devices in the middle. The hard part is identifying those endpoints when the host, internal backplane, external enclosure, and multiple devices form one electrical path.
First, draw the complete SCSI path
Before changing switches or adding a terminator, draw the chain from one physical end to the other. Include the host adapter, internal cable, internal drives, external connector, external cable, enclosure backplane, and every attached device. A connector on the outside of a chassis does not necessarily mark the electrical end of the bus.
In a simple external chain, the HBA may terminate the host end, and the last external device or an attached terminator may terminate the far end. In a combined internal and external arrangement, termination settings can change because the internal and external sections share one bus.
The endpoint rule
| Position | Termination state | Reason |
| First physical endpoint | Enabled | Absorbs signal energy at one end of the transmission line |
| Intermediate device | Disabled | Termination in the middle divides the bus and can disturb communication beyond that point |
| Last physical endpoint | Enabled | Completes termination at the opposite end of the bus |
Some HBAs, enclosures, and backplanes provide automatic termination. Others use a setting, jumper, removable resistor pack, or external terminator. Confirm the behavior in the equipment documentation rather than assuming that an unused connector is automatically terminated.
Passive, active, and LVD termination are not interchangeable labels
Passive termination
Traditional passive termination uses resistor networks tied to the termination power rail. It was used on early, slower single-ended buses, but voltage variation makes it less suitable as bus speed and complexity increase.
Active termination
Active termination uses regulated voltage and precision resistance to control the termination point more consistently. Equipment documentation commonly recommends active termination for faster single-ended systems. Do not mix active and passive terminators on the same bus unless the equipment manufacturer explicitly supports that configuration.
LVD and multimode termination
LVD buses require termination designed for LVD operation. Multimode LVD/SE terminators can sense the bus mode, but the cable and every device must still support the resulting electrical configuration. An old passive single-ended terminator is not a substitute for the termination required by an Ultra160 or Ultra320 LVD chain.
Four common topologies
| Topology | Likely endpoint A | Likely endpoint B | What you must verify |
| One internal chain | HBA/controller | Last internal connector, drive or cable terminator | Whether the cable has an integrated terminator and whether the last drive termination is enabled |
| One external device | HBA | External device or attached terminator | Whether both the HBA and device provide termination |
| Several external devices | HBA | Last device only | All intermediate devices must have termination disabled |
| Internal plus external devices | One end of the complete combined bus | Opposite end of the complete combined bus | The HBA may be in the middle and may need its termination disabled |
Symptoms that can point to a termination problem
Incorrect termination can contribute to intermittent detection, parity or CRC errors, timeouts, bus resets, reduced negotiated speed, or devices disappearing under load. Those symptoms are not proof of a termination fault: damaged connectors, excessive total bus length, incompatible signaling, poor cable construction, and duplicate SCSI IDs can create similar behavior.
When troubleshooting, change one controlled variable at a time. Record the original switches and jumpers, power down equipment as required by its documentation, and verify the topology before you replace parts.
A termination check you can perform before ordering
- List every device, adapter, and cable segment in physical order.
- Mark the two actual electrical endpoints.
- Record which component terminates each endpoint and whether it is automatic, switchable, or external.
- Identify the signaling mode: SE, HVD, LVD, or multimode LVD/SE.
- Add all internal and external cable lengths and compare the total with the equipment-specific limit.
- Check the device manuals before changing terminators or connecting unlike signaling types.
How cable selection fits into termination
A replacement SCSI-2 cable must match the narrow or wide interface and the equipment’s signaling requirements. A 68-pin SCSI cable may be used in several Wide SCSI modes, so the connector alone does not tell you which terminator or length limit applies. An internal SCSI ribbon cable may include a dedicated end terminator or depend on a drive or backplane setting. Review the applicable SCSI cable length limits before assuming termination is the only issue.
For a custom BespokeCable assembly, send the chain diagram, connector types, signaling mode, required length, equipment models, and termination arrangement. That information lets us review the cable construction and connector combination as part of the complete bus, not as an isolated pin-to-pin assembly.
Conclusion
Correct SCSI termination begins with topology. Identify the two endpoints, use termination appropriate to the electrical mode, disable intermediate termination, and follow the equipment limits for the total bus. If the chain is undocumented, document it before ordering a replacement cable or changing a terminator.
