Termination - erichelgeson/BlueSCSI GitHub Wiki

[!IMPORTANT] If you are looking for BlueSCSI v2 documentation please go here: https://github.com/BlueSCSI/BlueSCSI-v2/wiki

Termination is tricky to get right in any SCSI chain. Here is what you'll need to know for BlueSCSI.

What is termination

SCSI termination is the process of preventing the reflection of electrical signals from the ends of SCSI buses in order to ensure reliable operation. This termination is done passively via the use of various resistors connected to the signal lines at the connector end or actively through the application of a small amount of electricity. If termination is not done, the data signals themselves can reflect back from the ends of the bus and cause various anomalies in the data because of the pulse distortion or it results in outright data loss.

Via: https://www.techopedia.com/definition/17304/scsi-termination

When to enable

Termination is required at the end of a chain of SCSI devices. If BlueSCSI is the only device, termination must be on. If BlueSCSI is in the middle of a chain then it must be off.

The most common scenarios usually require termination to be enabled.

How to enable

Termination is enabled by the two jumpers next to the resistor networks. For the termination to work it must be supplied with termination power on the SCSI bus. BlueSCSI v1.1a, v2 Laptop and v2 DB25 models do not provide termination power to the SCSI bus. BlueSCSI v2 Desktop model has an option to provide termination power if needed. Your host controller usually supplies this power, but may not. Check the Supported-Systems page for more information on your system.

Termination Issues

Termination issue manifest themselves as the drives not showing up even though the LOG.txt shows the drives and there are no blinking LED indicators.

Low power

The Pull-Up resistor network is driven off the 5v supplied to the BlueSCSI. If you experience issues try powering the the BluePill via the USB connector.

NOTE: Early Macintoshes share the same 5v rail on TERMPOWER and the molex connection - in these cases powering it via the Molex/Berg connector will not change the weak termination. You should recap your mac or power it externally.

If in doubt measure the 5v on the external SCSI Port, Internal Molex, and on one of the BlueSCSI 5v lines (careful!) - If the BlueSCSI is below ~4.7v then it will still power up but may be too weak for termination to work properly.

Lastly you can use another drive or active termination device to terminate the chain.