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Created Termination (markdown)

Eric Helgeson 2021-07-06 13:44:19 -05:00
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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
## How to enable
Termination is enabled by the two jumpers next to the resistor networks.
## 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 in usually require termination to be enabled.
## 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.