Non Standard Serial

Hell all, I have a tap project where I am trying to monitor a serial connection that operates at 11 bits, 1 start bit, 9 data, and 1 stop.

I am getting a TON of framing errors (understandably) and wanted to see if there is a way to setup the tap to operate with these serial settings.

Thanks!

Hello,

The Serial Tap for IO Ninja (and the majority of standard USB-to-Serial adapters) does not support 9-bit UART communications. The upcoming Serial Tap Pro (to be released early next year) will support this feature natively, but in the meantime, here’s a workaround.

First, some background. The most common use case for the 9th bit in UART is marking the address byte in some industrial automation protocols. This important optimization enables slave nodes to inspect only the first (address) byte; if the address does not match, the slave can safely ignore all subsequent bytes until the next address byte (i.e., the next UART frame with the 9th bit set).

In these scenarios, the 9th bit is clear for most payload bytes and set only for address bytes. To monitor such communications using a Serial Tap (or a USB-to-RS485 adapter), use the settings as follows:

Baud rate: <specify-the-correct-baud-rate>
Data bits: 8
Parity: Space
Stop bits: 1

With these settings, address bytes will generate PARITY errors (the parity is set to "space," but the parity bit is 1), while the rest of the traffic will remain free of line errors.

Reporting of line errors is inherently asynchronous, so the PARITY errors won't necessarily align precisely with each address byte (BTW, Serial Tap Pro will address this, too, and line errors will be byte-precise). Still, you should see a PARITY error somewhere around the address byte.

Hope this helps; let me know if this workaround works for you!