Generic Serial Tap

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The Generic Serial Tap plugin should be used together with a dual-COM serial tap device, such as EZ-Tap RS-232 from Stratus Engineering Inc. Alternatively, you can easily solder your own tap.

The idea is to read all the lines of an ongoing serial (RS-232/RS-422/RS-485) communications using a pair of auxiliary “spy” serial links. A pair is needed because half of the lines (TX, DTR and RTS) can only be written to — so we need yet another connection to read those, as well.

After everything is connected, you can follow the steps below to start monitoring inside IO Ninja.

Basic Setup

In IO Ninja, click the “New Session” dropdown and select a new “Generic Serial Tap” session.

_images/generic-serial-tap-getting-started.png

In the DTE and DCE fields, select the ports for your pair of auxiliary “spy” serial links.

Start capturing by clicking the “Capture” button.

Monitor traffic in the log.

Adjust settings as needed via the “Settings” button (see “Settings” section below for details).

Settings

Setting	Description	Default
DTE	Specify a DTE serial port to monitor.
DCE	Specify a DCE serial port to monitor.
Baud rate	The serial baud rate.	38400 bps
Data bits	Serial data bit count (word size). See available options.	8 bits
Parity	Serial parity check type. See available options.	None
Stop bits	Serial stop bit count. See available options.	1 bit
Flip DTE/DCE	Flip logical sides of the tap cable.	False
Read mode (Windows-only)	Advanced read request-specific options. See available options.	Wait for the 1st char
Read max inter-char interval (ms)	Maximum inter-character delay during read request.	10
Read parallelism (Windows-only)	Maximum number of read requests to issue in parallel. Having more than one pending read at a time helps with increasing read throughput when incoming data arrives in rapid streams (after filling one user buffer, the kernel can immediately switch to the next one without any waiting). Increasing this number beyond 4 usually won’t yield any extra performance gains.	4
Read block size (B)	The size of each individual read block submitted to the underlying transport.	4KB
RX buffer size (B)	The full size of the incoming data (RX) buffer. Affects read throughput.	16KB
Keep read block size	Don’t merge read blocks in RX buffer. Incoming data blocks coming in quick succession can be merged together so that IO Ninja writes them to log as a whole. When this option is set to `True`, blocks are written to the log without merging, i.e., exactly as they are received from the underlying transport..	False
RX buffer full notifications	Toggle warnings in log about the incoming data (RX) buffer getting full.	False
Serial setting changes	Toggle notifications about serial setting (baud rate, data size, parity, stop bits, flow control) changes in the log.	on
DTR/RTS changes	Toggle notifications about control line (`DTR`, `RTS`) changes in the log.	on
DSR/CTS/DCD/RI changes	Toggle notifications about status line (`DSR`, `CTS`, `DCD`, `RI`) changes in the log.	on
Serial line errors	Toggle warning about serial line errors (`PARITY`, `FRAMING` and `BREAK`) in the log.	on

Data Bit Options

Option
5 bits
6 bits
7 bits
8 bits

Parity Options

Option	Description
None	No parity bit is used
Odd	Odd parity (parity bit is set when the number of logical ones in the UART frame is odd).
Even	Even parity (parity bit is set when the number of logical ones in the UART frame is even).
Mark	Parity bit is present and always set.
Space	Parity bit is present and always clear.

Stop Bit Options

Option
1 bit
1.5 bits
2 bits

Read Mode Options (Windows-only)

Option	Description
Check COMSTAT.cbInQue	Prior to reading, check the status of the incoming buffer of the serial driver. Only read the number of bytes available there.
Wait for the 1st character	Read requests return as soon as possible (as soon as at least one byte arrives).
Interval-based	Read requests keep accumulating bytes until the inter-character delay exceeds the limit specified by “Read inter-char interval (ms)” or the buffer is full.

Note

Typically, it’s best to leave the Read Mode setting at its default (Wait for the 1st character), unless compatibility issues require a change. The default setting is optimal in most situations.