Serial Tap

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Note

This page is for the Serial Tap Tap board. For the plugin, see the plugin docs.

Serial Tap is an affordable serial sniffer that can monitor RS232, RS48, and TTL-level UART communications. The Tap connects to your PC via a USB cable and is compatible with IO Ninja terminal/sniffer software.

Features

• Serial Tap is designed for use with IO Ninja software (http://ioninja.com/).

• Monitors TX, RX, RTS, CTS, DTR, and DSR lines [1] of serial ports.

• All inputs are connected to an 18-position, quick-release terminal block.

• Has three modes: RS232, RS485 [2], and UART (TTL [3]), selectable via a switch:

  • RS232 mode:

    • Two onboard DB9 connectors — male and female — offer true “cable wedge” sniffing [4].

    • Six jumpers for swapping and loopbacking signals within RX/TX, RTS/CTS, and DTR/DSR pairs [5].

    • Six bi-color LEDs indicate the status of monitored lines and allow distinguishing between the positive voltage (red), negative voltage (green), and zero voltage (off) states [5].

    • DB9 connectors additionally pass through DCD and RI signals (without monitoring them).

  • RS485 mode allows to monitor TX and RX signal pairs.

  • UART (TTL) mode allows to monitor UARTs of IC chips with logic levels from 3.3V to 5V.

• Full-speed (12Mbps) USB2.0 interface on a USB-C connector.

• Six yellow LEDs indicate the state of six monitored lines as they enter the USB controller. LEDs turn on when the lines go LOW [6].

• USB-powered, no additional external power necessary.

• Supplied with a USB-C cable and two DB9 gender changers.

• Compact, outside dimensions only 82 x 74 x 30 mm.

Block Diagram

Wiring Diagrams

RS485

RS232 (terminal block)

RS232 (DB9)

UART (TTL)

Wedge RS232 Monitoring

The Serial Tap allows you to insert (wedge) it in between two RS232 devices. Here is how this is done.

Let’s suppose that two serial devices are interconnected by a serial cable. Let’s also suppose that the first device has a DB9-F connector, while the second device has a DB9-M connector. The serial cable is, therefore, of the M-to-F type.

To wedge the Serial Tap between these two devices, you will need the second M-to-F cable:

Of course, DB9 genders on the serial devices may differ from the above example, and so your serial arrangement may be different. To aid you in the “wedging process,” each Serial Tap comes with two DB9 gender changers.

Jumper Pairs

The Serial Tap has an additional useful feature allowing you to swap and loopback the signals in TX/RX, RTS/CTS, and DTR/DSR signal pairs. To achieve this, two jumpers are provided for each of the three pairs. There are three standard jumper configurations:

• Normal — In this position, the lines are arranged in such a way that wedging the Tap between the serial devices does not change anything. Meaning, TX on one end goes to RX on another end, and vice versa.

• Swapped — This swaps signals in a pair. Meaning, TX goes to TX, and RX goes to RX.

• Loopbacked — Both serial devices “receive back” their own signals. Meaning, the TX line on each side “comes back” through the RX line.

The following diagram illustrates the jumper arrangements. The diagram shows the jumpers for the TX and RX signal pair. RTS/CTS and DTR/DSR jumpers work in the same way.

Common Mistakes

When setting up Serial Tap for RS485 communication, it’s crucial to avoid common wiring mistakes that can lead to malfunction or ineffective monitoring.

Miswiring Differential Pairs

A frequent error involves connecting the D+ and D− lines from one device to the TX+ and TX− inputs of Serial Tap, and the D+ and D− lines from another device to the RX+ and RX− inputs. This configuration assumes that Serial Tap will forward data between these lines, which it does not. Serial Tap is designed solely for passive monitoring and does not facilitate data forwarding between connected devices.

To correctly monitor RS485 communication, connect the D+ and D− lines from the RS485 bus directly to the corresponding monitoring inputs on Serial Tap. Ensure that the connections are made in parallel with the communication lines, allowing Serial Tap to observe the data without interfering with the bus operation.

Inappropriate Use of DB9 Connectors

Another common misuse is attempting to connect RS485 devices equipped with DB9 connectors, such as FTDI RS485 adapters, directly into Serial Tap’s DB9 ports. Serial Tap’s DB9 connectors are not designed to interface directly with RS485 devices in this manner. Doing so can result in improper operation or even damage to the equipment.

Instead, use appropriate breakout cables or adapters that match the pinout and electrical characteristics required for safe and effective monitoring. Always refer to the Serial Tap documentation and the RS485 device specifications to ensure compatibility and correct wiring practices.

Using the Serial Tap

Please see the documentation for the Serial Tap plugin.

Specifications

The Serial Tap is supplied with a USB cable and two DB9 gender changers.

Hardware Specifications [7] [8]

Parameter	Specification
USB port	USB 2.0, full-speed (12 MHz), USB-C connector
Maximum baudrate	2,764,800 bps
Operating temperature	0 to +60 degrees C
Operating relative humidity	10–90%
Mechanical dimensions	82 x 74 x 30 mm

Footnotes

[1]

A more accurate version of this statement would be: “Monitors two TX, two RTS, and two DTR lines,” but that would be somewhat confusing to the reader. For clarity, we chose to list the lines as they are commonly known: TX, RX, RTS, CTS, DTR, and DSR. Keep in mind, however, that when it comes to sniffers, these names are relative and depend on the view. For two interconnected serial devices, a TX line on one end is an RX line on the other end. If so, which line is a “TX” and which line is an “RX” from the sniffer’s point of view? Only you can answer this question. The Serial Tap itself has two inputs capable of monitoring serial data lines. Although they are both identical inputs, one is marked “TX” and another one is marked “RX.” The same goes for other signals — RTS, CTS, DTR, and DSR. All these lines are inputs of the Tap. We gave them “regular” names to make the use of the Serial Tap intuitively easier. The same goes for RS485 lines. RX+/- and TX+/- pairs are both inputs of the Serial Tap. This said, the Serial Tap incorporates two DB9 connectors, for which the distinction between TX and RX lines, RTS and CTS lines, and DTR and DSR lines is real. To give our terminology an anchored point of view, we have decided that the DB9-M connector on the left of the Tap is our “primary side” RS232 connector. For the entire board, all signal names are consistent with the standard pin assignment of this primary DB9-M connector. Of course, all “anchoring” disappears as soon as you abandon DB9s and plug into the terminal block lines.

[2]

These inputs can also be used for monitoring RS422 communications.

[3]

“TTL” here means “digital signals of microcontrollers, microprocessors, and other ICs.” TTL inputs of the Serial Tap are also compatible with CMOS circuitry. Most importantly, “TTL” serial signals are usually inverted with respect to RS232 signals: a LOW (negative voltage) on an RS232 line corresponds to a HIGH level on a serial TTL (CMOS) line, and HIGH (positive voltage) on an RS232 line corresponds to a LOW level on a serial TTL (CMOS) line.

[4]

TX, RX, RTS, CTS, DTR, and DSR lines of the primary DB9 connector located on the left of the Serial Tap are connected in parallel with RS232 inputs on the terminal block. This can be seen on the Block Diagram of the Serial Tap.

[5] (1,2)

Jumpers and bi-color LEDs on RS232 lines give the Serial Tap the second use as a simple mating and signal checking tool for serial devices.

[6]

This statement pertains to the latest revision D of the product. On the earlier (and now obsolete) revision C, yellow LEDs used to turn on when the lines went HIGH.

[7]

All specifications are subject to change without notice and are for reference only. Tibbo assumes no responsibility for any errors in this Manual and does not make any commitment to update the information contained herein.

[8]

Is the data you are looking for missing from the specifications table? Do not just assume that you know the answer — talk to Tibbo! Remember, that the ultimate responsibility for all decisions you make regarding the use and the mode of use of Tibbo products lies with you, our Customer.