Serial Tachometer: frequency meter utility

Updated 17-Feb-2011

Serial Tachometer is a freeware utility program for Microsoft Windows that uses standard PC serial port to measure pulse frequencies in range from 1 Hz to 11 kHz.

Principle of operation / Connecting to external hardware

The tachometer senses pulses on the RxD input of a standard PC serial port, making the PC hardware and device driver believe the pulses are valid characters by assessing average pulse width and "hunting" for an optimal baud rate. This imposes three important limitations:

• Detectable pulses must have sufficient amplitude, and certain polarity. Most serial ports accept anything from TTL levels (0 ... 5 V) to about ±15 V. The pulse must have positive polarity, i.e. active voltage is higher and quiet is lower. This corresponds to logical high (STOP bit) on RS-232C being a negative voltage, and logical low (START bit) being a positive voltage (when measured against signal ground GND).
• Each pulse width must be at least as wide as one bit at 115,200 bps (= 8.68 µs) and no wider than nine (9) bits at 75 bps (= 120 ms). The pulses must not vary in their width too quickly (baud rate steps up or down once a second as needed).
• Measurable frequencies lie between 1 Hz and 11 kHz. The lower limit is imposed arbitrarily by software design, and the upper limit is dictated by how many pulses (characters) can be handled (received) at 115,200 bps (the highest widely supported baud rate). Actual pulse frequencies may change rapidly (as long as pulse widths do not), and are shown averaged over a second, every second.

	It can be this primitive...

External hardware needs not to be sophisticated for the Serial Tachometer to work. If a mechanical contact is used to generate the pulses, only one resistor (almost any value >100 Ω will do), a few wires, and a cable with a female DB9 connector is all you need. Depending on the switch's "phase polarity", you need to select proper state of the DTR and RTS signal, respectively.

If the external circuit is galvanically connected to other hardware, it is a good idea to protect the RxD input with a resistor and Zener diodes or with an opto-isolator, as excessively high voltage may damage your computer. Even with diodes, serial port inputs don't usually survive direct connection to household mains (110 or 220 V AC). Since most modern computers come with serial ports integrated in their mainboard chipset, it's a whole mainboard that goes if you blow a port.

Although not designed for the purpose, DTR and RTS handshake outputs of a typical serial port can be used to power a low-consumption circuit (e.g. transistor side of an opto-isolator feeding the RxD input). The Serial Tachometer therefore supports custom setting of the handshake lines to desired polarity to provide suitable supply voltage. Again, it is a good idea to protect the powered circuit with serial diods, and the outputs from accidental shorting, by serial resistors, as state of the handshake lines is not guaranteed and can certainly change (resulting in inverted polarity) during boot or while running other applications accessing COM ports. Remember, typical output voltage on a handshake line is ±15 V, which can destroy chips designed to take only +5 V supply or lower.

Latest Public Release / Download / Installation / Removal

Download Release 1.1.14

You may also need Microsoft Redistributable libraries for VC9 Service pack 1

The application does not have an install/uninstall procedure. Simply copy the .exe file to a convenient utility directory in your system (or make one under Program Files) and run it from there. To replace an older version, simply override it with a new one.

To completely remove all traces of Serial Tachometer from a system, delete SerialTacho.exe. Then (using regedit or regedt32) delete the following key in the system registry:

HKEY_CURRENT_USER\Software\Hroch\SerialTacho

If the program was run under various users, you may have to remove registry entries from each user's profile.

Terms of use

The Serial Tachometer utility (hereinafter the Software) may be used and distributed by anyone free of charge under the following conditions:

1. Any use of the Software is at own risk of the user.
2. The author will not be liable for any consequences arising out of use or inability to use the Software for any purpose.
3. The Software and any associated documentation may not be modified in any way.
4. No third party may charge any fee for use and/or distribution of the Software.

The Serial Tachometer utility is Copyright (c) 2004-2011 by Sid Paral. All rights reserved.

System Requirements

• Intel Architecture (IA32) X86 - (Intel, AMD...; min 386) computer.
• A serial port that supports baud rates between 75 and 115,200 bps.
• Screen resolution at least 640x480 pixels, 16 colors
• Microsoft Windows 32-bit or 64-bit operating systems
• Tested on: XP (SP3), Windows 7 (Ultimate 64-bit)

Release Notes & History

Known issues and planned features

• Mousewheel pushbutton action as source of pulse data -- see Dynamometer Project.
• Switching baud rate during "hunting" may toggle output handshake lines, depending on device driver implementation. Make sure you protect external hardware from unexpected voltage/polarity if you use the line(s) to power control something.

Please e-mail the author with comments, suggestions, bug reports. Thanks.