The RS-232 Communication Protocol and your Alicat instrument

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The 55-year-old RS-232 protocol is the default method of talking to Alicat devices. Aside from RS-232, you could configure your instrument with RS-485, Profibus DPV1, ModBus RTU, DeviceNet, and Ethernet IP protocols. For the bulk of our customers however, the tried and true RS-232C communication standard will remain the go-to method for commanding and reading Alicat devices.

The Not-So-Standard RS-232 Standard

One of the main reasons that RS-232 has survived for over five decades is that it is a useful, but low level and rudimentary signal, with fairly loose operational guidelines. In 1962, the sole application for the RS-232 communication standard was connecting electromechanical typewriters and their host mainframes/modems, commonly known as ‘Teletype’ systems.

When more advanced electronic machines were developed subsequently, proprietary adaptations led to nonstandard pin assignments, connectors and signal voltage levels. For example the original specification called for a DB-25 connector, but in the last 30 years most RS-232 products have adopted a DB-9 connector (technically called DE-9M).

The ‘data’ being sent on RS232 lines are simply positive (+) and negative (-) voltage pulses relative to a ground reference. A group of +/- pulses sent by one device are carefully timed by the receiving device and decoded into whatever the hardware settings deem to be data bit packages. In other words, the RS-232 standard only defines a relatively loose general electrical framework to transmit and receive electrical pulses. What one does with all these pulses is ultimately up to the connected hardware. Things like character encoding, spacing, start bits, stop bits, bit order, error detection, bit transmission rate, etc. are not the responsibility of the RS-232 scope, and are established by the user’s connected circuitry, usually in the form of a serial communication port and its associated chips and transistors.


It’s the COM Port’s job to make sense of the pulses on behalf of the attached computer or peripheral. For reference, an RS-232 system must transmit from one device (sent on its Tx pin), to a receiving device (received on its Rx pin), and vice-versa. Do not try to connect Tx to Tx or Rx to Rx in an RS-232 three wire system! The only pin that is connected directly is the ground pin, which gives both ends a common reference point to measure the pulses from. Each RS-232 driver uses inversion logic and employs a single ended, bi-polar output voltage to feed to a UART (Universal Asynchronous Receiver/Transmitter). Because the system has three wires and two distinct channels of communication, it is considered a “Full Duplex” system. Data can be transmitted at the same time as it is being received.

RS-232, the Alicat Way

Understanding how loose the RS232 “standard” really is, you might be wondering how Alicat uses it. Alicat does offer the ubiquitous DB-9 or the “standard” DB-25 connector, but we can provide RS232 communication on any connector that is offered, such as DB-15, 6 pin industrial locking connectors, and of course the default 8 pin miniDIN jack.

Alicat RS-232 Specifications

However, our real departure from the standard, is how Alicat has exploited signal levels and allowed for multiple units to work on the same COM Port. Because Alicat devices neither accept nor produce negative voltages, a traditional +/- 15V RS232 is not possible. Fortunately, a positive only pulse of +5V can be made to replicate an RS232 waveform (logic high ‘mark vs. logic low ‘space’), readable by 99% of all UARTs used today.

Once UART serial ports went out of fashion in the early 2000’s, USB to serial converters took their place; most today use the FTDI chipset to replicate the COM Port. Alicat’s unique signal profile is also fully compatible with these devices.

In addition to bending the rules for signal level and polarity, Alicat has also designed a clever work-around for being able to use up to 26 units at once on a single serial COM Port. The technical term for this capability is called ‘Multi-Drop’ communication, and is supported by all Alicat units equipped with serial communication (whether RS-232, or the differential signal based RS-485). Through multi-drop communication, every device on the line is configured to have a unique identifying letter (A-Z), and every unit listens to the commands that have been sent. However, even though each device ‘listens’ to each command, a particular unit will only accept and respond to the command if the instruction begins with that instrument’s unique ID letter.

So you can read the current flow rate on unit “A”, give a new setpoint to the MFC “B”, and reset the totalizer on unit “C”; all while being hooked together on the same three wires (electrically in parallel).

Even though the RS-232 communication ‘standard’ itself is old enough for a place at the Smithsonian Institution, it is still heavily used today for all types of computer based systems that talk to various peripheral components. Employing the universal ASCII (American Standard Code for Information Interchange) character set as our language, Alicat instruments will continue to be sold with the robust, universal, and reliable RS-232 system for the foreseeable future. For basic connection and terminal examples please see our video and instructional here:

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