RS-232 communication with an Alicat
The RS-232 protocol is the default method of talking to Alicat devices. While instruments can also be configured using a variety of other protocols (RS-485, PROFIBUS, EtherCAT, Modbus RTU or TCP/IP, DeviceNet, PROFINET, and EtherNet/IP), the tried and true RS-232 communication standard remains the go-to method for commanding and reading Alicats.
The not-so-standard RS-232 standard
The RS-232 protocol was originally introduced over 50 years ago, in 1960. The primary reason RS-232 has survived so long is that it is a useful, low level, 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 subsequently developed, proprietary adaptations led to nonstandard pin assignments, connectors, and signal voltage levels. For example, the original specification called for a DB-15 connector, but in the last 30 years most RS-232 products have adopted a DB-9 connector (technically called DE-9M) instead.
How does it work?
The data being sent on RS-232 lines simply consists of positive (+) and negative (-) voltage pulses relative to a ground reference. A group of +/- pulses sent by one device is carefully timed by the receiving device, where it is then decoded using the connected hardware.
The RS-232 protocol is not used for character encoding, spacing, start bits, stop bits, bit order, error detection, bit transmission rate, etc. Rather, they are established by the user’s connected circuitry. This is usually in the form of a serial communication port and its associated chips and transistors.
The COM port makes 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. It is worth noting that in an RS-232 three wire system, you should not connect Tx to Tx or Rx to Rx.
The only pin that is connected directly is the ground pin, which gives both ends a common reference point from which to measure the pulses. 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 it is being received.
How Alicat uses RS-232
The RS-232 ‘standard’ is fairly loose, so how does Alicat use it? In addition to the ubiquitous DB-9 and the ‘standard’ DB-15 connector, Alicat can provide RS-232 communication on any offered connector. This includes DB-15, 6 pin industrial locking connectors, and the default 8 pin miniDIN jack.
Bending the rules for signal level and polarity
The real departure from the standard is found in how Alicat has exploited signal levels, enabling multiple units to work on the same COM port. Because Alicat devices neither accept nor produce negative voltages, a traditional +/- 15 V RS-232 is not possible. Fortunately, a positive only pulse of +5 V can be made to replicate an RS-232 waveform (logic high ‘mark’ vs. logic low ‘space’), readable by 99% of all UARTs used today.
UART serial ports are no longer used, and have been replaced by USB to serial converters. Today, the FTDI chipset is commonly used to replicate the COM port. Alicat’s unique signal profile is also fully compatible with these devices.
Using up to 26 units on a single serial COM port
Alicat has also designed a work-around enabling up to 26 units to be used at once on a single serial COM Port. The technical term for this capability is ‘multi-drop’ communication, and it is supported by all Alicat units equipped with serial communication (RS-232 or 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. Even though every device listens to the commands, a unit will only accept and respond if the instruction begins with that device’s unique ID letter. This means you can connect three flow controllers on the same three wires (electrically in parallel), and then read the current flow rate on unit ‘A’, give a new setpoint to ‘B’, and reset the totalizer on ‘C’.
Even though the RS-232 communication ‘standard’ is old enough for a place in the Smithsonian Institution, it is still heavily used for all types of computer based systems that need to 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.