How do ALCMS devices speak Modbus?
Modbus is a simple question-and-answer protocol: a master asks a device for a value or sends it a command, and the device replies, over Ethernet or a serial RS-485 line.
Modbus is a widely used industrial protocol: a simple question-and-answer language that lets a master (client), such as an ALCMS server or PLC, read values from and write commands to devices (servers) such as CCRs and monitoring units. Each request names a device address, a function (read or write) and the data wanted, and the device sends back a response.
It runs over two common media. Modbus TCP carries the messages over Ethernet, so it can travel on the same fibre-optic network as the rest of the ALCMS. Modbus RTU carries them over a serial RS-485 line to several devices on one bus, each with its own address. A gateway or PLC often converts between the two. The values in a device are held in four data tables: coils, discrete inputs, input registers and holding registers.
Modbus is simple and well supported, but it has no built-in security or authentication, so anyone who can reach the network can send commands. It is therefore kept on the isolated control network and changed only by authorised engineers. Which protocol a CCR or PLC uses, and what each register means, is set by the manufacturer's register map and the project design documents, and some systems use other protocols instead.
Frequently Asked Questions
Q1What is Modbus?
Modbus is a simple, open industrial communication protocol in which a master sends a request to a device and the device sends back a reply. It is used to read values such as status and alarm states and to send commands, and it is supported by many PLCs, power supplies and monitoring units.
Q2What is the difference between Modbus TCP and Modbus RTU?
Modbus TCP sends the messages over an Ethernet network, normally using port 502, and relies on the network for error checking. Modbus RTU sends them over a serial line such as RS-485 in a compact binary frame that ends with a CRC error check. The functions and data are of the same kind, so a gateway can convert between the two.
Q3What are coils and registers?
They are the four data tables inside a Modbus device. Coils are single-bit outputs that can be read or written, discrete inputs are single-bit read-only inputs, input registers are 16-bit read-only values such as measurements, and holding registers are 16-bit values that can be read or written, such as settings.
Q4Why is a register map needed?
Modbus defines only how a value is requested, not what it means. The same register number can be a current, a status word or a setting, depending on the device. The manufacturer's register map and the project documents give the address, scaling and meaning of each value, and the ALCMS is configured to match them.
Q5Is Modbus secure?
Not by itself. Standard Modbus has no authentication or encryption, so any device that can reach the network can read or write values. It is therefore kept on a separate control network, protected from office and internet networks, with changes limited to authorised engineers under change control.
Q6Is Modbus the only protocol used in airfield lighting control?
No. Many systems use Modbus because it is simple, but manufacturers also use other industrial protocols and their own proprietary ones. The project design documents state which protocol is used on each link, and a gateway can translate between them where needed.
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