Communication

The communication items register, in advance, the connections the controller uses to exchange data with external equipment and servers. A registered connection is then selected from the action of the same name.

MQTT Client

Enter the host and port. The default port is 1883 without encryption and 8883 with SSL/TLS. Connection Timeout (3 seconds by default) is the longest the client waits for the broker connection; once it expires, the connection is treated as failed. Client ID must be unique within the broker and is generated automatically if left empty. If the broker requires credentials, turn on User Authentication and enter the user and password; turning on SSL/TLS Encryption adds the client certificate, client private key, and CA certificate, each registered by uploading the file. With encryption on, every message exchanged with the broker is encrypted.

A Will Message is what the broker publishes on your behalf when the connection drops abnormally; set its topic (e.g. clients/device1/status), message, QoS, and retain flag. QoS 0 sends without checking for a response, QoS 1 resends until acknowledged (so duplicates are possible), and QoS 2 guarantees exactly one delivery (QoS 1 by default). Higher numbers are more reliable but use more network capacity. With Retain Message on, the broker keeps the Will message, so a subscriber that connects after the disconnection receives it immediately; retaining an “offline” message, for example, tells every later subscriber the state of this client.

Caution Clients sharing a client ID on the same broker disconnect each other. Give each controller and each registered client a distinct ID.

Modbus

Type selects RTU Master (the default), RTU Slave, TCP Master, or TCP Slave. RTU takes a serial port with baud rate, parity bit, stop bit, and data bits; TCP takes an IP address and port (502 by default). A TCP slave sets Max Connections, the number of masters that may connect at once (16 by default).

Field	Shown for	Description
Serial Port	RTU Master, RTU Slave	The serial port the device is connected to; find it with the search button or type it in (e.g. COM3 on Windows, /dev/ttyACM0 on Linux).
Baud Rate (bps), Parity Bit, Stop Bit, Data Bits	RTU Master, RTU Slave	Serial line settings, which must match the equipment on the other end. Baud rate is 1200 to 921600 (9600 by default), parity is None (default), Odd, or Even, stop bit is 1 (default) or 2, and data bits are 5 to 9 (8 by default).
Unit ID	RTU Slave	This slave's own number (1 to 247, 1 by default), also called the slave ID or address. When several slaves share a line, the master picks one by this number.
IP Address	TCP Master	The IP address of the TCP slave to connect to (e.g. 192.168.0.50). A trailing :port (192.168.0.50:502) is handled automatically.
Connection Timeout	TCP Master	The longest wait for the TCP slave connection (3 seconds by default); once it expires, the connection is treated as failed.
Max Connections	TCP Slave	The number of masters that can be connected at the same time (1 to 255, 16 by default).
Register Size, Bit Size	RTU Slave, TCP Slave	The size of the register area and the bit area of the slave memory (1 to 256 each, 8 by default).
Register Mapping, Bit Mapping	RTU Slave, TCP Slave	Map a Numeric variable to each register address and a Digital variable to each bit address.

When running as a master, the Unit ID of the slave to talk to is not a setting; it is given per request in the Modbus Communication action.

Running as a slave, set the Register Size and Bit Size and map a variable to each address. Grablo's slave memory does not separate inputs from outputs: it has only a register area and a bit area. Coils and discrete inputs use the bit area, holding and input registers use the register area, and addresses start at 0.

OPC UA Client

Enter the host, the port (4840 by default), and the Response Timeout (10 seconds by default), which is the longest the client waits for the server to answer a request; once it expires, the connection is treated as lost. If the server requires credentials, turn on User Authentication and enter the username and password. Turning on SSL/TLS Encryption adds a certificate file, a private key file, an Encryption Method, and a Security Mode. Grablo ships default certificate and key files (cert.der, key.der) that are already filled in, so they can be registered with the server and used as they are; upload your own files to replace them.

Field	Value	Description
Encryption Method	Basic256Sha256	The default. Strong symmetric encryption with 256-bit AES, using SHA-256 hashing to verify integrity.
	Aes128_Sha256_RsaOaep	128-bit AES with SHA-256 and RSA-OAEP key exchange, balancing performance and security.
	Aes256_Sha256_RsaPss	256-bit AES with SHA-256 plus RSA-PSS signatures, suited to high security requirements.
Security Mode	Sign Only	Messages are signed with the private key but not encrypted. Used on trusted networks.
Security Mode	Sign and Encrypt	The default. Messages are signed with the private key and encrypted with the recipient's public key. Used where security is required.

Caution The encryption method and security mode must match the OPC UA server's configuration. A mismatch fails the connection.

User Defined Communication

Nine types are supported: serial, TCP client and server, UDP client and server, shared memory, WebSocket client, and Bluetooth client and server. Serial takes a port and line settings, TCP, UDP, and WebSocket take a host and port, shared memory takes a name and size, and Bluetooth takes a local HCI index; a Bluetooth client also takes the remote MAC address of the server to connect to.

Field	Shown for	Description
Serial Port	Serial	The serial port the device is connected to; find it with the search button or type it in (e.g. COM3 on Windows, /dev/ttyACM0 on Linux).
Baud Rate (bps), Parity Bit, Stop Bit, Data Bits	Serial	Must match the equipment on the other end. The choices and defaults are the same as for Modbus (9600, None, 1, 8).
Host	TCP Client, UDP Client, WebSocket Client	The IP address or hostname of the server to connect to (e.g. 192.168.0.10, myserver.local).
Port	TCP and UDP client and server, WebSocket Client	The port to connect to or listen on (3000 by default).
Connection Timeout	TCP Client, WebSocket Client	The longest wait for the server connection (3 seconds by default); once it expires, the connection is treated as failed.
Shared Memory Name, Shared Memory Size (bytes)	Shared Memory	The name must match the peer program (e.g. grablo_shm1); the size is 1 to 65535 bytes (256 by default).
Local HCI Index	Bluetooth Client, Bluetooth Server	The number of the Bluetooth adapter on the controller. Use the scan button to find adapters; with a single adapter, keep the default (hci0).
Remote MAC Address	Bluetooth Client	The MAC address of the Bluetooth server to connect to, in any of the forms AA:BB:CC:DD:EE:FF, AA-BB-CC-DD-EE-FF, or AABBCCDDEEFF.
Receive Timeout	All types except UDP and shared memory	10 milliseconds by default. See below.

Receive Timeout is how long to wait for the next byte after the last one arrived. Once it expires, no further data is assumed to be coming, which makes this value decisive when exchanging frame-based data. A shared memory name must match the other side's; an existing shared memory with that name is reused, and otherwise a new one is created.

On a Linux controller, the hciconfig command in a terminal or SSH session shows each Bluetooth adapter's HCI index (hci0, hci1) and MAC address (BD Address).

Terminal output of hciconfig listing the hci1 and hci0 adapters with their BD Address — Example hciconfig output. hci0 and hci1 are the HCI indexes and BD Address is the MAC address.