Modbus TCP / UDP

Modbus is the most widely deployed industrial communication protocol. It connects PLCs, energy meters, variable-speed drives, and sensors over Ethernet using a simple request/response model.

Modbus TCP communicates over TCP/IP — reliable, connection-oriented, suitable for the majority of installations.

Modbus UDP uses a connectionless transport that is faster but without delivery guarantees — suited for time-critical or read-only applications on reliable local networks.

Connection parameters

Parameter	Description
IP Address	IP address of the Modbus device
Port	TCP/UDP port — default 502
Unit ID	Modbus slave address (1–247). Use 1 for most devices; 0 for TCP-only devices
Polling interval	How often tags are read from the device (ms)
Byte Order	Little Endian — least significant byte first (e.g. 0x1234 → 34 12). Big Endian — most significant byte first (e.g. 0x1234 → 12 34)
Bit Order	Standard — bit 7 to bit 0. Reverse — bit 0 to bit 7
Max Data Length	Maximum number of words read in a single request. Reduce for devices with limited read capacity
Max Data Gap	Maximum allowed gap between data blocks read continuously. Used for optimization — leave unchanged unless needed
Max wait time	Maximum time to wait for a response from the device (ms). Increase for slow networks or devices
Single value write only	When enabled, uses FC5/FC6 (single write) instead of FC15/FC16 (multiple write)

Address mapping

DataTalk uses a short addressing format. Addresses start from 0 (not from 1 as in Modicon notation):

Area	Modicon range	Access	Prefix	Example
Discrete Outputs (Coils)	00001–09999	Read/Write	O:	O:0 → 00001
Discrete Inputs	10001–19999	Read only	I:	I:0 → 10001
Input Registers	30001–39999	Read only	R:	R:0 → 30001
Holding Registers	40001–49999	Read/Write	H:	H:0 → 40001

Multiple devices on one line

When multiple devices share the same IP (e.g. RS-485 to Ethernet converter with multiple meters), each device is identified by its Device ID set in the tag’s address field in the Tag Database.

Example: four power meters on one converter — assign Device IDs 1, 2, 3, 4 per meter.

Batch optimization

Batch optimization groups multiple tags into a single Modbus request, reducing communication overhead and improving throughput:

Setting	Description
Max tag count	Maximum number of tags grouped into one request
Max wait time	Maximum time to collect tags before sending the batch

Larger batches mean fewer requests and better efficiency, but may add slight latency for individual tags.