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PDComm Modbus features

The PDComm Modbus SDK ships as its own library (pdcomm_modbus_ffi) and executables, independent of the SECS/GEM SDK. An endpoint acts as a client, a server or a gateway, and you drive it entirely with JSON commands.

Components

Section titled “Components”
Component Description
C ABI include/pdcomm/modbus/pdcomm_modbus.h. Pass JSON commands to pdcomm_modbus_call.
C++ wrapper Endpoint in pdcomm_modbus.hpp, header-only.
Qt and MFC adapters QtEndpoint in bindings/qt, MfcEndpoint in bindings/mfc
.NET binding Endpoint in bindings/dotnet/PDComm.Modbus
Simulator GUI PDComm Modbus Simulator, with six workspace tabs for testing servers, clients and gateways
Headless simulator pdcomm-modbus-headless, driven by JSON on standard input or by scenario files
MCP server pdcomm-modbus-mcp, which exposes the same commands as MCP tools for AI tools

The GUI, the headless simulator, the MCP server and the SDK share the same service code, commands, model and project format. Each process is still its own session: starting the MCP server doesn’t attach it to a GUI that’s already running.

Supported features

Section titled “Supported features”
Area Support
Transports Modbus TCP, RTU, ASCII
Roles Client, server, TCP-to-RTU/ASCII gateway
Function codes 1, 2, 3, 4, 5, 6, 7, 8, 11, 12, 15, 16, 17, 20, 21, 22, 23, 24, 43 (MEI 14)
Diagnostics sub-functions 0, 1, 2, 3, 4, 10, 11, 12, 13, 14, 15, 16, 17, 18, 20
Security Modbus/TCP Security: encrypted, authentication_only, role-based authorization, CRLs
Server data model Four tables, file records, FIFO queues, device identification objects, server ID, exception status
Observation The last 1000 requests and frames (trace), synthetic frame decoding (fault), diagnostics counters
Projects Save and restore the configuration and data model as pdcomm.modbus.project version 1 JSON
Diagnostic log Optional JSON Lines log (PDCOMM_LOG), off by default

Sizes and timing

Section titled “Sizes and timing”
Item Value
PDU 253 bytes
ADU TCP 260 bytes, RTU 256 bytes, ASCII 513 characters
Configured unit 1–247
timeout_ms 1–60000
ascii_timeout_ms 1–60000, default 1000
turnaround_ms 0–60000, default 100
JSON command 16 MiB per SDK call, 1 MiB per headless or MCP line
trace Last 1000 entries
fault input 4096 bytes
TCP server connections 32 at a time; closed after 120 seconds without traffic
Gateway queue 64 requests; exception 06 when full

Endpoint behavior

Section titled “Endpoint behavior”
Item Behavior
TCP framing The TCP transport uses the MBAP header.
Addresses address takes IP:port. Your application resolves host names to IP addresses before it passes them.
Sending requests Each request is sent once. Your application decides whether to send a timed-out request again; check the device state before you repeat a write.
Server unit A server endpoint answers the unit in its configuration.
Threads The SDK runs on your application’s threads. A server or gateway answers while your application keeps sending poll.
Register values Registers are 16-bit integers. Your application combines two registers for a 32-bit value.
Default server exceptions The default server answers MEI 13 (CANopen) and the user-defined function codes (65–72, 100–110) with exception 01.
RS-485 direction control The operating system and the adapter switch the line direction.

Check the features of a build

Section titled “Check the features of a build”

Depending on the delivery profile, an SDK can include only some of TCP, RTU, ASCII and Security. Configuring a transport or security that the build lacks fails with Protocol is excluded from this build. Check at startup:

{"ok":true,"result":{"ascii":true,"canopen":false,"diagnostic_logging":true,"diagnostics":[0,1,2,3,4,10,11,12,13,14,15,16,17,18,20],"functions":[1,2,3,4,5,6,7,8,11,12,15,16,17,20,21,22,23,24,43],"logging_schema_version":1,"mei13":"opaque_extension","project_version":1,"protocol":"modbus","rtu":true,"security":true,"tcp":true}}

Check the function codes and addresses of the device you connect in its manual.

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