FS-8704-03 – Modbus TCP Driver for FieldServer Gateways
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The FS-8704-03 Modbus TCP driver enables FieldServer and QuickServer gateways to exchange data with Modbus TCP/IP devices over standard Ethernet networks. This driver implements the Modbus TCP/IP protocol as defined by the Modbus Application Protocol Specification V1.1a and operates over TCP/IP using Ethernet connectivity.
This page summarizes the Modbus TCP driver based on its official protocol driver datasheet and expands on the technical details to provide additional context for engineers and system integrators evaluating or deploying the driver in real-world systems.
Driver Description
The Modbus TCP driver allows a FieldServer to transfer data to and from Modbus TCP/IP devices using Ethernet. Communication is performed using TCP/IP, with the driver using TCP port 502 by default. This port is configurable as required by the application.
The driver can emulate both a Modbus TCP client and a Modbus TCP server. It is capable of operating in both roles simultaneously on the same Ethernet port. When configured as a client, the driver supports multiple concurrent polls, allowing efficient communication with multiple Modbus TCP servers.
The Modbus TCP driver also supports Modbus TCP/IP broadcast messages and multiple server message handling. These capabilities allow it to function effectively in larger Ethernet-based Modbus networks where multiple devices may be exchanging data concurrently.
FieldServer Operating Modes
The driver supports both client and server operating modes, depending on how the FieldServer is configured within the system architecture.
When operating as a client, the FieldServer initiates communication by polling Modbus TCP servers for data. In multidrop configurations, only a single client node is permitted. When operating as a server, the FieldServer responds to requests from external Modbus TCP clients.
In server mode, up to 255 nodes are supported. Actual usable node counts may vary depending on electrical loading and network conditions.
Connection Facts
| FieldServer Mode | Nodes | Comments |
| Client | 1 | Only 1 client node allowed on Multidrop systems |
| Server | 255 | Actual electrical loading may reduce number of usable server nodes |
Addressing and Register Mapping
Modbus devices from different vendors often use different register addressing conventions. To accommodate these variations, the Modbus TCP driver supports multiple register mapping models.
The supported addressing models include Modicon 5-digit addressing, ADU (Application Data Unit) addressing, and PDU (Protocol Data Unit) addressing. Each model allows the user to select the addressing format that most closely matches the register documentation provided by the device manufacturer.
These addressing options affect how register numbers are interpreted when constructing Modbus requests, but they all rely on the same underlying Modbus TCP protocol behavior.
Compatibility
The Modbus TCP driver is compatible with a wide range of FieldServer, QuickServer, and ProtoCessor platforms. Supported platforms include ProtoCessor, ProtoCarrier, ProtoNode, ProtoAir, and multiple QuickServer model families.
This broad compatibility allows the driver to be used in both new installations and retrofit projects where existing Modbus TCP devices must be integrated into a multi-protocol system.
| FieldServer Model | Compatible |
| ProtoCessor | Yes |
| ProtoCarrier | Yes |
| ProtoNode | Yes |
| ProtoAir | Yes |
| QuickServer FS-QS-10xx | Yes |
| QuickServer FS-QS-12xx | Yes |
| QuickServer FS-QS-20xx | Yes |
| QuickServer FS-QS-22xx | Yes |
| QuickServer FS-QS-3x10-F | Yes |
Connection Information
Communication is performed over Ethernet using standard TCP/IP networking. The driver supports Ethernet speeds of 10Base-T and 100Base-T.
Because communication is Ethernet-based, the driver can operate across standard network infrastructure such as switches and routers, subject to normal TCP/IP networking rules.
Devices Tested
The Modbus TCP driver has been tested with a range of Modbus-capable devices and software platforms. Tested devices include Quantum PLCs, Fix Intellution, Wonderware InTouch, and GE Cimplicity. Testing has been performed both at the factory and in customer installations.
Other Modbus TCP devices may also be compatible. For unlisted devices, confirmation is typically performed during integration or by contacting the factory.
| Device | Tested (FACTORY, SITE) |
| Quantum PLCs | Customer |
| Fix Intellution | Factory |
| Wonderware Intouch | Factory |
| GE Cimplicity | Customer |
| Others | Contact factory |
Supported Function Codes
The driver supports a standard set of Modbus function codes used for reading and writing discrete values and registers. These include functions for reading discrete inputs, reading and writing holding registers, forcing single or multiple coils, reporting slave ID, exception status, and FIFO operations.
Support for these function codes allows the driver to interact with a wide range of Modbus TCP devices that expose digital points and register-based data.
| Function Codes | Description |
| 01 | Read Discrete Output Status (0xxxx) |
| 02 | Read Discrete Input Status (1xxxx) |
| 03 | Read Output Registers (4xxxx) |
| 04 | Read Input Registers (3xxxx) |
| 05 | Force Single Coil (0xxxx) |
| 06 | Preset Single Register (4xxxx) |
| 15 | Force Multiple Coils (0xxxx) |
| 16 | Preset Multiple Registers (4xxxx) |
| 17 | Report Slave ID |
| EX | Exception Status |
| FF | FIFO |
Supported Data Types
The Modbus TCP driver supports multiple data types commonly used in Modbus devices. Supported types include digital values, signed and unsigned integers, 32-bit floating-point values, 64-bit integer values, and double-precision floating-point values.
This range of data types allows the driver to represent both simple discrete points and more complex numeric values provided by Modbus TCP devices.
| Data Type | Comments |
| ASCII | 8-bit character |
| Digital | Digital |
| Float | 32-bit IEEE floating point |
| Long | Unsigned 32-bit integer |
| Signed | Signed 16-bit integer |
| Slong | Signed 32-bit integer |
| Unsigned | Unsigned 16-bit integer |
| Input_Reg_64bit | Long input data type |
| 64Bit_Reg | Long data type |
| Input_Double | Double-precision 64-bit IEEE 754 floating point input data type |
| Double_Reg | Double-precision 64-bit IEEE 754 floating point data type |
FAQ
What does the FS-8704-03 driver do?
It enables FieldServer and QuickServer gateways to communicate with Modbus TCP/IP devices over Ethernet.
Can the driver act as both client and server?
Yes. The driver can operate as a Modbus TCP client, a Modbus TCP server, or both simultaneously on the same Ethernet port.
Which Ethernet speeds are supported?
The driver supports 10Base-T and 100Base-T Ethernet connections.
Is register addressing flexible?
Yes. The driver supports multiple addressing models to align with different vendor register conventions.