Modbus Plus Protocol Integration and Legacy FieldServer Driver (FS-8700-02)
This page documents the FS-8700-02 FieldServer driver for Modbus Plus (Modbus+). Modbus Plus is a fieldbus used in many installed base industrial control systems, particularly in older PLC and SCADA deployments. In modern projects, Modbus Plus is most commonly encountered during retrofit, expansion, or migration work where existing Modbus Plus devices must remain operational while supervisory systems and IP infrastructure are updated.
The FS-8700-02 driver enables integration of Modbus Plus devices into a FieldServer-based solution, typically so that data can be mapped into other protocols or supervisory platforms. In practical terms, this driver is used when an engineer needs to preserve Modbus Plus communications while exposing points to a modern BAS/SCADA layer (for example BACnet/IP, Modbus TCP, or a data acquisition system).
From an implementation perspective, Modbus Plus differs from serial Modbus RTU and Ethernet-based Modbus TCP. It uses dedicated network hardware and a specific interface requirement on the gateway. That makes correct hardware selection and site validation an important part of scoping any Modbus Plus integration task.
FieldServer Driver – Fieldbus FS-8700-02 Modbus Plus
| Driver Code: | MB+ |
| Version: | 1.00 |
| Devices Supported: | Modbus+ devices |
| Interface: | SA85 (requires the 8915-01 Modbus Plus Single Port or 8915-19 Modbus Plus Dual Port Adapter Card) |
The driver is identified by the code MB+ and requires the SA85 interface, which in turn depends on the appropriate Modbus Plus adapter hardware. When reviewing a legacy site, it is important to confirm the existing Modbus Plus network wiring and topology, and to verify that the intended FieldServer platform supports the required adapter card arrangement.
Command/SubCommand Supported
| Command | 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) |
The supported command set is consistent with typical Modbus-style data access patterns (coils, discrete inputs, input registers, and holding registers). In an integration scenario, these commands are usually used to extract monitoring points (status, measurements, counters) and—where permitted by the system design—to write control values (setpoints, commands, enable/disable flags). Write capabilities should always be validated against the controlled equipment’s safety and operational requirements.
In addition to command support, one of the practical configuration considerations is station addressing and internal routing within the Modbus Plus network. Confirming addressing, device count, and overall network architecture is typically required before finalizing the point list and mapping.
User can configure FieldServer to handle Modbus Plus internal paths 1-8, each path can handle 254 Modbus Station addresses.
Modbus Integration Gateways and Tools
If you are planning a modernization effort or need to bridge legacy Modbus networks into BACnet/IP, Modbus TCP, or other supervisory systems, Chipkin maintains a set of Modbus-related integration resources, gateways, and tools.
Modbus Integration Gateways and Tools
When evaluating a Modbus Plus integration specifically, it is often helpful to confirm whether the broader project includes Modbus TCP, Modbus RTU, or other Modbus variants. Many sites operate mixed Modbus environments, and a consistent approach to register scaling, byte/word order, and point naming standards reduces commissioning time.
Frequently Asked Questions (FAQ)
Is Modbus Plus obsolete?
Modbus Plus is primarily found in existing installations rather than new designs. Many facilities continue to operate Modbus Plus networks reliably. The driver remains relevant for retrofit work where Modbus Plus devices must be maintained while higher-level systems are upgraded.
What hardware is required to interface with Modbus Plus using this driver?
The driver specifies an SA85 interface and notes that it requires either the 8915-01 Modbus Plus Single Port or 8915-19 Modbus Plus Dual Port adapter card. Confirm platform compatibility and physical interface requirements during project scoping.
What data objects can I read and write?
The supported commands include reading discrete outputs and inputs, reading output/input registers, and writing single or multiple coils/registers. Whether writing should be enabled depends on the application, equipment safety requirements, and the site’s control philosophy.
How many devices can be addressed?
The page states that FieldServer can be configured to handle Modbus Plus internal paths 1–8, and each path can handle 254 Modbus Station addresses. The practical device limit for a project depends on point count, update rates, and overall system architecture.
What information should I gather before commissioning?
At minimum, gather the Modbus Plus device list, station addresses, expected data (register/coil maps), and any requirements for scan rates. If the project includes protocol conversion into BACnet/IP or other systems, also define naming conventions, scaling rules, and any alarm/event handling expectations.