company news about Modbus Gateways Evolution Types and Selection Guide

Imagine industrial instruments and sensors as silent messengers, generating vast amounts of data. The efficiency and accuracy with which this data reaches control centers depend heavily on communication protocols. Modbus, a widely adopted protocol in industrial settings, plays this pivotal role. However, traditional Modbus RTU faces limitations in speed and scalability, prompting the development of network-based Modbus TCP. To bridge the gap between these protocols, Modbus gateways have emerged. This article explores four distinct types of Modbus gateways—from basic protocol converters to configurable gateways—highlighting their evolution, advantages, drawbacks, and practical selection criteria for engineers.

A Modbus gateway is a device that converts Modbus TCP protocol to Modbus RTU/ASCII protocol. In industrial automation, Modbus is extensively used for data acquisition from instruments and sensors. However, traditional RS485-based Modbus RTU (or ASCII) protocols suffer from limited transmission speeds and scalability. To meet growing networking demands, Ethernet-based Modbus TCP was introduced. Modbus gateways serve as the bridge between Modbus TCP and RTU/ASCII devices, playing a crucial role in industrial automation systems.

The evolution of Modbus gateways reflects the industrial sector's pursuit of higher efficiency and flexibility in data transmission. From basic protocol conversion to multi-host gateways, storage gateways, and configurable gateways, each type addresses specific challenges. Understanding their strengths and weaknesses helps engineers select the optimal solution for their applications.

The earliest Modbus gateways provided basic data format conversion. For example, a Modbus RTU (RS485 terminal) command to read one register at address 00 using function code 03 would be: 01 03 00 00 00 01 84 0a , where 84 0A is the CRC checksum. The corresponding Modbus TCP command (network side) would be: 00 00 00 00 00 06 01 03 00 00 00 01 . Simple serial servers use a "transparent transmission" mode, sending the same data ( 01 03 00 00 00 01 84 0A ) to both serial and network ports. To achieve RTU-to-TCP conversion, the CRC checksum ( 84 0A ) and header ( 00 00 00 00 00 06 ) must be removed.

While this basic conversion met initial needs, it has significant drawbacks:

• No multi-host support: Multiple Modbus TCP hosts cannot access the gateway simultaneously. Data collisions occur when Host A and Host B transmit concurrently, corrupting RS485 bus communications.
• Response interference: Responses to Host A are also sent to Host B, causing data contamination.

Though largely obsolete, understanding simple protocol converters provides context for gateway development. In configuration tools like ZLVircom, selecting "Modbus TCP to RTU" as the conversion protocol and disabling "multi-host" and "RS485 collision prevention" in advanced options enables this mode.

Multi-host Modbus gateways address the limitations of simple converters. When Hosts A and B access the gateway simultaneously, these gateways implement bus arbitration to prevent collisions. Additionally, responses to Host A are not forwarded to Host B, eliminating interference.

Communication steps:

1. The gateway checks bus availability upon receiving a query from Host A. If idle, the request is sent to the RS485 bus; otherwise, it waits.
2. The gateway records the querying host (Host A).
3. When the RS485 device responds, the gateway converts the data to Modbus TCP format and forwards it exclusively to Host A.

Advantages:

• Enables concurrent multi-host access.
• Prevents bus collisions and response interference.

Drawbacks:

• Slow response times: Hosts must wait 50–100ms for data due to RS485 bus latency.
• Timeout risks with many hosts: Slower responses increase timeout likelihood in large networks.

Early models like ZLAN5142 defaulted to multi-host mode. For newer devices (e.g., ZLAN5143 firmware ≥1.565), this mode is enabled by setting "RS485 bus collision wait time" to ≤2 seconds in configuration tools.

Storage gateways optimize speed by pre-fetching and caching data. Instead of querying devices in real time, they store frequently accessed registers locally, reducing response times to 1–2ms.

Key features:

• Self-learning: Automatically records requested registers without manual configuration.
• Preemptive queries: Periodically polls learned registers to update cached data.

Advantages:

• Near-instant responses for cached data.
• Zero-configuration operation.

Limitations:

• First-query latency: Initial requests still require RS485 bus access.
• Device compatibility: Aggressive polling may overwhelm some instruments. Adjusting "RS485 idle time" mitigates this.
• Stale data: Cached values may lag by milliseconds.
• Timing issues: Disrupts sequential read-write operations in time-sensitive applications.

Storage mode is the default for modern gateways like ZLAN5143. Issues 2–4 can be resolved by reverting to multi-host mode, albeit with slower performance.

Configurable gateways (e.g., ZLMB gateways) eliminate first-query latency by predefining register maps. Unlike storage gateways, they require manual configuration but offer superior efficiency.

Advantages:

• Fast responses for all queries, including initial requests.
• Command consolidation: Maps disparate registers into contiguous TCP addresses, enabling single-query data aggregation.
• Address remapping: Reorganizes discrete registers into logical blocks.

Drawbacks:

• Requires manual setup; storage gateways are preferable for simple deployments.
• Inherits storage-related issues like stale data and timing constraints.

Models supporting configurable mode (e.g., ZLAN5143 firmware ≥1.579, ZLAN7144 firmware ≥1.491) use dedicated configuration tools to define register mappings.

Configurable gateways offer the highest efficiency but require setup. Storage gateways provide a balance of speed and simplicity, while multi-host mode remains a fallback for incompatible devices. Simple protocol converters are rarely used due to collision risks.