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The main gateway connects to the slave gateway via a pigtail

Fiber and communications infrastructure knowledge brief Published 28 Aug 2023

Prepared for preliminary research by the ROGER FIBRE knowledge desk. Confirm optical interfaces, port or core capacity, electrical input and output, enclosure protection, thermal design and installation limits against current standards and product documentation.

A pigtail connection between a main gateway and a slave gateway typically uses a short RS-485 or serial cable to enable master-slave communication, allowing the main gateway to control or exchange data with the slave gateway.

Physical Connection

In industrial networks, a pigtail is a short cable that connects two devices, often used to bridge a main gateway (master) to a slave gateway or device. This connection usually involves twisted-pair RS-485 wiring, where the two lines (A and B) carry differential signals. Differential signaling ensures high noise immunity, stable communication in harsh environments, and reduced errors from ground potential differences, which is critical for reliable master-slave communication.

Role of the GatewaysMain Gateway (Master): Initiates communication, sends requests, and manages data flow to one or more slave gateways or devices. It may also handle protocol translation if connecting different network types, such as Modbus TCP/IP to Modbus RTU. Slave Gateway: Responds to requests from the main gateway and may interface with additional slave devices.

It acts as a bridge to extend the network or integrate devices that use different protocols.

Communication Mechanics

The pigtail allows the main gateway to transmit Modbus RTU frames to the slave gateway. The slave interprets these frames and responds accordingly. If the slave gateway connects to other devices, it can forward requests or aggregate data, effectively acting as a secondary bridge.

The main gateway uses addressing logic to ensure that messages reach the correct slave device on the network.

Practical Considerations

Cable Type: Use twisted-pair cables with a characteristic impedance of ~120 ohms for RS-485 networks. Termination: Proper termination resistors at both ends of the RS-485 bus reduce signal reflections. Length: Pigtails are typically short to minimize signal degradation, but RS-485 supports long-distance communication if needed.

Protocol Translation: If the main gateway is on TCP/IP and the slave gateway is on serial RTU, the gateway handles conversion transparently. In summary, the pigtail provides a reliable physical link for master-slave communication, enabling the main gateway to control, monitor, or exchange data with the slave gateway while maintaining signal integrity and protocol compatibility.

The main gateway connects to the slave gateway via a pigtail - ROGER FIBRE ATELIER [PDF]

The main gateway connects to the slave gateway via a pigtail

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Technical note

This reference is intended for preliminary fiber and communications infrastructure research. Optical interface, fiber type, port or core capacity, electrical input and output, enclosure protection, cooling, installation limits and applicable standards must be verified for the specific project.

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