Author: Christopher Gobok, Product Marketing Engineer, Mixed Signal Products, Linear Technology
Introduction
PROFIBUS-DP (Distributed Peripherals) fieldbus standards have been around for over two decades. However, the requirements of the physical layer are often unclear, leading to confusion in transceiver selection. Despite this ambiguity, PROFIBUS has become a highly successful fieldbus solution—possibly the most successful—with more than 50 million devices deployed worldwide.
When deploying a new system, it's crucial to ensure that the transceivers used are designed according to the latest and most accurate interpretation of the PROFIBUS-DP standard. The LTC2877 from Linear Technology addresses this need by following the latest IEC 61158-2 PROFIBUS-DP standard and incorporating an advanced retention mechanism to improve compatibility and reliability between master and controlled devices.
PROFIBUS-DP Basics
The faster and simpler PROFIBUS-DP standard was introduced in 1993, based on the slower and more complex PROFIBUS FMS (Fieldbus Message Specification). It also has a "younger brother" called PROFIBUS-PA (Process Automation), which uses Manchester Code Bus Powered (MBP) transmission and includes bus-powered functionality, making it ideal for hazardous environments. PROFIBUS-DP is currently the most widely used version due to its plug-and-play features, flexibility, and cost-effectiveness, making it suitable for a wide range of applications—from managing sensors and actuators in industrial plants to communicating with flow meters at railway stations.
PROFIBUS-DP supports communication over various media, including copper wire, fiber optics, and even infrared. However, the most common medium is twisted pair connected to TIA/EIA-485-A (RS485) transceivers. This makes sense given RS485’s high-speed differential signaling and robust performance over long distances in noisy industrial environments. In a linear topology, up to 30 slaves can be connected per segment using RS485 transceivers like PLCs. A hub or repeater can expand the network to connect up to 124 slaves. Each segment must be terminated at both ends with active terminators, and all slaves are hot-swappable, as each has a unique network address.
95% RS485, 5% Different
While PROFIBUS-DP uses most of the TIA/EIA-485-A (RS485) standard, there are key differences that are sometimes overlooked, leading to system issues. Not all RS485 transceivers and cables are compatible with PROFIBUS-DP, and vice versa. Differences in cabling, termination, signal naming, and drive requirements exist. Ignoring these can lead to performance problems or even device failure.
Although RS485 doesn’t specify cabling, 120Ω shielded twisted pair is commonly used. However, PROFIBUS-DP recommends 150Ω shielded twisted pair. This small impedance difference means different cables are often needed. Additionally, PROFIBUS-DP specifies maximum cable lengths depending on the baud rate, ranging from 1200m at 9.6kbps to 100m at 12Mbps.
Termination also differs. RS485 typically uses 120Ω resistors at both ends, while PROFIBUS-DP recommends a 171Ω termination network. This network includes two 390Ω bias resistors and a 220Ω termination resistor, creating an effective resistance of 171Ω. Though not perfectly matched to the 150Ω cable, this slight mismatch results in a small voltage bump at the receiving end, which is generally acceptable.
Another key difference is pin naming. In most RS485 transceivers, pin A is the non-inverting input/output, and pin B is the inverting one. However, PROFIBUS-DP defines the opposite. This discrepancy arises from ambiguities in the original TIA/EIA-485-A standard, and designers may interpret it differently. This can cause issues when mapping pins for mixed RS485 and PROFIBUS-DP projects.
The differential driver output voltage (VOD) is another area of confusion. RS485 requires a VOD between 1.5V and 5V, measured at the driver terminals. PROFIBUS-DP, however, requires a VOD of 4V to 7V, measured at the far end of the cable with proper terminations. Many RS485 transceivers only specify a minimum VOD, without a maximum, which may not meet PROFIBUS-DP requirements. Testing with a PROFIBUS-DP load is essential to ensure compliance.
Protecting PROFIBUS-DP
Noise, faults, ESD, EFT, and surges are common challenges in industrial environments. While the TIA/EIA-485-A standard offers limited guidance on protection, the LTC2877 provides exceptional built-in protection. Its receiver can handle a common-mode range of –25V to +25V, ensuring safe operation under harsh conditions. Replacing a standard transceiver with the LTC2877 can eliminate many field failures without requiring external protection components.
ESD protection is especially critical. The LTC2877 offers ±26kV HBM protection on bus pins, even when unpowered. It also provides ±52kV protection against ground voltage when not powered. These levels make it highly resistant to electrostatic discharges, a common issue in industrial settings.
Electrical fast transients (EFT) are another concern. The LTC2877 meets the highest IEC 61000-4-4 level (level 4), handling up to 2kV on bus pins. For lightning surges, external protection such as MOVs, TVS diodes, and GDTs is usually required. However, the LTC2877’s ±60V rating makes it easier to integrate external surge protection components effectively.
Conclusion
PROFIBUS-DP remains a dominant force in fieldbus communication due to its reliability, flexibility, and widespread adoption. Designing with the LTC2877 ensures compliance with the latest specifications and enhances system stability. While other transceivers may claim compatibility, they often fall short in addressing the subtle but critical differences between RS485 and PROFIBUS-DP. The LTC2877 not only handles these differences but also provides robust protection against a wide range of electrical threats. Building a PROFIBUS-DP prototype has never been easier with this advanced transceiver.
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