Profibus DP Rules at a Glance
This page summarizes some key Profibus DP network design rules that are considered in the simulator. Adhering to these rules is crucial for a stable network.
Segment Lengths vs. Baud Rate
The maximum permissible length of a copper Profibus DP segment decreases as the baud rate increases. Exceeding these lengths can lead to signal degradation, attenuation, and communication errors.
| Baud Rate (kbps/Mbps) | Max. Segment Length (meters) |
|---|---|
| 9.6 kbps | 1200 m |
| 19.2 kbps | 1200 m |
| 45.45 kbps | 1200 m |
| 93.75 kbps | 1200 m |
| 187.5 kbps | 1000 m |
| 500 kbps | 400 m |
| 1.5 Mbps (1500 kbps) | 200 m |
| 3 Mbps (3000 kbps) | 100 m |
| 6 Mbps (6000 kbps) | 100 m |
| 12 Mbps (12000 kbps) | 100 m (often 50m recommended in practice) |
Note: These are typical values for Profibus DP Type A cable. Actual lengths can also depend on cable quality, number of devices, and environmental noise.
Stub Lines (Spur Lines)
Stub lines are branches off the main segment. They introduce impedance mismatches and can cause reflections, especially if unterminated or too long. Their use is restricted, particularly at higher baud rates.
| Baud Rate | Max. Individual Stub Length | Max. Total Stub Length (All stubs combined) |
|---|---|---|
| Up to 187.5 kbps | Up to 20 m (use with caution) | ~50-100 m (depends on segment length) |
| 500 kbps | Up to 6.6 m | ~10-15 m |
| 1.5 Mbps | Up to 6.6 m (often 0m recommended for direct device connection) | 6.6 m |
| ≥ 3 Mbps | 0.3 m (effectively no stubs, or extremely short pigtails) | 0.3 m (effectively no stubs) |
General Stub Line Rules:
- Stub lines should always be kept as short as possible.
- At baud rates of 1.5 Mbps and above, it's strongly recommended to avoid stub lines altogether or use active components like repeaters or hubs to create new segments.
- Unterminated stubs are a major source of problems. If a stub is unavoidable, it should ideally be terminated if its length is significant (e.g., > 0.5m - 1m, depending on baud rate).
Termination
- Every Profibus DP segment **must** be terminated at both physical ends with active termination resistors.
- Termination consists of a resistor network (typically 220 Ohm, 390 Ohm, 390 Ohm) that provides a defined impedance and bias voltage to keep the bus in a defined idle state.
- **Missing Termination:** Leads to strong signal reflections, making communication unreliable or impossible.
- **Double Termination (Over-Termination):** If termination is active in the middle of a segment (e.g., on a device that's not at an end, or on a connector with termination switched on), it can severely attenuate the signal.
Other Important Considerations
- Number of Devices: Max 32 devices (including repeaters, masters, slaves) per segment. Up to 126 devices total on a network (addresses 0-125, with some reserved). Repeaters are needed to extend beyond 32 devices or to exceed segment length limits.
- Cable Type: Use specified Profibus cable (e.g., Type A, shielded twisted pair, characteristic impedance ~150 Ohms).
- Shielding: Proper shield connection to ground at one or multiple points (depending on grounding philosophy) is critical for EMC immunity. A broken shield makes the network highly susceptible to noise.
- Grounding: A consistent grounding concept is essential to avoid ground loops and potential differences.
- EMC/Noise: Running Profibus cables near high-power lines, motors, VFDs, or other sources of electromagnetic interference can induce noise and corrupt signals.
-
Short Circuits:
- Line A or B to Ground: Will pull the respective line to ground potential, severely distorting the differential signal and likely halting communication on that segment.
- Line A to Line B: Will create a low impedance path, collapsing the differential voltage and halting communication.