Comms Cable Selector
Pick a comms cable and check the run length.
Questions
Each answer describes what this tool calculates. The result is a design aid. Verify against the current standard.
What does the comms cable selector do?
It names the cable construction for a 24 V DC, 4-20 mA, RS-485, Ethernet or PROFINET link, and says whether the run length is inside the limit for that interface. It also checks the sheath, the screen and the bending radius against the environment.
How long can an RS-485 segment be?
The tool reports PROFIBUS DP segment lengths from a cable maker manual: 1000 m up to 187.5 kbps on standard bus cable, 400 m at 500 kbps, 200 m at 1.5 Mbps and 100 m from 3 Mbps up. Flexible and torsion cable runs shorter.
What does the RS-485 limit of 32 count?
Unit loads, not devices. One unit load draws 1 mA under a common-mode stress of 12 V. A receiver above 12 kilohm input resistance presents only part of one, so a line can carry more than 32 receivers.
What termination does RS-485 need?
The termination resistance equals the characteristic impedance of the line. The held source works to 120 ohm at each end, so match it to the cable you use. On a short line that counts as a lumped-parameter model, termination is not needed.
What length limit applies to 24 V DC and 4-20 mA?
Neither carries a published length. A 24 V DC run is limited by volt drop, so use the AmpSize voltage drop tool at 24 V. A 4-20 mA loop is limited by the loop burden and the transmitter supply voltage.