通信电缆选型

How this is calculated#

The tool picks the cable construction for a control or data link. It checks your run length against the limit for the interface. No AmpSize figure here comes from a printed Standard. The figures come from maker data and course notes, which the tool labels as secondary sources.

1. Interface. You pick 24 V d.c., 4–20 mA, RS-485, Ethernet or PROFINET. For RS-485 you also pick the data rate and the bus cable family.
2. Length limit. The tool reads the longest run for that interface. For RS-485 the limit falls as the data rate rises.
3. Length check. It compares your run with the limit. It reports the run as inside or over the limit.
4. Construction. It gives the pairs, the cores and the smallest core size. It says whether a screen is required, and checks the screen you enter.
5. Environment. It checks the sheath and the bend radius for a fixed or a flexing installation. For Ethernet it checks the category.
6. Termination. For RS-485 it gives the termination at each end and the unit loads per line.

Worked example#

• RS-485 at 187.5 kbps over 1200 m. The limit is 1,000 m, so the run is over the limit. The screen and environment checks pass. Add a repeater or shorten the run.

Limits#

• The figures are secondary source and maker data. They are not a Standard. Confirm each one with the device maker.
• The tool does not size power conductors. Use the DC cable size calculator for a 24 V supply.
• The tool does not check segregation from power cables. Use the segregation checker.
• The termination value suits typical cable. Match it to the cable you use.
• The 32 unit loads on an RS-485 line are not a device count.

See also: Control panels and PLC wiring, 24 V DC instrument loop voltage drop and Loop power.