Specifications

23
Schneider Electric
Choice of the Physical
Communication Medium (continued)
Example of a choice of medium
b Characteristic impedance: 120 Ω .
b Attenuation: a minimum voltage of 0.2 V on the receiver is required for a minimum
voltage of 1.5 V at the transmitter output:
v Thus attenuation will be:
- Att = 20 Log (Vout/Vin); Max. att. = 17 dB over 1200 m.
v Calculation of maximum network length:
- length (in m) = 1000 x [1-(1- α / α c)]
α : 14 dB/km
α c: attenuation of cable used (in dB/km)
b Resistance per unit length: the application guide for standard EIA485 (TSB89)
stipulates that maximum resistance should in no case exceed 390 Ω (for the
conditions defined in this guide and at the given use temperature).
v therefore, length (in m) = 1000 x [1-(1-R/Rl)]:
R: 0.325 Ω /m
Rl: Resistance per unit length of the cable used (in Ω /m).
b Capacitance between conductors:
v rise time or decay time for the signal Tr <0.3 Tui, where Tui (Time unit interval)
= 1/Br (Baud rate).
Transmission times are between 10% and 90% of the total amplitude of the signal
Vss (approximately 2
τ , where τ =Rl x Cl)
Therefore Cl < 0.3 / (Br x Rl x 2).
b Temperature of use: > 70°C (Maximum ambient temperature in the enclosure).
Application with an RS485 link at 19200 Bds on a 1000 m bus
b Characteristic impedance: 120 Ω.
b Cable attenuation = 20 dB/km at 19200 bds:
v length = 1000 x [1-(1-14/20)],
v maximum length = 700 m,
v since this length is incompatible with the specified 1000 m bus, a cable with a
maximum attenuation of 14 dB/km is required.
b Resistance per unit length: 0.213 Ω/m (at the temperature of use):
v length = 1000 x [1-(1-0.325/O.213)],
v maximum length = 1525 m.
b Capacitance between conductors:
v Cl < 0.3/ (19200 x 213 x 2),
v Cl < 36.6 nF, i.e. < 36.6 pF/m.
b Temperature of use: > 70°C.
Example: BELDEN ref 9842 for applications RS232/RS485.
Cabling Guidelines