Delta RMC101 - Manual (Page 498)

RMC100 and RMCWin User Manual
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• For RS-485 (4-wire, point-to-point or multi-drop), terminate the receivers of each end device in
the chain, and the transmitter of the last slave (but not master).
Termination is not required on all differential networks, but it does typically extend the maximum
cable length. The following chart shows the maximum cable lengths at various baud rates with
and without termination:
Note: The maximum cable length is the length of the entire network and not just the distance
between nodes on the network.
Termination vs. Cable Length:
Baud Rate
Max
Unterminated
Cable Length
(ft)
Termination
Requirements Max Cable
Length (ft)
115,200 475 Required
beyond 475 ft 3250
57,600 950 Required
beyond 950 ft 4000
38,400 1900 Required
beyond 1900 ft 4000
19,200 3750 Required
beyond 3750 ft 4000
9,600 4000 Not required 4000
4,800 4000 Not required 4000
2,400 4000 Not required 4000
Cable Length Derivation
The values presented in the chart above are based on 24AWG cable with capacitance of 16 pF/ft and
the following reasoning. Signals travel through a cable at approximately 66% of c or 0.66 ft/ns. It is
assumed that a signal transition will dampen out after three round trips in the cable. This damping
must occur before the bit is sampled or within half a bit time. One bit time is equal to the reciprocal of
the baud rate.
Example:
Compute the cable length for 115,200 baud RS422.
First, we compute a half bit time at this baud rate.
Half
Bit
Time
= 0.5 * 1 / 115200
= 4,340 ns
Next, we convert this time to the distance the signal would travel in this time, assuming a speed
of 0.66 ft/ns as described above: