Delta RMC101 User Manual
Page 499
Serial (RS-232/422/485) 5.5
Communications
5-249
Distance
=
4,340 ns *
0.66 ft / ns
=
2890 ft
Since it requires three round trips for the signal transition to dampen and each round trip is twice
the length of the cable, the total distance in feet is divided by six to get the final unterminated
cable length:
Length
=
2890 ft / 6
=
482 ft
This value is then rounded down to allow for inexact cable velocities and damping rates, giving us
475 ft.
Biasing
RS-422 and RS-485 indicate a binary 1 when the A line is at least 200 mV negative with respect
to B, and a binary 0 when A is at least 200 mV positive with respect to B. It is important that the
lines always be in a known state, not only when being driven. Biasing forces the network into a
known state when the lines are idle and therefore otherwise not driven.
Hardware revisions 2 and later of the RMC SERIAL contain a 68 pF capacitor in series with the
120W termination resistor. This keeps the resister from loading the network when the network is
idle, and maintains a known state without biasing. However, the software selectable biasing
circuit can be used to bias the line when resistive termination is present on the other end of the
network.
Hardware revision 1 of the RMC SERIAL module does not have the capacitor in its termination
circuit and therefore will require biasing.
Biasing forces a valid state onto the network by allowing current to flow across the termination
resistor.
Example:
This example assumes that there is a single master and two RMCs on the network. Compute the
voltage across a 120W termination resistor when using 1150W biasing resistors.
First, we calculate how much DC resistance will be between the biasing resistors. Calculating the
parallel resistance of all DC terminations and node input impedances does this. For a single master
and two RMCs we have the following components:
• Master load: 1 unit load, which is defined as 12 kW.
• RMC loads: ¼ unit load each, which is 48 kW.
• Termination Resister in the RMC: 120W
Therefore, putting all of the resistances in parallel yields the following:
Termination
=
120W ||
12kW ||