Delta RMC101 User Manual download pdf (Page 499)

100

101

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 ||

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Delta RMC101 User Manual Page 499

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