Termination and Biasing are concepts that only apply to differential wiring. As such, they only apply to RS-485 and not RS-232.
Note:
Delta recommends you always use biasing on a RS-485 network. To determine whether you need termination, read the Termination Concept section in this topic.
Selecting Termination and Biasing on the RMC75
The RMC7xS termination and biasing can be independently selected with jumpers on the 8-pin RS-485 connector. The locations where the jumpers should be installed are marked on the label.
To select termination: Insert a jumper between pins +T/R and Trm Jpr.
To select biasing: Insert a jumper between pins SCmn and Bias Jumper.
The Termination Concept
Cable termination is a way of absorbing transmitted energy at the end of a network. This prevents signal reflections from bouncing back towards the transmitter and potentially upsetting signal quality and communications.
The termination resistor should match the characteristic impedance of the cable being terminated. The effective impedance of the RMC7xS's termination resistor and biasing resistors is 114Ω. Therefore, cabling with impedance of 100Ω to 120Ω is recommended.
Termination should be placed at the end of the network for each wire pair. For RS-485 (2-wire, point-to-point or multi-drop), terminate the wire pair at each end of the network. The diagram in Serial Network Topologies shows the correct location of the termination.
Termination and Cable Length
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 Terminated 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:
|
Distance |
= |
4,340 ns * 0.66 ft / ns |
|
|
= |
2890 ft |
Since three round trips are required 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.
The Biasing Concept
RS-485 indicates 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.
A known state is forced by allowing current to flow across the termination resistor. Therefore, biasing is usually selected on the RMC that also has a termination. However, some masters only have termination, in which case the user may want to only select biasing on an RMC close to the master. The current will then flow across the master's termination resistor.
The RMC7xS requires biasing in order to be in a known state when the lines are idle. The biasing forces a binary 1.
Example:
This example assumes that there is a single master and two RMCs on the network. Compute the voltage across a 120Ω termination resistor when using 1150Ω 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 kΩ.
RMC loads: unit load each, which is 48 kΩ.
Termination Resister in the RMC: 120Ω
Therefore, putting all of the resistances in parallel yields the following:
|
Termination Resistance |
= |
120Ω || 12kΩ || 48kΩ || 48kΩ |
|
|
= |
118Ω |
Then, we calculate how much DC resistance the network has between power rails:
|
Total Resistance |
= |
1150Ω + 118Ω + 1150Ω |
|
|
= |
2418Ω |
Next, we calculate how much current is flowing through this DC resistance:
|
Current |
= |
5VDC / 2418Ω |
|
|
= |
2.068mA |
Finally, we calculate the voltage drop across the termination resistor:
|
Voltage |
= |
2.068mA * 118Ω |
|
|
= |
244mV |
This value is greater than the 200mV difference required by the TIA/EIA standards and constitutes a valid binary 0 state.
See Also
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