Biasing forces the network into a known state when the lines are idle and therefore otherwise not driven. It is important that the lines always be in a known state, not only when being driven. 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. This value is then rounded down to allow for inexact cable velocities and damping rates, giving us 475 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: 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: One bit time is equal to the reciprocal of the baud rate.Ĭompute the cable length for 115,200 baud RS422.įirst, we compute a half bit time at this baud rate. This damping must occur before the bit is sampled or within half a bit time.
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It is assumed that a signal transition will dampen out after three round trips in the cable. Signals travel through a cable at approximately 66% of c or 0.66 ft/ns. The values presented in the chart above are based on 24AWG cable with capacitance of 16 pF/ft and the following reasoning. Note: The maximum cable length is the length of the entire network and not just the distance between nodes on the network. The following chart shows the maximum cable lengths at various baud rates with and without termination: Termination is not required on all differential networks, but it does typically extend the maximum cable length. The diagrams in Serial Network Topologies show the correct locations of the termination:įor RS-422 (4-wire, point-to-point), terminate the receiver of each wire pair.įor RS-485 (2-wire, point-to-point or multi-drop), terminate the wire pair at each end of the network.įor 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 should be placed at the end of the network for each wire pair. Therefore, cabling with impedance of 100W to 120W is recommended. The effective impedance of the RMC SERIAL's termination resistor and biasing resistors is 114W. The termination resistor should match the characteristic impedance of the cable being terminated. This prevents signal reflections from bouncing back towards the transmitter and potentially upsetting signal quality and communications. The values of the resisters in the Rx/Tx biasing circuit are 2.2 kW instead of 1.15 kW.Ĭable termination is a way of absorbing transmitted energy at the end of a network. There is no capacitor in the Rx/Tx termination circuit. There is no biasing or termination on the Tx wire pair. The values of the resisters and capacitors in version 2 and later of the RMC SERIAL's termination/biasing circuit are shown in the diagram below:Īs described above, hardware revision 1 differs in its biasing and termination. In the RMC Configuration dialog box, click Close. If the module could not be reset automatically, you may be prompted to reset the module manually.
#PARALLEL TO SERIAL CONVERTER RC NET WORD UPDATE#
The Update Module Configuration dialog box will be displayed to indicate the progress. This step will save the settings to Flash memory. Note: As described above, hardware revision 1 does not allow biasing and termination to be independently selected, and only allows enabling them on the Rx/Tx pair. The diagram in the dialog box will update accordingly. In the Serial Module Options dialog box, ensure that RS-422/RS-485 (4-wire) or RS-485 (2-wire) is selected.Ĭheck or un-check the Enable Biasing Circuit and Enable Termination check boxes for the transmitter and receiver (for 4-wire) or transceiver (for 2-wire). In the Slots list, click the Serial line.
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On the Tools menu, click Module Configuration. To change the termination and biasing options for the RMC SERIAL module: For RMC SERIAL hardware revision 1, biasing and termination must be enabled and disabled together and are only available on the Tx/Rx wire pair and not on the Tx pair. First, however, we will describe the options provided by the RMC SERIAL module.įor RMC SERIAL modules with hardware revision 2 or later, biasing and termination can be independently selected for both wire pairs. The Termination and Biasing concepts are described in detail below. As such, they only apply to RS-422 and RS-485 and not RS-232. Termination and Biasing are concepts that only apply to differential wiring. RS-422/485 Termination and Biasing RS-422/485 Termination and Biasing