RECEIVER CIRCUIT PC-BOARD LAYOUT
A proper receiver-circuit layout starts with the RS-422 encoder's input connector. The differential signal pairs A A—, B —B, and Index —Index must occupy adjacent pins on the connector. That configuration minimizes signal imbalance by ensuring that the differential pair's returning signal-current paths overlap and cancel. Figure 4 shows typical component placements. To ensure that each pc-board trace has the same parasitic capacitance, route each differential pair of traces close together, with equal lengths, and with symmetrical bends.
To minimize inductive and capacitive crosstalk on the digital outputs and provide lower inductance, differential RS-422 signals from the connector and receiver circuit should be laid over a solid ground-plane layer within the pc board. No high-current signals should flow in this ground plane.
The high-speed current switching in motion-controller circuits can produce common-mode noise. Using filters and bypass capacitors helps to reduce the effect of common-mode voltages coupled onto the power-supply lines. You should place a 0.1-µF bypass capacitor close to the receiver's VCC input. To minimize inductance in the bypass loop, the capacitor's ground lead should connect directly to the solid ground plane, as should the IC ground pin through a via placed adjacent to it. Finally, to minimize noise coupling to the receiver circuits, avoid routing receiver traces near or adjacent to any power circuits.
ENCODER SIGNAL CABLE
Because the differential signals from a quadrature encoder are balanced, they can be transmitted on regular paired cable, but twisted-pair cable is preferred. Twisted-pair cables have very low-inductance coupling and a constant impedance up to several megahertz for exceptionally high-speed performance in motion-control systems. Twisted-pair cables also help reduce radiated and received electromagnetic interference (EMI).
Both shielded and unshielded twisted-pair cables are available. Unshielded cable is smaller, costs less, weighs less, and can bend in a smaller radius. But shielded twisted-pair cable must be used for the differential quadrature-encoder signals. Shielded twisted-pair cable provides better common-mode rejection, because the shield offers additional protection from EMI. The nonideal twists in an actual unshielded twisted-pair cable allows for a dramatic increase in EMI noise. The shield wire should be connected to the receiver's ground plane at the encoder-input connector.
The encoder's signal cable should not carry power-level signals—or any other signals for that matter. Nor should it be routed close to or parallel to other cables or conduits that carry power-level signals or other noisy signals, including 60-Hz power.
Modern, high-speed servo-control systems operate with encoders that use data rates up to several megahertz. At such high rates, the encoder-signal cable must be properly terminated with a terminating resistor or network at the receiver end. Ideally, the terminating resistor has the same value as the cable's characteristic impedance.
Because only one transmitter (encoder output) is on the RS-422 network (one transmitter and one receiver), the transmitter doesn't require a terminating resistor. However, ringing and reflections on a nonterminated receiver input can restrict the data throughput to several hundred kilobits per second. Matching the characteristic impedance of a cable to within ±20% is usually more than adequate. Figures 2 and 3 demonstrate the proper termination of encoder cables.
Recommended Reading:
Barnes, John R., Electronic System Design, Interference And Noise Control Techniques, Prentice-Hall, Englewood Cliffs, N.J., 1987.
"New RS-485 IC Increases System Reliability and Fault Detection in Motor-Control Circuits," www.maxim-ic.com.
Thomas, Sokira J., and Jaffe, Wolfgang, Brushless DC Motors, Electronic Commutation and Controls, TAB Books Inc., Blue Ridge Summit, Pa., 1990.
RECEIVER CIRCUIT PC-BOARD LAYOUT
A proper receiver-circuit layout starts with the RS-422 encoder's input connector. The differential signal pairs A A—, B —B, and Index —Index must occupy adjacent pins on the connector. That configuration minimizes signal imbalance by ensuring that the differential pair's returning signal-current paths overlap and cancel. Figure 4 shows typical component placements. To ensure that each pc-board trace has the same parasitic capacitance, route each differential pair of traces close together, with equal lengths, and with symmetrical bends.
To minimize inductive and capacitive crosstalk on the digital outputs and provide lower inductance, differential RS-422 signals from the connector and receiver circuit should be laid over a solid ground-plane layer within the pc board. No high-current signals should flow in this ground plane.
The high-speed current switching in motion-controller circuits can produce common-mode noise. Using filters and bypass capacitors helps to reduce the effect of common-mode voltages coupled onto the power-supply lines. You should place a 0.1-µF bypass capacitor close to the receiver's VCC input. To minimize inductance in the bypass loop, the capacitor's ground lead should connect directly to the solid ground plane, as should the IC ground pin through a via placed adjacent to it. Finally, to minimize noise coupling to the receiver circuits, avoid routing receiver traces near or adjacent to any power circuits.
ENCODER SIGNAL CABLE
Because the differential signals from a quadrature encoder are balanced, they can be transmitted on regular paired cable, but twisted-pair cable is preferred. Twisted-pair cables have very low-inductance coupling and a constant impedance up to several megahertz for exceptionally high-speed performance in motion-control systems. Twisted-pair cables also help reduce radiated and received electromagnetic interference (EMI).
Both shielded and unshielded twisted-pair cables are available. Unshielded cable is smaller, costs less, weighs less, and can bend in a smaller radius. But shielded twisted-pair cable must be used for the differential quadrature-encoder signals. Shielded twisted-pair cable provides better common-mode rejection, because the shield offers additional protection from EMI. The nonideal twists in an actual unshielded twisted-pair cable allows for a dramatic increase in EMI noise. The shield wire should be connected to the receiver's ground plane at the encoder-input connector.
The encoder's signal cable should not carry power-level signals—or any other signals for that matter. Nor should it be routed close to or parallel to other cables or conduits that carry power-level signals or other noisy signals, including 60-Hz power.
Modern, high-speed servo-control systems operate with encoders that use data rates up to several megahertz. At such high rates, the encoder-signal cable must be properly terminated with a terminating resistor or network at the receiver end. Ideally, the terminating resistor has the same value as the cable's characteristic impedance.
Because only one transmitter (encoder output) is on the RS-422 network (one transmitter and one receiver), the transmitter doesn't require a terminating resistor. However, ringing and reflections on a nonterminated receiver input can restrict the data throughput to several hundred kilobits per second. Matching the characteristic impedance of a cable to within ±20% is usually more than adequate. Figures 2 and 3 demonstrate the proper termination of encoder cables.
Recommended Reading:
Barnes, John R., Electronic System Design, Interference And Noise Control Techniques, Prentice-Hall, Englewood Cliffs, N.J., 1987.
"New RS-485 IC Increases System Reliability and Fault Detection in Motor-Control Circuits," www.maxim-ic.com.
Thomas, Sokira J., and Jaffe, Wolfgang, Brushless DC Motors, Electronic Commutation and Controls, TAB Books Inc., Blue Ridge Summit, Pa., 1990.