Table of contents
PCB Modular Design 16——RS232, RS485 Interface Module PCB Layout and Routing Design Specifications
RS232 interface module
1. Interface overview
In data communication, computer network and distributed industrial control system, it is often necessary to use serial communication to realize data exchange. RS-232 is the earliest serial interface standard, which has been widely used in short distance (<15M) and lower baud rate serial communication.
The EMC design scheme with RS232 interface is designed from three aspects: interface schematic diagram, structure design, and cable design.
2. EMC design of the interface circuit schematic diagram
Anti-static design of RS232 interface
Figure 1
Overview of anti-static design of RS232 interface interface circuit design:
RS232 interface is mostly used for communication between computer and equipment, for data monitoring and debugging.
In the process of application, the communication cable is easy to couple external interference, which will affect the signal transmission to a certain extent, and the interference inside the board may also form external radiation through the cable.
Based on the principle of EMC, this scheme has carried out related interference suppression and anti-sensitivity design, and solved the EMC problem from the design level.
Circuit EMC design instructions:
(1) Circuit filter design points:
L1, L2, C1, and C2 form a filter circuit,
L1, L2 are filter beads, the recommended value is 600Ω/100Hz, used to suppress high-frequency interference on the circuit;
C1, C2 are filter capacitors, used to filter out the on-line interference, the value of the capacitor is 330pF;
R1 and R2 are 100 ohm current-limiting resistors, which can be increased according to the actual application;
C3 is the jumper capacitance between the interface ground and the digital ground, the typical value is 1000pF, and the withstand voltage It is required to reach more than 2KV, and the capacitance of C3 can be adjusted according to the test situation;
(2) Key points of circuit protection design:
D1 and D2 are TVS tubes, which form a protective circuit to prevent the chip from being damaged due to the impact of large interference energy and static interference on the circuit during hot swapping; when selecting circuit protection TVS devices, pay attention to the TVS start-up voltage ≥
15V *1.2=18V.
Remarks for interface circuit design:
If the device is a metal case, and the single board can be divided into an interface ground independently, then the metal case is directly electrically connected to the interface ground, and the single board ground is connected to the interface ground through a 1000pF capacitor; if the device is a non-metal
case , then the interface ground PGND is directly electrically connected to the board ground GND.
3. Connector design
RS232 signal sequencing design of DB type metal connector
Figure 1 Structure design of DB type connector
The way of connecting the RS232 connector to the body:
(1) Precise milling processing technology is used when the panel is opened, so that the shape of the hole is more suitable for the placement of the connector, avoiding gaps in places where the hole is cut inaccurately, and thereby reducing electromagnetic interference radiation; it has been proved by tests that precise milling and opening Processing can improve the electromagnetic compatibility by 12~18%;
(2) The joint between the body and the metal connector should add shrapnel, so that the two can maintain good electrical conductivity when they are joined. The specific lapping method is shown in the figure above:
4. Cable design
RS232 interface signal cable Requirements for cable design: DB type metal connector
conventional The requirement is not less than 90%; (2) The signal transmission line and the ground wire are arranged in the same group adjacent to each other ; Closely combined, try to choose a thick and long magnetic ring.
Routing design:
(1) When routing RS 232 cables, it is required to keep away from other strong interference sources, such as power modules
; Mixed bundles with other cables.
Figure 2 Lap joint of DB type metal connector
Bonding of cables to metal connectors:
(1) The shielding layer of the shielded cable needs to be overlapped with the metal connector at 360°;
5. RS-232 conventional pin definition:
①Carrier detection
② Data receiving
③Data sending
④The data segment is ready
⑤Signal pin
⑥Data setting completed
⑦Request to send
⑧Allow sending
⑨Ring display
6. RS-232 knowledge points
①The signal level of the interface is high, which is easy to damage the chip of the interface circuit, and because it is incompatible with the TTL level, it needs to use a level conversion circuit to connect to the TTL circuit.
② The transmission rate is low. During asynchronous transmission, the baud rate is 20Kbps.
③ The interface uses a signal line and a signal return line to form a common ground transmission form. This common ground transmission is easy to form individual citizens, so the anti-noise interference is weak.
④The transmission distance is limited, the maximum transmission distance standard value is 50 feet, but in fact it can only use about 50 meters.
RS485 interface module
1. Schematic design scheme
1. Design scheme of 6KV lightning protection circuit with RS485 interface
Figure 1 RS485 interface lightning protection circuit
Overview of interface circuit design:
RS485 is used for communication between equipment and computers or other equipment. In product applications, its wiring is mostly mixed with power supply, power signals, etc., and there are potential EMC hazards.
Based on the principle of EMC, this scheme has carried out related interference suppression and anti-sensitivity design, and solved the EMC problem from the design level.
2. Circuit EMC design instructions
(1) Key points of circuit filter design:
L1 is a common-mode inductor, which can attenuate common-mode interference, suppress internal and external interference on the board, improve the anti-interference ability of the product, and reduce the external radiation through the 429 signal line , the selection range of common mode inductance impedance is 120Ω/100MHz ~2200Ω/100MHz, and the typical value is 1000Ω/100MHz;
C1 and C2 are filter capacitors, which provide a low-impedance return path for interference, which can effectively reduce the external common-mode current and filter external interference at the same time; the range of capacitance selection is 22PF~1000pF, and the typical value is 100pF; if the signal The line has insulation and withstand voltage requirements for the metal shell, so the two filter capacitors for the differential line to ground need to consider the withstand voltage; when there are multiple nodes on the circuit, consider reducing or removing the value of the filter capacitor.
C3 is the jumper capacitance between the interface ground and the digital ground, the typical value is 1000pF, and the capacitance of C3 can be adjusted according to the test situation;
(2) Key points of circuit lightning protection design:
In order to meet the IEC61000-4-5 or GB17626.5 standard, common mode 6KV, differential touch 2KV lightning protection test requirements, D4 is a three-terminal gas discharge tube to form the first level of protection circuit, use It is used to suppress the common mode and differential mode surge interference on the line, and prevent the interference from affecting the next-level circuit through the signal line;
The nominal voltage VBRW of the gas discharge tube is required to be greater than 13V, the peak current IPP is required to be greater than or equal to 143A; the peak power WPP is required to be greater than or equal to 1859W;
PTC1 and PTC2 are thermistors to form the second-level protection circuit, with a typical value of 10Ω/2W;
in order to ensure the smooth conduction of the gas discharge tube, this resistance must be increased to divide the pressure to ensure that most of the energy passes through the gas. The discharge tube is gone;
D1~D3 are TSS tubes (semiconductor discharge tubes) to form a third-level protection circuit. The nominal voltage VBRW of the TSS tube is required to be greater than 8V, the peak current IPP is required to be greater than or equal to 143A; the peak power WPP is required to be greater than or equal to 1144W
;
(3) Remarks for interface circuit design:
If the device is a metal shell and the interface ground can be divided independently by the single board, then the metal shell is directly electrically connected to the interface ground, and the single board ground is connected to the interface ground through a 1000pF capacitor; if the device is a non-metal shell, then the interface ground
PGND It is directly electrically connected to the board digital ground GND.
2. PCB design scheme
1. RS485 interface circuit layout
Figure 1 RS485 interface filter and protection circuit layout
Scheme features:
(1) Protection devices and filter devices should be placed close to the interface and should be placed compactly and neatly. According to the rule of first protection and then filtering, try to avoid wiring when routing (2 )
The common mode inductance and jumper capacitance should be placed in the isolation zone.
Solution analysis:
(1) Do not route wires around the interface and the interface filter protection circuit, and do not place high-speed or sensitive devices;
(2) The projection layer under the isolation belt must be hollowed out, and wiring is prohibited.
2. RS485 interface circuit ground division design
Figure 1 RS485 interface ground division design
Program features:
(1) In order to suppress the internal noise of the single board through the RS485 interface, and to enhance the anti-interference ability of the single board to external interference, a filter device is added at the RS485 interface for suppression, and the size of the filter device is divided into Interface ground;
(2) Capacitors can be selectively added in the isolation zone as the connection between the two grounds. The value of capacitors C4 and C5 is recommended to be 1000pF. The common mode inductor CM and capacitor filter are connected in series on the signal line and connected to the interface ground Connect GDT and TVS tubes in parallel for protection; and all protection devices are placed close to the interface, and the common mode inductor CM is placed in the isolation zone. The specific layout is shown in the figure.
case analysis:
(1) When there is a circuit with poor compatibility or incompatibility between the interface and the board, it is necessary to perform "ground separation" processing between the interface and the board, that is, according to different port voltages, level signals and transmission rates to set the ground respectively. "Splitting the ground" can prevent the superposition of the return signal of the incompatible circuit and prevent the impedance coupling of the common ground wire;
(2) The phenomenon of "splitting the ground" will cause the impedance to increase when the return signal crosses the isolation zone, which will cause a huge EMC Risk, so capacitors are used to provide a return path for the signal between the isolation bands.
3. Structural design scheme
1. Lap connection between RS485 connector and body
Figure 1 Rectangular Connector Design
The overlapping method of the rectangular connector RS485:
Shielding material (conductive cloth or stainless steel reed) is installed in the gap between the shielding connector and the shielding body to ensure the shielding effectiveness of the gap. This shielding material needs to be customized and processed according to the shape of the shielding connector, and conductive cloth material is generally recommended; or When making the mold of the body, the opening of the connector is embedded to increase the contact area between the connector and the body and improve the shielding effect. The overlapping method is as shown in the figure above:
2. RS 485 signal pin definition
Figure 2 RS 485 signal
RS485 connector signal pin definition
(1) If it is a single set of RS485 signals, then select two adjacent pins as differential signals;
(2) If multiple sets of RS485 signals are in the same connector, then each set A ground pin should be defined between the signals to isolate each other. As shown in FIG:
4. Cable design scheme
1. Lap connection of RS485 connector and cable
Figure 1 Rectangular connector
Lap connection between the cable and the rectangular connector RS485: (1) The shielding
layer of the shielded cable needs to be lapped at 360° with the metal connector;
A separate "tail" phenomenon occurs.
2. RS485 signal wiring
Figure 2 RS 485 signal
Cable design:
(1) The RS485 signal cable adopts the shielding method of the mesh braided shielding layer, and the braiding density of the mesh braiding layer is required to be no less than 90%; (2)
When wiring internally, the differential cable adopts twisted pair transmission, and the twisted pair twist distance is generally 3 times the diameter of the signal cable; the wiring method is shown in the figure above:
(3) Both ends of the cable need to be treated with magnetic rings. The inner diameter of the magnetic ring should be closely combined with the outer diameter of the cable. Try to choose thick and long magnetic rings.
Routing design:
(1) When routing the RS485 cable, it is required to stay away from other strong interference sources, such as power modules;
(2) It is best to route the cable alone or keep a distance of more than 10cm from other analog and power cables, and must not be connected with other cables Mixed bundles.