CN-0373: Isolated USB to Isolated RS-485/Isolated RS-232 Interface1/27/2022481 K
Design & Integration Files• Schematic
• Bill of Materials
• Gerber Files
• Layout Files (PADS)
• Assembly Drawing Download Design Files 1.37 M
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- EVAL-CN0373-EB1Z ($53.50) EVAL-CN0373-EB1Z
Features & Benefits
- Isolated USB to Isolated RS-485/RS-232 Interface
- Small Footprint
- Robust Interface
Markets and Technologies
Documentation & Resources
Circuit Function & Benefits
The circuit shown in Figure 1 provides a completely isolated connection between the popular USB bus and an RS-485 or RS-232 bus. Both signal and power isolation ensures a safe USB device interface to an industrial bus or debug port, allowing TIA/EIA-485/232 bus traffic monitoring and the convenience of sending and receiving commands to and from a PC that is not equipped with an RS-485 or RS-232 port.
Isolation in this circuit increases system safety and robustness by providing protection against electrical line surges and breaks the ground connection between bus and digital pins, thereby removing possible ground loops within the system.
The TIA/EIA RS-485 bus standard is one of the most widely used physical layer bus designs in industrial and instrumentation applications. RS-485 offers differential data transmission between multiple systems, often over very long distances. RS-485 communication offers additional robustness through differential communication when compared to the RS-232 standard.
TIA/EIA RS-232 devices are widely used in industrial machines, networking equipment, and scientific instruments. In modern personal computers, which are often used for debugging network problems, USB has displaced RS-232 from most of its peripheral interface roles, and many computers do not come equipped with RS-232 ports. The circuit in Figure 1 offers a robust and compact solution for both RS-232 and RS-485 interfaces.
The circuit in Figure 1 provides highly integrated and robust isolation of fully TIA/EIA-485/232 compliant transceivers. The ADM3252E 2.5 kV rms isolated RS-232 and the ADM2587E 2.5 kV rms isolated RS-485 solutions are the smallest available in the industry. The ADuM3160 provides market-leading USB port 2.5 kV rms isolation, and is easily integrated with low and full speed USB-compatible peripheral devices. The FTDI FT2232H (USB to UART converter) facilitates transmission via UART to RS-485 or RS-232 bus ports. TIA/EIA RS-232 devices are widely used in industrial machines, networking equipment, and scientific instruments. Applications for RS-485 include process control networks; industrial automation; remote terminals; building automation, such as heating, ventilation, air conditioning (HVAC) and security systems; motor control; and motion control.
In these real-world systems, lightning strikes and power source fluctuations can cause damage to communications ports by generating large transient voltages. Isolation in the Figure 1 circuit increases system safety by providing protection against these electrical line surges.
Figure 2 shows a photo of the isolated USB to FTDI isolated RS-232/isolated RS-485 circuit. It is possible to transmit from the USB port to both RS-485 and RS-232 ports simultaneously, if required, or to just one port.
The ADuM3160 provides 2.5 kV rms digital isolation of the data signals on the D+ and D− USB bus input to the FTDI FT2232H (USB to UART converter). The isolated USB output DD− and DD+ signals are connected to the DM and DP pins of the FTDI, respectively. The FTDI FT2232H can transmit data via UART to RS-232/RS-485 depending on which PC virtual COM port (VCP) is chosen. The ADuM3160 VBUS1 power is supplied via the USB cable connection. The ADuM3070 provides a regulated isolated power source. Power (+6 V) and ground for the ADuM3070 are connected via the J1 barrel socket connector. The ADuM3160 VBUS2 pin is supplied with 3.3 V isolated power by the ADuM3070. The ADuM3070 3.3 V output also supplies the primary power for the ADM3252E 2.5 kV rms isolated RS-232 and the ADM2587E 2.5 kV rms isolated RS-485 transceiver.
The ADM2587E 2.5 kV rms isolated RS-485 transceiver is the industry-leading signal and power isolated solution. This transceiver is capable of operating at 3.3 V or 5 V. Data transmits on the TxD pin, and it is received on the RxD pin. Both the driver and receiver outputs can be enabled or disabled, that is, put into a high impedance state, by changing the logic levels on the DE and RE pins, respectively.
The FTDI output pin BDBUS0 is connected to the TxD data input pin of the ADM2587E. The FTDI output pins BCBUS0, PWREN#, and BDBUS1 are connected to the DE, RE, and RxD input pins of the ADM2587E, respectively. The DE and RE pin state can also be configured via the LK1 and LK2 jumpers. For each link, Position A connects the logic pin to 3.3 V, Position B connects the logic pin to GND, and Position C connects the logic pin to the FTDI output pins.
The ADM2587E can transmit and receive bus data via the A, B, Y, and Z RS-485 inputs/outputs. An RS-485 bus cable can be connected via the J2 five-way connector. Disconnecting the LK5 and LK6 jumpers places the ADM2587E in a full duplex configuration, where data can be transmitted via the Y and Z pins, and received via the A and B pins. Connecting the LK5 and LK6 jumpers places the ADM2587E inputs/outputs in a half duplex configuration, which is where bus data can only be transmitted or received, but not simultaneously.
The ADM3252E is a high speed, 2.5 kV, fully isolated, dual channel RS-232/V.28 transceiver device that is operational from a single 3.3 V or 5 V power supply. The RS-232 interface supports full-duplex communication and provides CTS and RTS for hardware handshaking via the J5 five-way connector or the J3 connector. J3 provides an RS-232 cable connector for connecting to industrial equipment.
The ADM3252E transmitter inputs (TINx) accept TTL/CMOS input levels from the ADBUS0 and ADBUS2 output pins of the FTDI. The TINx inputs are inverted and coupled across the isolation barrier, where they are transmitted as EIA/TIA-232E bus signals via the J3 or J5 connector. The ADM3252E receiver inputs (RINx) accept EIA/TIA-232E signal levels from the J3 or J5 connector. The RINx inputs are inverted and coupled across the isolation barrier to appear at the ROUTx pins. The ROUTx pins are connected to the ADBUS1 and ADBUS3 inputs to the FTDI.
Two separate ADP190 circuits are used as soft start circuitry to power the ADM2587E and ADM3252E after the FTDI FT2232H has fully powered up. Jumper LK7 links the FTDI FT2232H PWREN# pin to the EN pin of the ADP190 circuits via the 74AHC1G14W5-7 inverter. The ADP190 EN pin must be high to turn on the power switch; drive EN low to turn off the power switch. Connecting LK7 in Position A (Position B disconnected) means that the ADP190 EN pin is always high/enabled. Connecting LK7 in Position B (Position A disconnected) means that the ADP190 EN pin is triggered via the inverted PWREN# signal.
The FTDI FT2232H data sheet provides a comprehensive listing of the pin functions for RS-232 communications. The LK3 and LK4 jumpers allow handshaking options for RS-232. When LK3 is connected, ADBUS4 (DTR#) is connected to ADBUS5 (DSR#). When LK4 is connected, ADBUS4 (DTR#) is connected to ADBUS6 (DCD#).
Circuit Evaluation & Test
Apply 6 V to the J1 barrel socket connector to power the EVAL-CN0373-EB1Z board. The voltage can be checked on the VCC_REG test point at the output of the ADP7102 regulator. The ADP7102 output voltage should measure 5 V. The 5 V is routed to the ADuM3070 and T2 transformer, which supply the 3.3 V outputs to power the RS-485, RS-232, and FTDI circuits. Check that the ISO_VCC test point measures 3.3 V.
A complete transmit and receive path can be tested by connecting the RS-232 and RS-485 outputs to an Analog Devices ezLINX™ iCoupler® Isolated Interface Development Environment board. Alternatively, a USB to RS-232/RS-485 transmit test can be performed by using the Tera Term open source terminal emulator program. It is possible to transmit from the USB port to both RS-485/RS-232 ports simultaneously, if required, or to just one port.
Connect a USB cable from the laptop/PC to the J4 connector. Attach an oscilloscope probe to the ISOTxD test point next to the ADM3252E transceiver, as shown in Figure 3. Open the PC application software and select COM5, then click OK, as shown in Figure 4.
Load the RS-232 data to be transmitted by clicking Send under the File menu. Choose any large data size file for continuous test transmission and click Send, as shown in Figure 5.
Observe the isolated RS-232 signal on an oscilloscope by probing the ISOTxD test point on the EVAL-CN0373-EB1Z board (see Figure 6).
A similar test can be performed for the ADM2587E transceiver. Connect a USB cable from the laptop/PC to the J4 connector. Attach an oscilloscope probe to the Y and Z test points next to the ADM2587E transceiver as shown in Figure 3. Open a second Tera Term emulator window, and select COM6 for RS-485 transmission, as shown in Figure 7.
Load the RS-485 data to be transmitted by clicking Send under the File menu. Choose any large data size file for continuous test transmission and click Send, as shown in Figure 8.
Observe the isolated RS-485 signals on an oscilloscope by probing the Y and Z test points, or the differential bus signal by using the oscilloscope Y − Z Math function, as shown in Figure 9.
A complete design support package including schematic, assembly, bill of materials, and layout files is available at http://www.analog.com/CN0373-DesignSupport.
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