The circuit shown in Figure 1 provides a fully isolated connection between the widely used USB bus and the RS-485 or RS-232 bus. Signal and power isolation ensures a secure interface between USB devices and industrial buses or debug ports, allows monitoring of TIA/EIA-485/232 bus traffic, and facilitates sending commands to or from PCs not equipped with RS-485 or RS-232 ports. The PC receives the command.

The isolation in this circuit provides electrical line surge protection and breaks the ground connection between the bus and digital pins, increasing system safety and robustness, thereby eliminating possible ground loops in the system.

In industrial and instrumentation applications, the TIA/EIA RS-485 bus standard is one of the most widely used physical layer bus design standards. RS-485 provides differential data transmission between multiple systems, which are often far apart. Compared to the RS-232 standard, RS-485 communication provides additional robustness through differential communication.

TIA/EIA RS-232 devices are widely used in industrial machines, network equipment and scientific research instruments. In modern personal computers (PCs are often used to debug network problems), most peripheral interfaces have adopted USB instead of RS-232, and many computers are no longer equipped with RS-232 ports. The circuit in Figure 1 provides a robust and compact solution for RS-232 and RS-485 interfaces.

The circuit in Figure 1 provides a fully TIA/EIA-485/232 compliant, highly integrated, and robust isolation transceiver. The ADM3252E 2.5 kV rms isolated RS-232 and ADM2587E 2.5 kV rms isolated RS-485 solutions are the industry’s smallest solutions. The ADuM3160 provides market-leading 2.5 kV rms isolation of USB ports, allowing easy integration with low-speed and full-speed USB-compatible peripherals. FTDI FT2232H (USB to UART converter) can facilitate transmission through UART to RS-485 or RS-232 bus port. TIA/EIA RS-232 devices are widely used in industrial machines, network equipment and scientific research instruments. Applications for RS-485 include: process control networks, industrial automation, remote terminals, building automation (e.g., HVAC, security systems), motor control, and motion control.

In these real systems, lightning strikes and power supply fluctuations can cause damage to communication ports by generating large transient voltages. Isolation of the diagram circuit provides protection against these electrical line surges, increasing system safety.

Figure 2 shows a photo of an isolated USB to FTDI isolated RS-232/isolated RS-485 circuit. If necessary, this circuit can achieve simultaneous transmission from the USB port to the RS-485 and RS-232 ports, or it can transmit to only one port.

The ADuM3160 provides 2.5 kV rms digital isolation of the USB bus D+ and D− input data signals 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 FTDI respectively. The FTDI FT2232H can send data via a UART to RS-232/RS-485 interface, depending on the selected PC Virtual COM Port (VCP). The ADuM3160 provides VBUS1 power via the USB cable connection. The ADuM3070 provides a regulated isolated power supply. The ADuM3070's power supply (+6 V) and ground are connected through the J1 barrel jack connector. The ADuM3160 VBUS2 pin is powered from the ADuM3070's 3.3 V isolated supply. The ADuM3070 3.3V output also provides main power for the ADM3252E 2.5 kV rms isolated RS-232 and ADM2587E 2.5 kV rms isolated RS-485 transceivers.

The ADM2587E 2.5 kV rms isolated RS-485 transceiver is the industry-leading signal and power isolation solution. The transceiver can operate at 3.3 V or 5 V. Data is sent via the TxD pin and received via the RxD pin. The outputs of both the driver and receiver can be enabled or disabled into a high-impedance state by changing the corresponding logic levels on the DE and RE pins.

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 states are also configurable 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 pin.

The ADM2587E can send and receive bus data via the A, B, Y, and Z RS-485 inputs/outputs. The RS-485 bus cable can be connected through the J2 five-way connector. Disconnecting the LK5 and LK6 jumpers places the ADM2587E in a full-duplex configuration where data can be sent on the Y and Z pins and received on the A and B pins. Connecting the LK5 and LK6 jumpers places the ADM2587E input/output in a half-duplex configuration, where only bus data can be sent or received, not both at the same time.

The ADM3252E is a high-speed, 2.5 kV, fully isolated, dual-channel RS-232/V.28 transceiver operating from a single 3.3 V or 5 V supply. The RS-232 interface supports full-duplex communication and provides hardware handshaking for CTS and RTS through the J5 five-way connector or the J3 connector. J3 provides RS-232 cable connectors for connecting industrial equipment.

The ADM3252E transmitter input (T _INx ) supports FTDITTL/CMOS input levels from the ADBUS0 and ADBUS2 pins of FTDI. The T _Inx input is inverted and coupled across the isolation barrier and then sent as an EIA/TIA-232E bus signal through the J3 or J5 connector. The ADM3252E receiver input (R _INx ) supports EIA/TIA-232E signal levels from the J3 or J5 connector. The R _Inx input is inverted and coupled across the isolation barrier to the R _OUTx pin. The R _OUTx pin is connected to the ADBUS1 and ADBUS3 inputs of FTDI.

Use two separate ADP190 circuits as soft-start circuits and power the ADM2587E and ADM3252E after the FTDI FT2232H is fully powered up. Jumper LK7 connects the FTDI FT2232H PWREN# pin to the EN pin of the ADP190 circuit through the 74AHC1G14W5-7 inverter. The ADP190 EN pin must be high to turn on the power switch; driving EN low turns off the power switch. Connecting LK7 in position A (disconnected in position B) means the ADP190 EN pin is always high/enabled. Connecting LK7 in position B (disconnected in position A) indicates that the ADP190 EN pin is triggered by the PWREN# inverted signal.

The FTDI FT2232H data sheet provides a complete list of pin functions for RS-232 communications. LK3 and LK4 jumpers support RS-232 handshake option. When LK3 is connected, ADBUS4 (DTR#) is connected to ADBUS5 (DSR#). When LK4 is connected, ADBUS4 (DTR#) is connected to ADBUS6 (DCD#).