Anatomy of a USB to Serial Cable: Signal Chain, Bridge IC, UART, RS232 and RS485 explained

The appearance of the USB to serial cable externally is very easy. At one end there is a connection to a laptop with USB, and at the other end there is a connection to a serial device with RS232, RS485, RS422, TTL, DB9, RJ45 or terminal wires. To most users, it is just a handy method of obtaining a COM port. However, in reality and electrically, a USB to serial adapter is not merely a cable.

The internal part of the housing will receive the USB signal, which will be recognized by the operating system, and will then go through a process where a USB serial bridge IC converts it to UART data, which is subsequently converted back to the proper electrical standard, including RS232, RS485 or TTL UART. This entire path is known as the USB to serial signal chain.

Learning this signal chain will help us understand why some two USB to serial cables with similar connector may be different in their behavior in industrial automation, PLC programming, Modbus RTU communication, firmware debugging and field maintenance.

What Happens Inside a USB to Serial Cable?

On average, a USB to serial cable consists of various functional blocks. The initial block is USB interface that interacts with the host computer. The next block is the USB serial bridge IC, including FTDI, Silicon Labs CP210x, WCH CH340 or Prolific PL2303. The subsequent block is the internal UART engine, which generates serial data frames. The last block is the line driver, which translates UART-level signals to RS232, RS485, RS422, or TTL output.

That is to say, a USB to serial cable is not just a passive wire; it is a working electronic device. The cable has to support the USB protocol communications on one end and the conventional serial communication on the other end.

On a regular USB to RS232 cable, the bridge IC produces UART data and RS232 transceiver changes logical signal to positive and negative voltage levels. A USB to RS485 cable sends UART signal to a differential RS485 driver. On a USB to TTL cable, the UART signal can be connected directly to the output pins with the logic levels of 3.3V or 5V.

It is the reason why internal chip, driver support, baud rate accuracy, latency behavior, and protection circuit are all important.

USB Enumeration: How the Computer Detects the Cable

To send serial data through a USB to serial cable, the computer has to recognise the cable as a USB device first. It is known as USB enumeration.

When enumerated, the USB to serial adapter indicates its vendor ID, product ID, device class and other descriptors to the operating system. Using this data, Windows, Linux, or macOS loads the appropriate driver and makes a virtual COM port.

In other words, an example of an FTDI-based USB to serial cable might be called a USB Serial Port in the Windows Device Manager. A CP2102 based adapter can load the Silicon Labs driver. A CH340-based cable can need a WCH driver based on the operating system version.

This step is significant since most issues related to USB to serial cables begin in this step. In case the driver is absent, outdated, or incompatible, the serial device will not be visible. When the bridge IC is fake or incorrectly programmed, the computer could recognize the USB device but it would not generate an operational COM port.

To industrial users, the stability of USB enumeration is highly important. A technician in a factory or a field engineer would not like to diagnose drivers each time a USB to RS232 or USB to RS485 cable gets connected to a new laptop.

The USB Serial Bridge IC Is the Core Component

The main element in the USB to serial cable is the USB serial bridge IC. This chip will convert USB data to UART data and control the manner in which the host computer interacts with the serial side.

The common bridge IC families are FTDI FT232, Silicon Labs CP2102 and CP2102N, WCH CH340, and Prolific PL2303. The properties of each family of chips vary according to support of drivers, accuracy of baud rate, behavior of latency timers, the size of the FIFO buffers, and long-term reliability.

The majority of chips will be able to operate at a basic terminal communication level of 9600 or 115200 baud. However, the differences are more significant in an industrial setting. The Modbus RTU system might demand steady timing. Firmware flashing software could require consistent DTR and RTS commands. A data logger with a high speed could have to generate an exact baud rate and have enough buffer space.

It is because of this reason that professional users may have an interest in the chip on the USB to serial cable besides the connection that is visible.

An excellent USB serial bridge chip may enhance the stability of drivers, decrease the rate of communication errors, enable the use of non-standard baud rates, and offer more control over the signals of the serial interface, including RTS, CTS, DTR and DSR.

UART: The Middle Layer Between USB and Serial Standards

Once the data has come in through the USB port, it will need to be translated as a UART communication. The name of the Universal Asynchronous Receiver/Transmitter is UART. The simplest serial communication form that is used by RS232, RS485, RS422, and TTL serial devices is UART.

One UART frame typically consists of a start bit, data bits, an optional parity bit, and a stop bit. The commonest format is 8N1 with 8 data bits, no parity and 1 stop bit. But in industry, other configurations are possible. In particular, there are several Modbus RTU devices that support even parity or two stop bits.

Configuration mismatch is a widely known issue of USB to serial communication. The outcome would be corrupt data, no response, or intermittent communication when the baud rates, parity, data bits, or stop bits do not match between both sides.

The UART layer also clarifies the importance of the accuracy of baud rates. UART is asynchronous so that two devices have no common clock. The timing of both parties needs to be done independently. When the USB serial bridge IC outputs a baud rate that has too much error, the receiving device can take a false sample of the bit and introduce framing errors.

The vast majority of USB to serial converters perform well at commonly used baud rates, including 9600, 19200, 38400 and 115200. The quality of the bridge IC is significantly more critical when transmitting at custom baud rates or high-speed serial communication.

TTL, RS232 and RS485 Are Electrically Different

Serial communication is often believed by many users to be purely a protocol issue yet the physical layer should also be considered. TTL, RS232 and RS485 are not equivalent electrical standards.

Logic-level UART signals are typically outputted by a USB to the TTL cable which has a standard voltage level of either 3.3V or 5V. Microcontroller development, embedded debugging, Arduino, ESP32, STM32, and firmware programming are some of the common uses of this kind of cable. The danger is voltage mismatch. There is a possibility that a 5V TTL signal will destroy the device if it is connected to a 3.3V MCU pin.

The USB to RS232 cable uses an RS232 transceiver which produces positive and negative voltages. RS232 is a single-ended system and is commonly employed in point-to-point communication. Its widespread use is in PLCs, CNC machines, medical equipment, POS systems, measurement devices, and legacy industrial devices.

The USB to RS485 cable has a differential driver. The data is transmitted through two wires that are commonly referred to as A and B. Because of its use of differential signaling, RS485 has better noise immunity and a greater transmission distance than RS232 or TTL. It is this that makes USB to RS485 cables useful in Modbus RTU, energy meters, HVAC controllers, building automation, access control system and industrial field devices.

The key here is that the cable connecting the USB port to the serial port should have both matching protocols and electric interfaces. A device on RS485 cannot be plugged straight into a TTL output regardless of whether the baud rate and data format are identical.

RS485 Direction Control and Modbus RTU Timing

Direction control is one of the most significant issues in USB to RS485 cable design. The typical implementation of RS485 is half-duplex, so that a device could send and receive on the same set of wires, but not simultaneously.

RS485 driver has to put itself in transmit mode to send data. Once it has sent data, it has to free the bus so that another device can reply. It is managed by DE and RE pins of RS485 transceiver.

Other USB to RS485 adapters apply RTS control. The design in question has software or the driver that switches the RTS signal to switch between transmit and receive. It may be effective, but its effectiveness will depend on operating system timing and the application behavior.

Better USB to RS485 cables may provide automatic direction control. The adapter senses outgoing UART data and activates the RS485 driver only when required. It automatically frees the bus after the final stop bit has been transferred. It is extremely critical with Modbus RTU communication, in which response time might be critical.

Should the direction control be weak, the symptoms can be absent replies, bus collision, arbitrary CRC errors, or higher baud rates communications failure.

Latency: Why USB Serial Communication Is Not Instant

One of the things that a USB to serial cable needs to be able to control is latency. Every byte of the UART is not sent immediately by USB as a single event. Rather, they are stored in buffers and transmitted over USB in packets.

This bridge IC contains internal FIFO buffers. The operating system driver has buffers as well. Most USB serial bridge ICs have a latency timer which determines the time by which buffered data must be forwarded to the host. It is more effective, but it may interfere with timing critical protocols.

When using a general RS232 terminal, several milliseconds of latency will probably not be an issue. When using Modbus RTU, industrial polling, or real time device control, latency may be significant. The application could be confused by the frame timings if the USB to serial adapter takes too much time to pass the received bytes.

There is one explanation of why industrial USB to serial cables could be more effective compared to generic low-cost adapters. They tend to employ bridge ICs and driver settings with more predictable latency and improved response behaviour.

Isolation and Protection in Industrial USB to Serial Cables

Industrial setting is electrically noisy. Motors, inverters, relays, long cables, and various ground potentials may produce the circumstances that destroy typical USB to serial adapters.

Because of this, galvanic isolation, ESD protection, surge protection, and common-mode filtering are possible features of industrial-grade USB to RS485 and USB to RS232 cables.

The galvanic separation between the USB side and the serial side isolates them. It is helpful in preventing ground loop current to harm the laptop, USB controller or attached device. Isolation is particularly important in RS485 networks when the devices are located at large distances or supplied with power by different electrical sources.

ESD protection prevents the adapter against electrostatic discharge when plugging, unplugging and field wiring. Surge protection is critical in long RS485 cables as they can accumulate high-energy transients.

The protective circuitry is not always externally obvious, but does have a significant impact on long term reliability. A cheap USB to serial cable can be used at a desk, but will not function in a cabinet in a factory. This kind of cable made with industrial use in mind can operate under more extreme electrical environments.

Why Two USB to Serial Cables Can Perform Differently

Even though two USB to serial cables have the same appearance, they can be built in fundamentally different ways inside. One of them might be based on a reliable FTDI or CP2102N bridge IC, whereas another could be based on an inexpensive clone chip. One may be able to provide precise baud rates, whereas the other may generate timing errors when operating at higher speeds. One USB to RS485 cable could have automatic direction control, whereas another would rely on software RTS timing.

The other distinctions are the quality of drivers, FIFO buffer capacity, setting of latencies, ESD protection, shielding of cables, quality of connectors and accuracy in voltage levels.

It clarifies the reason why one USB to serial adapter will perform well with PLCs, Modbus devices and embedded boards whereas another will result to random disconnects or unreadable data. The connector is just a visible component. It is the signal chain that lies within that defines the actual performance.

Conclusion

USB to Serial cable is an active conversion device, not a passive wire. Internally, USB data goes through enumeration, USB serial bridge IC, UART framing, and a line driver to TTL, RS232, RS485 or RS422.

The internal design also directly impacts the stability of industrial automation, Modbus RTU, PLC programming, and firmware debugging. The quality of the Bridge IC, accuracy of baud rate, latency, RS485 direction control, isolation and protection are factors that decide if the cable is reliable in application.

Author

Franck Yan
Founder | Farsince Connectivity Solutions

Franck Yan is the founder of Farsince and has more than 13 years of experience in the cable and connectivity industry, working closely with global customers on data center, industrial, and network connectivity solutions.