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Serial to USB adapters are typically compliant with the USB 2.0 specification, with a data rate of 12 Mbps and support USB features such as suspend (device sleeps), resume (device wakes) and remote wake-up (device wakes host). The maximum open-circuit voltage specified by the RS232 standard is 25 V, but normally signal levels 5 V, 10 V, 12 V, and 15 V. RS232 determines the communication between the DTE and DCE using DB9 and DB25 connectors. The D-sub connectors (DB9, DB25) comes with male and female cable. DB9 connector has 9 pins and DB25 connector has 25 pins with each pin having its own function. DB9 Male and Female Pinouts DB25 Pinout Functional Description Standard RS232 cables connect a DTE (Data Terminal Equipment) such as a desktop computer to a DCE (Data Communications Equipment), typically a modem, printer or special-purpose peripheral. These "straight through" serial cables connect pins on the sending device to the same pins on the receiving device (e.g. pin 1 to pin 1, pin 2 to pin 2 and so on).
The electrical characteristics of RS232 defines the specifications related to voltage levels, line impedance and rate of change of signal levels. Voltage Levels RS-232 is a standard that was introduced in the early 60s for serial data transmission and still widely in use mainly because it’s multipurpose, easy to operate and maintain, accessible, and widely supported. It defines not only electrical signal characteristics, such as signal transmission rate, voltage levels, cable length, timing and short-circuit behavior, but many other things as well, including interface’s mechanical characteristics, connectors, and pinouts.If parity bit is used, a maximum of 8 bits can be transmitted. If parity isn’t used, then 9 data bits can be transmitted. After the data is transmitted, the transmitter sends the stop bits. It can be either 1 bit or 1.5 bits or 2 bits long. The following image shows the frame format of the RS232 protocol. Also, the original RS232 standard specifies the maximum length of the cable as 15 meters but the revised standards specify the maximum length in terms of capacitance per unit length. Mechanical Specifications The Computer and Modem communicate with each other using RS232 interface and the communication between the modems is established using telecommunication links. Typically, the logic ‘1’ in RS232 will be -12V and logic ‘0’ will be +12V. All the above mentioned voltages are with respect to a common ground ‘GND’ pin. Any voltage between -3V and +3V is considered to be an undefined logic state. All these specifications provide us different parameters like common voltage levels, signal levels, pin wiring specifications, control data between the host device and its peripheral etc. Let us see about the different specifications in detail. Electrical Characteristics
There may be more signaling for error correction, but that's not required. So an RS-232 port can be easily made using I/O pins in any microcontroller, the only thing you'll need is voltage conversion since RS-232 lines are 12V and microcontrollers usually work at 3.3V. was (and to some extent still is) a very useful powerful flexible way of connecting computing device to peripherals.
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Though RS232 is considered to be a complete standard, many manufacturers may not abide by the standards. Some manufacturers may implement the complete specifications and some implement just a partial specification. In the above diagram, the transmitter and receiver have their own buffer. The transmit buffer holds the characters to be sent to the receiver. While the receive buffer holds the characters received from the transmitter. If the transmitter sends data at a higher speed, the receiver may fail to receive. In this case, character ‘C’ is missed by the receiver. To avoid this, handshaking is used. Handshaking allows the transmitter and receiver device to agree before the communication is going to start. Hardware Handshaking
The electrical specifications for RS232 are updated in the year 1969. It specifies the electrical voltages, slew rate, line impedance, operation mode, and baud rate.The flow control of data transmission and reception is done using hardware handshaking. It uses control signals DTR, DSR, RTS and CTS signals. Typically, when establishing communication between a computer and modem RTS and CTS signals are used.
Since RS232 is considered a complete standard, it defines more than electrical and mechanical characteristics. RS232 standard also defines the functions of different signals used in the interface. This type of handshaking uses two ASCII characters for start-stop communication. Hence, this is known as software flow control. Software handshaking use XON/XOFF character to control the serial communication. ‘XON’ represents Ctrl+S or ASCII character 11, whereas ‘XOFF’ represents Ctrl+Q or ASCII 13. This handshaking requires 3 wires. They are TXD, RXD and signal GND. The Procedural Specifications of RS232 specify the sequence of operations that must be carried out when a DTE and DCE are connected. There are other pins for control, but they are not necessarily used. Their main function is for buffer retention control.Software Handshaking in RS232 involves two special characters for starting and stopping the communication. These characters are X-ON and X-OFF (Transmitter On and Transmitter OFF). To avoid situations like this, we need some sort of Handshaking mechanism (either Software or Hardware Handshaking). Hardware Handshaking When you plug a USB to Serial Adapter like the USA-19HS into a Windows 10 machine, it should appear in the Device Manager under Ports (COM & LPT). If you do not see this heading, go to the View menu and make sure Show Hidden Devices is selected. If you still do not see your adapter, you will need to install the manufacturer's driver for your device.
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