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This is basically in brief the software protocol of the UART. How about the physical layer standards that actually defines the level of voltages and so on? There are quite a number of different standards that utilizes similar protocol. For instances, TTL level UART, RS-232, RS-422, RS-485 and etc. The TTL UART Most microcontrollers with UART uses TTL (Transistor-transistor Logic) level UART. It is the simplest form of UART. Logic 1 is represented by 5V and logic 0 by 0V.

Logic Voltage Low 0V High 5V Voltage level for TTL level UART The TTL level UART is commonly used in the communications between microcontrollers and ICs. Only 2 wires are required for the full duplex communications as illustrated in the picture below on figure 1. Figure 2: Serial Communication Between 2 devices The RS-232 The RS-232 is a standard for serial binary data signals connecting between a Data Terminal Equipment (DTE) and a Data Communication Equipment (DCE). One of the significant differences between TTL level UART and RS-232 is the voltage level. Valid signals in RS-232 are ±3 to – ±15V, and signals near 0V is not a valid RS-232 level. Logic Voltage Low +3 to +15V High -3 to -15V Voltage level for RS-232 Besides voltage level, the RS-232 also has a few extra pins specifically designed for the communication between PC and modem. The pinouts of the DB-9 and their functions are shown below in table 1.

Pin Signal abreviation Signal Name DTE (PC) 1 DCD Data Carrier Detect In 2 RXD Receive Data In 3 TXD Transmit Data Out 4 DTR Data Terminal Ready out 5 GND Signal Ground – 6 DSR Data Set Ready in 7 RTS Request to Send out 8 CTS Clear to Send In 9 RI Ring Indicator In Table 1: Pinout and diagram of DE9 connector (DB9 connector), commonly used for serial ports (RS-232). The DTE (PC) has the male connector (shown below), and the DCE (peripheral) has the female. Relient K The Birds And The Bee Sides Raritan here. The maximum cable length for RS-232 is about 15m (50ft), but in practice depends on baud rate, cable specific capacitance and ambient noise. The table below contains some rules-of-thumb for recommended distances.

Baud Rate Maximum cable length 56000 2.6m (8.5ft) 38400 3.7m (12ft) 19200 7.6m (25ft) 9600 15m (50ft) 4800 30m (98ft) 2400 60m (196ft) Table 2: Recommended distances Interfacing between TTL level UART and RS-232 From the above discussions, we know that microcontrollers use TTL level UART (5V for logic 1 and 0V for logic 0) while the PC serial port uses RS-232. Since both standards uses similar software protocol, both of them are able to communicate via UART. However, because of the differences in voltage level and polarity, we will need a level shifter to interface the TTL level UART with the RS-232. Nowadays, this can be easily done with the commonly available IC such as the MAX2322 from. The diagram below on figure 3 illustrates how the MAX232 IC can be connected to the UART of a microcontroller (TX of PIC connected to T1IN of MAX232 and RX pin to R1OUT of MAX232) and a personal computer with DB9 connector. Figure 3: The use of MAX232 IC to convert TTL levels from microcontroller to RS232 level EUSART Configuration with MPLAB Code Configurator MPLAB is phasing out the PIC18F Peripheral Library which is no longer included in XC8 v1.35. In this version, you have to download and install them separately into your compiler and they are now called.