Specifications

Chapter 6 - Samples
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Serial Communication
SCI is an abbreviation for Serial Communication Interface and, as a special subsystem, it exists on
most microcontrollers. When it is not available, as is the case with PIC16F84, it can be created in
software.
As with hardware communication, we use standard NRZ (Non Return to Zero) format also known
as 8 (9)-N-1, or 8 or 9 data bits, without parity bit and with one stop bit. Free line is defined as
the status of logic one. Start of transmission - Start Bit, has the status of logic zero. The data
bits follow the start bit (the first bit is the low significant bit), and after the bits we place the Stop
Bit of logic one. The duration of the stop bit 'T' depends on the speed of transmission and is
adjusted according to the needs of the transmission. For the transmission speed of 9600 baud, T
is 104 uS.
1. CD (Carrier Detect)
2. RXD (Receive Data)
3. TXD (Transmit Data)
4. DTR (Data terminal Ready)
5. GND (Ground)
6. DSR (Data Set Ready)
7. RTS (Request To Send)
8. CTS (Clear To Send)
9. RI (Ring Indicator)
Pin designations on RS232 connector
In order to connect a microcontroller to a serial port on a PC computer, we need to adjust the
level of the signals so communicating can take place. The signal level on a PC is -10V for logic
zero, and +10V for logic one. Since the signal level on the microcontroller is +5V for logic one,
and 0V for logic zero, we need an intermediary stage that will convert the levels. One chip
specially designed for this task is MAX232. This chip receives signals from -10 to +10V and
converts them into 0 and 5V.
The circuit for this interface is shown in the diagram below:
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