The SmartMotor uses an asynchronous serial interface often described as a "three wire implementation of RS232."
Asynchronous communications require a stable environment: if any bit of a transmission is lost, there is a high probability that the remainder of any transmitted message will be misunderstood. It is therefore necessary to carefully define the structure of the transmission. A serial bit is a defined period of time, and the state of the bit can either be a 1 or a 0, depending on the voltage state of the transmit line during the duration of the serial bit.
When the RS232 is in the idle state (waiting to transmit a message) it rests in the high (on) state. When a character is to be transmitted the TxD line is brought low for a carefully defined period of time, the time period of the start bit is a function of the BAUD rate. This low state is called the start bit.
Immediately following the start bit are a number of data bits. The SmartMotor must use 8 data bits, and this is the meaning of the "8" in the string "8-N-1."
Following the data bits in the is an optional parity bit. The parity bit used in some transmission schemes to check the validity of received characters on a byte-by-byte basis. Since this is an optional process which slows down communications, it is not implemented in the SmartMotor. The "N" in "8-N-1" stands for no parity bit. Following the optional parity bit is a bit called the stop bit. On ancient teletypes and other old machinery, a delay was required to ensure that the received character could be handled before the next character arrived. For this reason a stop bit was added at the end of every transmitted character. Depending on the delay needed, either one, one-and-a-half or two stop bits are added. The SmartMotor uses the minimum required period, 1 stop bit (8-N-1).
Following the above description, the SmartMotor uses 10 bit data structure to transmit an eight bit ASCII character. The SmartMotor requires a maximum of one signal change to transmit one bit, and since the baud rate is defined as the number of signal changes per second, this means that the data transmission rate is (BAUDRATE) /10 = characters per second.
Baud Rate Characters per Second
2400 240
4800 480
9600 960
19800 1980
38400 3840