SECTION 6. 9-PIN SERIAL INPUT/OUTPUT

FIGURE 6.7-1. Transmitting the ASCII Character 1

If the computer/terminal is configured as DCE equipment (pin 2 is an input for RD), a null modem cable is required. See the SC32A manual for details.

6.7.3COMMUNICATION PROTOCOL/TROUBLE SHOOTING

The ASCII standard defines an alphabet consisting of 128 different characters where each character corresponds to a number, letter, symbol, or control code.

An ASCII character is a binary digital code composed of a combination of seven "bits", each bit having a binary state of 1 (one) or 0 (zero). For example, the binary equivalent for the ASCII character "1" is 0110001 (decimal 49).

ASCII characters are transmitted one bit at a time, starting with the 1st (least significant) bit. During data transmission the marking condition is used to denote the binary state 1, and the spacing condition for the binary state 0. The signal is considered marking when the voltage is more negative than minus three volts with respect to ground, and spacing when the voltage is more positive than plus three volts.

Most computers use 8-bits (1 byte) for data communications. The 8th bit is sometimes used for a type of error checking called parity- checking. Even parity binary characters have an even number of 1's, odd-parity characters have an odd number of 1's. When parity checking is used, the 8th bit is set to either a 1 or a 0 to make the parity of the character correct. The CR10 ignores the 8th bit of a character that is receives, and transmits the 8th bit as a binary 0. This method is generally described as "no parity".

To separate ASCII characters a Start bit is sent before the 1st bit, and a Stop bit is sent after

the 8th bit. The start bit is always a space, and the stop bit is always a mark. Between characters the signal is in the marking condition.

Figure 6.7-1 shows how the ASCII character "1" is transmitted. When transmitted by the CR10 using the SC32A RS232 interface spacing and marking voltages are positive and negative, as shown. Signal voltages at the CR10 I/O port are 5V in the spacing condition, and 0V in the marking condition.

BAUD RATE

BAUD RATE is the number of bits transmitted per second. The CR10 can communicate at 300, 1200, 9600, and 76,800 baud. In the Telecommunications State, the CR10 will set its baud rate to match the baud rate of the computer/terminal.

Typically the baud rate of the modem/computer/ terminal is set either with dip switches, or programmed from the keyboard. The instrument's instruction manual should explain how to set it.

DUPLEX

Full duplex means that two devices can communicate in both directions simultaneously. Half duplex means that the two devices must send and receive alternately. Full duplex should always be specified when communicating with Campbell Scientific peripherals and modems. However, communication between some Campbell Scientific modems (such as the RF95 RF modem) is carried out in a half duplex fashion. This can affect the way commands should be sent to and received from such a modem, especially when implemented by computer software.

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