RS232’s greatest benefit is its universality; most personal computers can use this protocol to send and receive serial data. The RS232 protocol specifies the use of two separate grounds, a signal ground and a protective (or chassis) ground. Two asynchronous communications ports named Serial1 and Serial2 can each be configured for RS232 or RS485 protocols. Typing a carriage return at the terminal should now produce the familiar ok response via the Serial2 port. We can gain insight into the operation of the RS232 protocol by examining the signal connections used for the primary serial port in the above table. The secondary channel is very useful for debugging application programs that communicate with other computers or I/O via the primary channel. The primary and secondary serial communications ports are accessible through the PDQ Board's 10 pin, dual row Communications Header (H2) and through the Docking Panel's 10 pin, right-angle, rs485 cable dual row Communications Header (H1) and individual DB-9 Serial 1 and Serial 2 connectors. In fact, the program works the same as it did before, but now it is using the secondary serial port instead of the primary port -- and you didn’t even have to recompile the code!

As described below, the RS485Transmit() function controls the RS485 data direction of each serial port. The RS485 data direction of Serial1 and Serial2 are controlled by the PJ0 and PJ1 processor pins, respectively. The Serial1 and Serial2 ports have identical communications capabilities, although more of the Serial1 signals (both RS232 and RS485) are made available on the Docking Panels headers and connectors. The Serial 1 and Serial2 ports can be configured for either RS-232 or RS-485 communications at standard baud rates up to 115200 bits per second. Moreover, if Serial2 is running full duplex at 4800 baud, any other interrupt service routine that takes longer than 100 µs is likely to cause a problem. At the start of a transmitted character, the service routine takes about 65 µs. In the middle of a character, each interrupt service routine takes about 35 µs. The mating 10-pin connectors that join the H6 header of the PDQ Board to the H4 header of the Docking Panel are typically not accessed directly, and are not discussed in detail here.

These factors are discussed in more detail in The UART Wildcard: Modem Handshaking Signals. The AVR1284-3U board uses removable UART interfaces called ComBoards. These signals are commonly called A and B in RS485 systems. At the UART, a logic high (5 volts) is called a mark, and a logic low (0 volts) is called a space. High (mark) parity means that the parity bit is always logic 1 at the UART, and low (space) parity means that the parity bit is always logic 0 at the UART. Parity checking is not often used, because it is not a robust method of error detection. The above parity settings will also determine how incoming data is interpreted (whether the most significant bit is considered a parity bit or part of the data being transmitted, and how many bits total to expect in each byte). For seven data bits with a parity bit, M would be cleared (equal to zero), and PE would be set in order to make the most-significant bit of a normal eight-bit byte be used by the serial port as a parity bit.

The M bit, with mask 0x10, determines whether eight or nine bits total are transmitted with each byte, regardless of whether or not the most-significant bit is a parity bit. Each serial port can be configured for the RS232 or RS485 protocol, and runs at standard baud rates up to 115,200 bits per second. Having a second serial port is also handy for system debugging. Since both channels can operate simultaneously and independently, serial debugging can be performed while the application program is communicating via its primary channel. The primary serial channel can operate at standard speeds up to 19200 baud and can be configured for either RS232 (the default) or RS485 operation. Although the maximum standard baud rate of the primary serial port is 19200 baud, nonstandard baud rates of over 80 Kbaud can be attained by the 68HC11's on-chip UART and the onboard RS232 driver. After a factory cleanup, Serial1 is the default serial port for program development and downloading.