Connecting to the Balloon Serial Port
The balloon serial console is the basic level of access to the device. You may well not need it for normal use but almost any development work is likely to need it and it is the only way to recover if any of the basic stuff breaks (bootloader, kernel, rootfs) such that the machine does not boot.
There are many different connection arrangements depending exactly what case and add-on boards your balloon is mounted in. Different CPLD/FPGA logic variants can also affect this (TCL variants pass the serial through to the mainboard, inverting signals on the way).
See device-specific information for exactly where to plug things in. This page gives generic details for the 'PC' end of the connection.
The balloon is configured for 115200 baud, 8N1 (8 dta bits, no parity, 1 stop bit), no hardware flow control, no software flow control. Only 3 wires are connected (to the normal console port): GND, RX, TX.
The port is exposed both at 3.3V TTL levels (on backplane connector, and on J12) and a RS232 levels (on power connector and on J11). The TTL levels are useful for talking to other attached hardware. The RS232 levels are needed for talking to another computer down a potentially long cable.
You need something that talks to a plain serial port. Much 'serial' software is actually concerned with talking to a modem attached to a serial port, which is a slightly different/more complex matter. On Linux we use minicom or screen. screen is simpler, minicom provides convenient serial upload (xmodem/zmodem) functionality. Hyperterm on Windows (XP or earlier). For Vista and Windows7 you need to download one as hyperterm is no longer supplied.
If your machine is old enough to have a real serial port then connect the balloon directly to that. Normally a serial extension cable is needed as the balloon is rarely within 8cm of the port. Configure the comms software to use /dev/ttyS0 (or sometimes /dev/ttyS1) as the device.
On most modern hardware you will need a USB<->serial adaptor. Most of these work fine on Linux by just plugging them in (they usually have a 'prolific' chip inside). Configure comms software to use /dev/ttyUSB0 as the device.