Sunday, March 28, 2010


Booting an embedded target requires some cabling that may not be handy. A typical commercial embedded development kit includes cables; however, they tend to get lost over time. If the board in question doesn’t have an integrated display (or display adapter) and keyboard connector, at least a null modem serial cable is required. It’s best to locate these components now, because you’ll need them.

Serial (for Console)
Nearly every embedded development board has a serial console connector in the form of a DB-9 connector. Most boards require a null modem cable that swaps the send and receive pins and connects the clear to send and request to send pins. You can buy a null modem cable at any self-respecting electronics store and most big-box office supply stores. When confronted with the task of finding a null modem cable in a jumble of cables at work, look for one with female connectors on both ends; this doesn’t guarantee that the cable in question is properly wired, but chances are that it is. You can also purchase a null modem converter that makes a regular straight-through cable a null modem. Some boards use a USB connection for the serial console. Linux and Windows include device drivers to make communicating over USB essentially the same as using serial. From a cabling perspective, all you need to do is find a USB cable with the right connectors. The USB specification ensures that the device and computer are wired properly; there's no need for special cabling.

A network connection is also a common way for a board to communicate with the outside world. The Linux kernel is large enough that downloading via a serial cable (even if the connection is fast) is slow. When the boot loader uses the network adapter, downloading the kernel takes just a few seconds.

You can attach an embedded board with an Ethernet port to the network using a standard network cable. However, if the board is connected directly to another network adapter, the Ethernet equivalent of a null modem cable is necessary; it’s called a cross-over cable. This Ethernet cable is wired to the serial null modem: the data wires are swapped from input to output, and the flow control pins are crossed. These cables are readily available at electronics stores but are difficult to find elsewhere. Unlike serial cables, a null modem converter for Ethernet cables isn't readily available. In a pinch, using a Ethernet hub is a workaround, because when it moves data from one port to the next, it does roughly the same as a null modem converter.

Source of Information : Pro Linux Embedded Systems

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