Making Things Talk

THE TOOLS 33

Figure 1-6

Configuring a serial connection in PuTTY.

Making_Things_Talk.indb 33 11/13/09 4:49 PM

34 MAKING THINGS TALK

Serial ports aren’t easily shared between applications.

In fact, only one application can have control of a serial

port at a time. If PuTTY or the screen program has the

serial port open to an Arduino module, for example, the

Arduino programming application can’t download new

code to the module. When an application tries to open

a serial port, it requests exclusive control of it either by

writing to a special file called a lock file or by asking the

operating system to lock the file on its behalf. When it

closes the serial port, it releases the lock on the serial

port. Sometimes when an application crashes while it’s

got a serial port open, it can forget to close the serial

port, with the result that no other application can open

the port. When this happens, the only thing you can

do to fix it is to restart the operating system, which

clears all the locks (alternatively, you could wait for the

operating system to figure out that the lock should be

released). To avoid this problem, make sure that you

close the serial port whenever you switch from one

application to another. Linux and Mac OS X users should

get in the habit of closing down screen with Ctrl- ...