Page 336 - Hacking Roomba

Chapter 14 — Putting Linux on Roomba 317

USB Serial Port Drivers

The Linux USB serial device drivers that have been packaged for OpenWrt are: ftdi_sio,
pl2303, belkin_sa, and mct_u232. These four drivers enable you to communicate with
90 percent of the USB-to-serial adapters out there and all of the generic ones.
Figure 14-9 shows a common generic USB-to-serial adapter. The actual chip for the adapter is
embedded in the hood of the RS-232 connector. These are available from various online retailers
like NewEgg.com for around $9. There’s no indication as to which chip one uses; you have to
plug it in to find out. But for only $9 it’s a pretty cheap experiment, and if it isn’t supported you
can probably use it on a fully-fledged PC.
The Keyspan adapter you’ve used in the previous chapters doesn’t use any of these drivers.
There does exist an open-source Linux driver for the Keyspan adapter. It just hasn’t been
packaged for OpenWrt yet. There’s no reason to think that it wouldn’t work. Most users of
USB serial on OpenWrt are connecting to GPS devices and PDAs, both of which tend to use
the same USB-to-serial chips as the generic adapters.
The RooStick uses the Silicon Labas CP2103 chip, which appears to work with the standard
Linux cp2101 driver. Unfortunately that’s another driver that hasn’t been packaged for use
with OpenWrt yet. There’s no reason to think it couldn’t be and it may be part of the next
OpenWrt distribution.

On the positive side, the Arduino board uses an FTDI chip that is supported by the ftdi_sio
driver. This means you could program an Arduino board to do something based on serial com-
mands and then plug the board into the WL-HDD. This is very powerful. It’s so easy to add
sensors and actuators to Arduino that Arduino could become a sort of I/O co-processor for the
WL-HDD.

There have been sporadic reports of problems with combinations of some WL-HDDs with certain
USB-to-serial adapters. If you have problems with one serial adapter, try another. If you think the
problem is due to the WL-HDD, try the Linksys WRTSL54GS discussed in Chapter 15. It’s almost
twice as expensive but has a more advanced USB interface with fewer reported problems. This is
the disadvantage of hacking consumer electronics: there’s no guarantee the vendor will keep the
device the same.

Installing USB Serial Port Drivers in OpenWrt
Thanks to ipkg, installing new kernel modules is a snap. Kernel modules are packaged up just
like normal programs. The two kernel module packages needed to get serial working are:

kmod-usb-ohci: Contains the drivers usbcore and usb-ohci needed to talk to the
USB interface.
kmod-usb-serial: Contains the general driver usbserial and the device-specific
drivers ftdi_sio, pl2303, belkin_sa, and mct_u232.