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The USB Multiheaded Monster...
We can do a little better than 127 devices by using additional USB controllers. Although most PCs are equipped with multiple USB ports, these ports often are on the same controller, and the 127 device limit constrains both ports. Even if we add more controllers, there is an upper limit of 256 USB-serial devices allowed, or r </= 256. This is a limitation of the number of allowable minor numbers for the assigned USB-serial major number, which is 188. More industrious users probably could modify the driver to allow additional majors. Adding additional controllers also may be required in cases where the serial ports have a high baud rate; keep in mind the USB bus is limited to 12Mbps. Timing issues also may keep the usable number of USB-serial adapters below the theoretical limit, but adding controllers is relatively easy and inexpensive.
The other limitation of using USB devices is the interconnect length; cables are limited to a maximum length of five meters. Effective cable length can be extended by using an active device, such as a powered hub or powered extension cable, though even an active extension cable counts as a device. The total depth is limited to seven tiers, counting the root and the bottom device. This means there is a maximum of six 5m interconnects for a total length of 30m. Although 30m is sufficient to reach each corner of our server room, the reach can be increased another 15m by using shielded RS-232 cable to connect the USB-serial adapter to the server console port. Other ways of extending the RS-232 signal are available; for example, use a pair of RS-422 adapters with an effective range of about 1.3km.
http://www.linuxjournal.com/article/6518
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