Industrial Ethernet and USB-to-Serial Adapters
Configurable and managed switches are becoming more popular in the world of Industrial Ethernet. Typically such switches are accessed through either a serial port or by a web browser. However, to optimize security, local control should be exercised over the management information -- and this normally involves a null-modem cable connection between the switch and the computer used for setting the management options. But problems may arise when laptops are used for this function.
It is becoming harder to find a laptop that has a serial port -- and under these circumstances the Industrial Ethernet installer or supervisor must employ an alternate method of connecting the laptop to the serial port on the switch. Two types of serial adapters are commonly available: a PC Card (PCMCIA) adapter or a USB adapter.
PC Card adapters are more expensive than USB adapters (sometimes hundreds of dollars more), but they are always much faster and typically more reliable. Furthermore, the low-cost choice of a USB-to-serial adapter may not even work at all. A PCMCIA-to-serial adapter offers a hardware solution whereas a USB-to-serial device requires both hardware and software for the conversion. Running the conversion through the USB adapter's software complicates the data transfer.
Depending on serial baud rates and computer programs competing for processor time, a USB adapter can easily lose sync or drop bits. Although PC Card adapters cost a lot, spending hours solving a USB adapter issue can cost more. USB adapters vary considerably in quality. Some people have tried several different types to no avail. One adapter may work for one switch but not another and vice-versa.
Our engineering department has tried some USB adapters that worked and some that did not. Surprisingly, even a $40 USB-to-serial adapter from Radio Shack did the job. But be forewarned: a USB adapter could easily take up to five times as long to transfer data as a PC Card solution.
USB-to-serial adapters that worked for us:
Radio Shack --- model 26-183
Belkin --- model F5U103
PC Card-to-serial adapters that worked for us:
Quatech --- model SSPR-100
Quatech --- model DSPR-100
3 Comments:
Bill-
Terrific info. You just saved me from a disaster!
Thanks,
Jim
I'm curious when you say they "did not work". What was the symptom? We are currently having problems with these adapters and we've tracked it down to massive (2Vpp) noise on the Rxd or Txd lines when the comm port isn't open.
Regarding the "did not work" question: First of all, thank you for your information. It is much appreciated.
However, we did not examine the causality in detail. We only observed that, with the problematic models, communication failed reliably due to dropped bits.
Perhaps other readers will post their findings as you did and the mystery will be further unveiled.
Post a Comment
<< Home