USING SI-SCOPE WITH USB CONVERTERS
SI_SCOPE has built in support for USB serial adapters. They are inexpensive, easy to use, and are a great option for systems without expansion slots such as netbooks. Before using a USB serial adapter you should be aware of the differences between polled devices on a shared USB bus and interrupt driven addressable devices such as PCMCIA or ExpressCards.
When using a USB adapter SI_SCOPE will not be able to capture and timestamp at the interrupt level. As a result it is possible that the time stamps between two characters may appear as if the characters were received at the same time if the time interval between the two characters is smaller than the poll time of the USB device.
The poll time of the USB device may also cause data to display out of sequence when monitoring a bidirectional (full duplex) link if the response times between the two devices you are monitoring are less than the USB poll time. For example if the DTE device sends an “ATV1” command to a modem (DCE device) and the modem responds with the string “OK” it is possible that the first byte of the modem response (the 'O') may be displayed before the last byte of the DTE command (the '1') if the modem were to respond faster than the USB poll time. This situation can only occur on bidirectional data transmitted between the two devices within the USB poll time and will only effect the sequencing of the events. The source of the event (DTE or DCE) will be correctly displayed.
For most users this is not an issue and the convenience and low cost of the USB hardware makes this an attractive option. If you are going to utilize a USB converter for RS232 analysis we recommend using the VSCOM adapters we offer bundled with SI-SCOPE. These durable small footprint adapters can be configured to a latency time of 2 milliseconds making it one of the best converters to use.