USING THE DUAL PORT PASSIVE MONITORING CABLE WITH USB CONVERTERS
SI_SCOPE has built in support for USB serial converters when using the Dual Port Passive Monitoring Cable (PMC). Before using a USB serial converter with your SI-SCOPE PMC you should be aware of the differences between USB serial converters and interrupt driven addressable devices such as PCMCIA and ExpressCards or a specialized USB device such as our UltraTap Cable.
When using a USB serial converter SI_SCOPE cannot use its specialized device driver and instead must resort to the standard Windows driver to see the converter. As a result SI-SCOPE will not be able to capture and timestamp at the interrupt level making it possible that the time stamps between two characters appear as if they were received at the same time if the interval between the two characters is less than the poll time of the USB converter.
The poll time of the USB serial converter 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 serial converters makes this an attractive option. However if you are trying to reverse engineer an unknown protocol or if inter character timing is important to your application we recommend using an interrupt driven serial card with your PMC or using our UltraTap Cable which connects to your system through a USB port but handles RS232 data capture and time stamping through specialized embedded electronics in the cable.