SER0_TX not functioning properly
During prototype hardware test it was observed that the SER0_TX signal was not functioning properly. This is due to the absence of a 4.7kΩ pull-down resistor on SER0_TX in the design of the PXIeCOMe rev 0.4.
Explanation
Within the COMe-cKL6 user guide the following comment regarding SER0_TX can be found: “20 V protection circuit implemented on-module, PD on carrier boards needed for proper operation.” (Table 26: Connector X1A Row A Pin Assignment (A1- A110), page 51). Unfortunately, this is not documented for the COMe-bBD6.
This pull-down resistor is not present on the PXIeCOMe and explains the measured results. The reason why the pull-down resistor is not present, is because the PXIeCOMe is designed for COM Express Type 6 Revision 2.0 and up. So including the SER0 and SER1 ports and without protection against the 12V from a Revision 1.0 module. This protection, which is present on the Kontron COMe Eval Carrier T6, has a pull-down of 4.7kΩ. But the PXIeCOMe lacks the pull-down.
A second reason why the protection has not been implemented on the PXIeCOMe is to not limit the RS232 baud rate. The COM Express Carrier Design Guide Rev. 2.0 states: “Conformance to the protection scheme defined in COM.0 Rev 2 for pins recovered from the 12V pool results in a transfer rate limit of about 10kbaud. If your situation requires higher speeds, contact your Module vendor for possible work-arounds. The work-arounds likely involve sacrificing the 12V protection as a tradeoff for higher speeds.” And the SER0_TX and SER1_TX ports of the COMe interface are defined as 3.3V outputs.
To properly use the SER0_TX and SER1_TX on a COM Express 2.0 (and up) carrier, like the PXIeCOMe, at higher serial speeds, a 4.7kΩ pulldown is needed.
Fix
After applying the 4.7kΩ patch resistor (0402) between J6.9 and J6.10 on the top layer, SER0_TX is functioning properly.