Straight serial cable is Tx to Tx and Rx to Rx. Null is Tx to Rx and Rx to Tx.

If I understand correctly, in theory one device should be DCE and one is DTE, where the devices are 'expecting' there to be a straight through connection between them.

"The serial port was originally intended to be used for connecting DTE to DCE which makes cabling simple: just use a straight-thru cable. Thus when one connects a modem one seldom needs to worry about which pin is which. But people wanted to connect DTE to DTE (for example a computer to a terminal) and various ways were found to do this by fabricating various types of special null-modem cables. In this case what pin connects to what pin becomes significant."