to connecting instrument via rs232 to Digi transport WR31 which cable we use, strait or cross cable?

Hi

the wr31 is a DCE device

http://ftp1.digi.com/support/cabling/dte_vs_dce.pdf

so it will depend on what you are connecting to

normaly a straight cable if the other end is DTE
this will also depend on the standard and the pinout on the actual interfaces

here is a link to the user guide

https://www.digi.com/resources/documentation/digidocs/90001019/default.htm#references/r_transport_wr31_serial_pinout.htm?Highlight=serial

http://ftp1.digi.com/support/cabling/signals.pdf