If the cable from the lap top keeps the DSR (pin 6) at the control turned on, and your cable that has 6,8, and 20 bridged does not, then I'd say that the lap top cable probably has pin 4 from the lap top connected to 6 at the control, as suggested by dcoupar. Pin 4 of the DB9 connector is equivalent to pin 20 of the DB25 connector.
To check why the usual cable pin out is not working, check the voltage between pin 20 and 7 of the DB25 connector at the control. For logic 1 it should be +3 to +25v. If not, then you may have an issue with the cable between the DB25 female connector and the I/O board or the port its self. A work around could be to do as dcoupar suggested and power the DSR pin of the control from the DTR (pin 4) of the PC.