I mapped out the corresponding pins on both ends of the Guralp seismometer cable. Here is the diagram:

The circular 26-pin end of the cable (that plugs into the seismometer) is labeled as above. The other end (the 39-pin end) is not physically numbered, so I just came up with a numbering system. They are both pictured on the non-cable end of the connector. The colored circles indicate the pin pairs.

FROM JENNE, 30JULY2009:  the Dsub end is 37 pin, not 39.