There are several considerations when determining whether the deck box and CTD underwater unit will be compatible.
(1) In most cases (with the exception being (2), below), instruments with the “-plus” designation are compatible with each other, but the “-plus” variants are not compatible with the variants that do not have “-plus” in their model number (i.e., an SBE9plus CTD must be used with an SBE11plus, and cannot be used with an older SBE11 deck unit).
(2) If you have an SBE9/11plus system with the serial uplink feature installed, then both the deck box and the CTD must have the same hardware configuration from the factory (either enabled or disabled). Otherwise, no telemetry will be received from the CTD by the deck box.
(3) For older instruments that do not have “-plus” in their model number, you need a matching pair of SBE9 and SBE11. There was no standard configuration, and different CTDs and deck units could have telemetry word/rate differences (4/24, 8/24, 12/24, etc.) and power differences (standard low power or high power). You would need to consult the original documentation that shipped with the instruments or send them to Sea-bird service for a repair evaluation to determine compatibility.