[Exposition]
The 1772-SD2 was an A-B Universal Remote I/O scanner module for the PLC-2/20 and PLC-2/30 controllers that allowed them to scan up to seven 1771 style I/O chassis.
Usually the 1771 chassis had 1771-ASB RIO network adapters in them, but there are many A-B devices that emulate an I/O chassis adapter, like the old PanelViews and variable frequency drives.
The 1771-DCM emulates an I/O chassis adapter, but fits into a chassis with a PLC controller like a PLC-5 or a Mini-PLC2. The 1771-DCM emulate a single "Rack" of 8 Words of discrete I/O memory, and accepts Block Transfers of up to 64 words.
The 1771-DCM has been functionally obsolete since the PLC-5 Enhanced controllers gained the ability to configure one channel as a RIO Adapter in the early 1990's, and the 1771-DCM went completely out of production in 2010.
[/Exposition]
There is no successor module to the 1771-DCM that performs the same functions in the same form factor.
I can think of three approaches to the problem.
1. Use a PLC-5 Enhanced, Ethernet, or ControlNet controller that has an available RIO channel to be configured in Adapter Mode.
2. Use a ControlLogix chassis with ControlLogix CPU, I/O, and a 1756-RIO module configured for Adapter Mode. This is not the 1756-DHRIO, but rather the newer 1756-RIO that does have an Adapter Mode.
3. Use a standalone data gateway that functions in RIO Adapter Mode, specifically the QTS AN-X module. They have a marketing/sales/support relationship with Prosoft now, and Prosoft has their own family of older RIO gateways.
You're going to have to go into the project with your eyes open and your oscilloscope fired up. It's very unlikely that anyone has tested a PLC-2 scanner with the AN-X. Even though the RIO protocol is very old and very robust, there could still be problems mixing very old and very new hardware.
If this were my project, I would not undertake it without also having a detailed upgrade plan for the PLC-2 control system. "Run to failure" is an acceptable plan as long as you've got enough information and preparation to handle the failure.
