A-B 1771-DCM

A-B  1771-DCM  Direct Communication Module

The 1771-DCM is a chassis-mounted single-slot I/O module that allowscommunication between a supervisory processor and local processors(Figure 2.1). The local processor can be chassis-mounted or configuredas a local or remote system.

The 1771-DCM passes data table values such as command bits, statusbits, and data blocks between supervisory and local processors. Thesupervisory processor typically writes commands and/or data table valuesto the local processor, and reads resulting status, diagnostic data, and datavalues from the local processor (or vice versa).

The supervisory processor communicates serially with the 1771-DCM asthough it were a 1771-ASB Adapter Module, and addresses the1771-DCM as though it were addressing a separate I/O chassis. Youselect the mode of transfer between the supervisory processor and1771-DCM: discrete transfer (up to eight words) or block transfer (up to64 words). Either mode includes one status word.

The local processor communicates with the 1771-DCM over thebackplane using block transfers and addresses the 1771-DCM as anintelligent I/O module. When the local processor is remotely configured,it communicates with the 1771-DCM via block transfer through itsscanner-adapter link.

Choosing 1771DCM or DataHighway

The 1771-DCM and data highway allow communication between PCprocessors. Consider the following factors when determining theappropriate choice. Use the 1771-DCM to: 

  • Transfer data table words 

  • Obtain predictable transfer times 

  • Transfer up to eight words when transfer time over a remote I/Ochannel is adequate, in addition to the transfer time of the localprocessor and 1771-DCM 

  • Transfer up to 64 words when transfer time for block transfers over aremote I/O channel is adequate, in addition to the transfer time of thelocal processor and 1771-DCM 

  • Decrease data highway traffic

At least one processor must have a remote I/O scanner.

You cannot transfer ladder diagram programs nor messages stored in themessage area of memory using the 1771-DCM.

Other models：1771-NT2 ，1771-OA ，1771-OAD ，1771-OB ，1771-OBN ，1771-OD ，1771-OD16 ，1771-ODD ，1771-OFE1 ，1771-OFE