Because of the uncertainty and indeterminacy of time delay when exchanging data by industrial switchboards，the process layer network cannot provide data to the relay system.Based on media access control（MAC）simulation technology of field-programmable gate array（FPGA），data exchange units with characteristics of explicit transfer delay and equi-spaced exchange are designed，constructing sampled value（SV）network by data exchange units instead of industrial switchboards.Not only can the solution realize the sharing of sampling data at the process layer，but also the structure of sampling network for the relay system is simplified.The theory and schematic design of data exchange units are described in detail.