Envoy Configuration and Management Manual

Introduction to Envoy

Envoy Configuration and Management Manual—429861-001

2-8

Multipoint Data Link

A somewhat more complex network consists of several terminals or computers

connected in parallel to a single communications line. This configuration is a multipoint

data link; each terminal or computer is a station in the network. In this case, the

application program in the host or supervising computer addresses each terminal or

computer by issuing the appropriate station address to the communications line.

In a multipoint data link (Figure 2-4

on page 2-9), one station is the supervisor. The

supervisor controls all communications over the link; all other stations are tributaries.

Communication can occur only between the supervisor and a tributary; tributary-to-

tributary communication is not permitted. Each tributary has a unique station address

that is defined at the application level in both the supervisor and the individual

tributaries. Transmissions between the supervisor and a tributary (in either direction)

always contain the particular tributary’s station address.

To solicit data transfers from the tributary stations, the supervisor periodically polls each

tributary station in the multipoint link. A tributary station cannot transmit data until it is

polled.

Figure 2-3. Point-to-Point Nonswitched Line

Privately Owned Cable or

Nonswitched Line

StationStation

CDT006.CDD