Siemens SIMATIC TIWAY 1 - Manual (Page 14)

OPERATION AND APPLICATION
UNILINK HOST ADAPTER 2–4
USER MANUAL
2.3 FUNCTIONAL ORGANIZATION AND OPERATING MODES
The UNILINK Host Adapter is organized into two primary functions; the Host
Interface Unit (HIU) function and the Network Manager (NM) function. An
adapter can be configured with one of these functions or they can be combined to
form a Master Host Interface Unit (MHIU). The MHIU, HIU, and NM are three
of the six configurable operating modes.
In addition to the above modes, there are two Emulate Host Adapter (EHA)
modes and a Standalone Network Manager (SANM) mode.
2.3.1 Host Interface Unit Function
The primary purpose of the Host Interface Unit (HIU) function is to establish
communications between the host computer and the secondary devices on a
TIWAY I network. The HIU uses the memory in the adapter for macro buffers
and to store host-to-secondary statistics as shown in Figure 2–4. Macro buffers
are used to store macro commands and responses. Macros provide a method for
connecting, disconnecting, or sending a Primitive to a secondary device without
delays in host processing. Macros are discussed further in Paragraph 2.10.1.
Commands are received from the host computer and checked for validity. Then
the host requests are processed and queued to the Network Manager function
for routing to the appropriate secondary device. The Master Host Interface Unit
(MHIU) mode combines the HIU function and the Network Manager function.