Z80 Datasheet, PDF (275/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

Z80 CPU Peripherals
User Manual
255
Table 7. Bisync Receive Mode (Continued)
Function
Termination
Typical Program Steps
Redefine Interrupt Modes and Sync
Modes
Update Modem Controls
Disables Receive Mode
Comments
SDLC (HDLC) OPERATION
Overview
The Z80 SIO allows processing of both High-level Synchronous Data
Link Control (HDLC) and IBM Synchronous Data Link Control (SDLC)
protocols. In this chapter only Synchronous Data Link Control (SDLC) is
covered because of the similarity between SDLC and HDLC.
The SDLC mode is bit oriented which differs from Synchronous Bisync
protocol, which is character oriented. Therefore, SDLC mode allows trans-
parent operation and variable message length. The bit-orientation SDLC is
a flexible protocol because it can process longer message length and bit
patterns. IBM document GA27-3093 has more information about SDLC
protocol.
The SDLC message, called the frame (Figure 113), is opened and closed by
flags that are similar to the sync characters in Bisync protocol. The Z80 SIO
handles the transmission and recognition of the flag characters that mark
the beginning and end of the frame. Although the Z80 SIO can receive
shared-zero flags, it cannot transmit them. The 8-bit address field of an
SDLC frame contains the secondary station address. The Z80 SIO has an
Address Search mode that recognizes the secondary station address,
allowing it to accept or reject the frame.
UM008101-0601
Serial Input/Output