Z80 Datasheet, PDF (245/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

Z80 CPU Peripherals
User Manual
225
during which the Z80 SIO tries to match the assembled character in the
receive shift register with the nag pattern in WR7. When the first flag char-
acter is recognized, all subsequent data is routed through the same path,
regardless of character length.
Although the same CRC checker is used for both SDLC and synchronous
data, the data path taken for each mode is different. In Bisync protocol, a
byte-oriented operation requires that the CPU decide to include the data
character in CRC. To allow the CPU ample time to make this decision, the
Z80 SIO provides an 8-bit delay for synchronous data. In the SDLC mode,
no delay is provided because the Z80 SIO contains logic that determines the
bytes on which CRC is calculated.
The transmitter has an 8-bit transmit data register that is loaded from the
internal data bus and a 20-bit transmit shift register that can be loaded from
WR6, WR7, and the transmit data register. WR6 and WR7 contain sync
characters in the Monosync or Bisync modes, or address field (one char-
acter long) and flag respectively in the SDLC mode. During Synchronous
modes, information contained in WR6 and WR7 is loaded to the transmit
shift register at the beginning of the message and, as a time filler, in the
middle of the message if a Transmit Underrun condition occurs. In the
SDLC mode, the flags are loaded to the transmit shift register at the begin-
ning and end of message.
Asynchronous data in the transmit shift register is formatted with start and
stop bits and is shifted out to the transmit multiplexer at the selected clock
rate. Synchronous (Monosync or Bisync) data is shifted out to the transmit
multiplexer and also to the CRC generator at the x1 clock rate.
SDLC/HDLC data is shifted out through the zero insertion logic, which is
disabled while the flags are sent. For all other fields (address, control, and
frame check) a 0 is inserted following five contiguous 1s in the data stream.
The CRC generator result for SDLC data is also routed through the zero
insertion logic.
UM008101-0601
Serial Input/Output