Z80 datasheet(268/330 Pages) ZILOG

268 / 330 page

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UM008101-0601

Serial Input/Output

248

Transmit Transparent Mode

Transparent mode (Bisync protocol) operation is made possible by the

ability to change Transmit CRC Enable on the fly and by the additional

capability of inserting 16-bit sync characters. Exclusion of IDLE characters

from CRC calculation can be achieved by disabling CRC calculation

immediately preceding the IDLE character transfer to the Z80 SIO.

In the case of a Transmit Underrun condition in the Transparent mode, a

pair of DLE-SYN characters are sent. The Z80 SIO can be programmed to

send the DLE-SYN sequence by loading a IDLE character to WR6 and a

sync character to WR7.

Transmit Termination

The Z80 SIO is equipped with a special termination feature that maintains

data integrity and validity. If the transmitter is disabled while a data or sync

character is being sent, that character is sent as usual, but is followed by a

marking line rather than CRC or sync characters. When the transmitter is

disabled, a character in the buffer remains in the buffer. If the transmitter is

disabled while CRC is being sent, the 16-bit transmission is completed, but

sync is sent instead of CRC.

A programmed break is effective as soon as it is written to the control

In all modes, characters are sent with the least-significant bits first. This

requires right justification of transmitted data if the word length is less than

eight bits. If the word length is five bits or less, the special technique

described in the Write Register 5 discussion (Z80 SIO Programming

section) must be used for the data format. The states of any unused bits in a

data character are irrelevant except when in the Five Bits Or Less mode.

If the External/Status Interrupt Enable bit is set, transmitter conditions such

as “starting to send CRC characters,”“starting to send sync characters,” and

CTS changing state cause interrupts that have a unique vector if Status

Affects Vector is set. This interrupt mode may be used during block transfers.

Z80 Datasheet(PDF) 268 Page - Zilog, Inc.