Z80 Datasheet, PDF (266/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

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insertion of sync characters when there is no data to send. CRC is not
calculated on the automatically inserted sync characters. When the CPU
detects the end of message, a Reset Transmit Underrun/EOM command
can be issued. This action allows CRC to be sent when the transmitter has
no data. In this case, the Z80 SIO sends CRC, followed by sync charac-
ters, to terminate the message.
There is no restriction as to when in the message the Transmit Underrun/
EOM bit can be reset. If Reset is issued after the first data character has
been loaded, the 16-bit CRC is Sent and followed by sync characters the
first time the transmitter has no data to send. Because of the Transmit
Underrun condition, an External/Status interrupt is generated whenever the
Transmit Underrun/EOM bit becomes set.
In the case of sync insertion, an interrupt is generated only after the first
automatically inserted sync character has been loaded. The status indicates
the Transmit Underrun/ EOM bit and the Transmit Buffer Empty bit are set.
In the case of CRC insertion, the Transmit Underrun/EOM bit is set and the
Transmit Buffer Empty bit is reset while CRC is being sent. When CRC has
been completely sent, the Transmit Buffer Empty status bit is set and an
interrupt is generated to indicate to the CPU that another message can begin
(this interrupt occurs because CRC has been sent and sync has been
loaded). If no more messages are to be sent, the program can terminate
transmission by resetting RTS, and disabling the transmitter (WR5, D3).
Pad characters may be sent by setting the Z80 SIO to eight bits/transmit
character and writing FF to the transmitter while sending CRC. Alterna-
tively, the sync characters can be redefined as pad characters during this
time. The following example clarifies this point.
1. The Z80 SIO interrupts with the Transmit Buffer Empty bit set.
2. The CPU recognizes that the last character (ETX) of the message has
already been sent to the Z80 SIO by examining the internal program
status.
UM008101-0601
Serial Input/Output