Z80 datasheet(280/330 Pages) ZILOG

280 / 330 page

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7UGT /CPWCN

UM008101-0601

Serial Input/Output

260

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 is sent and the flag is loaded. If

no more messages are to be sent, the program can terminate transmission

by resetting RTS, disabling the transmitter.

In the SDLC mode, reset the Transmit Underrun/EOM status bit immedi-

ately after the first character is sent to the Z80 SIO. When the Transmit

Underrun is detected, this ensures that the transmission time is filled by

CRC characters, giving the CPU enough time to issue the Send Abort

command. This procedure also stops the flags from going on the line

prematurely and eliminates the possibility of the receiver accepting the

frame as valid data. For example, the data pattern, immediately preceding

the automatic flag insertion, could match the CRC checker, thereby giving a

false CRC check result. The External/Status interrupt is generated when-

ever the Transmit Underrun/EOM bit is set as a result of the Transmit

Underrun condition.

The transmit underrun logic provides additional protection from premature

flag insertion if the proper response is given to the Z80 SIO by the CPU

interrupt service routine. The following example illustrates this point:

1. The Z80 SIO interrupts with the Transmit Buffer Empty status bit set.

2. The CPU does not respond in a timely manner, which causes a

Transmit Underrun condition.

3. The Z80 SIO starts sending CRC characters (two bytes).

4. The CPU eventually satisfies the Transmit Buffer Empty interrupt

with a data character that follows the CRC character being

transmitted.

5. The Z80 SIO sets the External/Status interrupt with the Transmit

Underrun/EOM status bit set

6. The CPU recognizes the Transmit Underrun/EOM status and

determines from its internal program status that the interrupt is not

“end of message”.

Z80 Datasheet(PDF) 280 Page - Zilog, Inc.