Specifications

Serial Communications (16C452)
Data Communications Equipmnent
(DCE)
ZT 8808A/8809A COM 2
POWER-UP
Data Terminal Equipment
(DTE)
OTHER
POWER-UP
Do I xmit?†
Is CTS set?
Assert RTS
Recv byte
Last byte?
De-assert RTS
Last byte?
Xmit byte
Is CTS set?
Assert RST
no
no
yes
no
no
Do I xmit?†
Is CTS set?
Assert RTS
Recv byte
Last byte?
De-assert RTS
Last byte?
Xmit byte
Is CTS set?
Assert RST
no
no
yes
no
no
TXD
RXD
RTS
CTS
GND
J2
TXD
RXD
RTS
CTS
GND
USER
ZT 8808A/8809A
†A communications priority scheme must be provided at this junction to prevent both devices
from transmitting at the same instant. The system designer must assign either the DCE or the
DTE to transmit only after having received a message indicating it is okay to transmit.
Figure 8–1. Establishing Serial Communications.
Both the transmit and the receive loop test for the last byte by
specifying a data string length and/or an End-Of-String (EOS)
character in the software. When the data transfer is complete, the RTS
line is deasserted. When the ZT 8809A was receiving data, this action
told the DTE it was no longer "clear to send" data. When the
ZT 8809A was transmitting, it told the DTE that the ZT 8809A was
no longer "requesting to send" data. The software should loop back
and repeat the process again.
8-5