Zilog Z80230 - Manual (Page 76)

SCC/ESCC
User Manual
UM010903-0515 Interfacing the SCC/ESCC
69
Z85X30 Receive Request Release
Test Functions
The SCC contains two other features useful for diagnostic purposes, controlled by bits in WR14.
They are Local Loopback and Auto Echo.
Local Loopback
Local Loopback is selected when WR14 bit D4 is set to 1. In this mode, the output of the transmit-
ter is internally connected to the input of the receiver. At the same time, the TxD pin remains con-
nected to the transmitter. In this mode, the /DCD pin is ignored as a receive enable and the /CTS
pin is ignored as a transmitter enable even if the Auto Enable mode has been selected. Note that
the DPLL input is connected to the RxD pin, not to the input of the receiver. This precludes the use
of the DPLL in Local Loopback. Local Loopback is displayed schematically in Figure on page
70.
Auto Echo
Auto Echo is selected when bit D3 of WR14 is set to 1. In this mode, the TxD pin is connected
directly to the RxD pin, and the receiver input is connected to the RxD pin. In this mode, the /CTS
pin is ignored as a transmitter enable and the output of the transmitter does not connect to any-
thing. If both the Local Loopback and Auto Echo bits are set to 1, the Auto Echo mode is selected,
PCLK
/REQ
/RD
D7- D0 Receive Data