Serial Communications (16C452)
SERIAL COMMUNICATIONS PROTOCOL
The following paragraphs describe the functioning of a serial data link
between the ZT 8809A and a terminal or other computer.
The
ZT 8809A is shipped configured as Data Communications Equipment
(DCE) for both serial ports 1 and 2. DCE is usually a device such as a
modem that would talk to a computer like a PC through the PC’s
COM1 port.
One general principle for DCE and DTE: if one piece of equipment is
configured in one sense (DCE, for example), the equipment with
which it is communicating must be configured in the opposite sense
(DTE, for example). The ZT 8809A employs a cable that converts the
frontplane serial signals to a female 25-pin D-type connector as
defined in the EIA Standard RS-232-C, Section 3.1, formally defining
the ZT 8809A as DCE. Both serial ports are jumper configurable for
DTE or DCE; refer to Appendix A for these jumper assignments.
The protocol shown in Figure 8-1 on page 8-5 could be used to link
the ZT 8809A as Data Communications Equipment (DCE) to a host
computer (DTE). Only the Request-To-Send (RTS) and Clear-To-
Send (CTS) handshake lines are used, although both RTS/CTS and
Data-Set-Ready
(DSR)
and
Data-Terminal-Ready
(DTR)
are
provided.
Both the transmit loop and the receive loop test for the CTS signal
and assert the RTS signal, but in opposite order from each other. In
the receive loop, CTS is set as a result of the other device asserting
RTS, indicating that the other device wants to transmit. Notice that
RTS and CTS are crossed at the interface. The ZT 8809A can monitor
the CTS signal via the Modem Status register, bit 4. The software
should loop until CTS is set. Refer to Table 8-3 on page 8-18 for a
definition of all the serial port registers.
8-3
