Quatech SSP-300, SSP-200 user manual Half Duplex Operation, Options

6.2 Half Duplex Operation

The SSP-200/300’s ports may be configured for either full duplex or half duplex operation. By default, the RS-422/485 ports are configured for full duplex operation with the RS-422/485 output drivers always enabled.

In half duplex mode, the RS-422/485 transmitter may be enabled and disabled via the RTS (request to send) or DTR (data terminal ready) signals. Both RTS and DTR are controlled through the Modem Control Register of the 16750.

3 Options:

Half Duplex

-RTS

-DTR

16C550

Full Duplex

MUX

Data Out

RS-422/485			+
	Driver		-
	Active Low
	Output Enable	Aux Out
			+
RS-422/485
	Driver		-

NOTE: One of these three options must be selected via software configuration. Full duplex operation is the default mode.

Figure 0. RS-422/485 Driver Enable Options

If RTS is selected as the signal to enable the output drivers, setting 'bit 1' of the Modem Control Register (to logic '1') will enable the output drivers and clearing 'bit 1' of the Modem Control Register (to logic '0') will force the outputs into a high impedance state.

Similarly, if DTR is chosen as the signal to enable the output drivers setting 'bit 0' of the Modem Control Register (to logic '1') will enable the output drivers and clearing 'bit 0' of the Modem Control Register (to logic '0') will force the outputs into a high impedance state.

Hardware Information 6-3