Z80 Computer Serial Interface and Timer Card

Schematic , Z80 CPU , Z80 UI

     The 1740 uses the Z84C3008PEC CTC and the Z84C4410VEC SIO to implement a 2 channel full duplex RS232 interface for the Z80 computer with independently programmable BAUD rates for each channel. This card also provides a timer to interrupt the processor at periods between 30.5uS and 2S (Using a 32.768kHz clock) as well as two 8 bit output ports connected to 16 LEDs.

Hardware Features:

• 3.0 inch wide by 2.5 inch high 4 layer PWB with a 40 pin (2 x 20 x 0.1) edge mounted female connector to mate with the Z80 Backplane or user circuit.
• Two full duplex RS232 ports; RS232A and RS232B.
• Both ports support hardware hand shaking using the CTS and RTS signals.
• RS232A port connection is jumper selectable between a DSUB-9M connector or an FT232RL USB interface.
• RS232B port connection is implemented with a DSUB-9M connector.
• The DSUB-9M connectors are wired as DTE, i.e. the same pinout as an RS232 port found on a PC for direct connection to devices such as modems. To connect to a PC, a null modem adapter or cable is needed.
• The DSUB-9M serial ports use MAX232 drivers.
• The Z80 CTC is mapped to I/O ports 0xF4 to 0xF7.
• The Z80 SIO is mapped to I/O ports 0xF8 to 0xFB.
• The two 8 bit output ports for driving the 16 LEDs are mapped to output ports 0xFC and 0xFD.

Baudrate generation

• CTC channels 0 (RS232A) and 1 (RS232B) divide a 7.3728MHz clock to derive the serial port bit clocks.
• With a divider 48, using the SIO Clk/16 mode the baud rate range for both ports is 50 to 9,600bps.
• With a divider 1, using the SIO Clk/16 mode the baud rate range for both ports is 19,200 to 460,800bps.
• Note: The SIO Clk/1 mode does not work without external bit synchronization hardware so higher baud rates can't be achieved with this SIO option: The SIO has to have a clock of at least double the baud rate so that it can sample the bits following the start condition at 0.5 bits, 1.5 bits and so on otherwise it would sample at the uncertain bit trailing edges.

Timer function

• CTC channels 2 and 3 provide the user with a programmable interval timer with interrupt capability.
• Channel 2 is supplied with an on board 32.768kHz clock and then channel 2's output is fed into the input of channel 3.
• In counter mode (Using the 32.768kHz clock) this provides for output intervals ranging from 30.5uS to 2S that function normally while the CPU is single stepped.
• In timer mode (Using the 8MHz CPU clock) this provides for intervals ranging from 2uS to 2.0972S that will single step when the CPU is single stepped.