SC520, v1.0, User Guide

The SC520, serial card houses a 68B50 ACIA chip, which provides a TTL level asynchronous serial port for use with FTDI style serial adapter cables.

Jumper Settings

The illustration below shows the default jumper shunt positions which set the card as follows:

  • The base I/O address is 0xA2 (ACIA occupies 0xA2 and 0xA3)
  • Serial port set to 115200 baud
  • Power is supplied by the Z50Bus (not SC520)
  • Interrupt not connected (see software requirements)
Default jumper shunt positions

Address Selection

Jumper shunts on JP1 allow the base address of the card to be set. The card occupies two I/O addresses. Some firmware and software support two cards.

PortAddressJumper shunt(s)
PrimaryBase address 0xA2
Range 0xA2 and 0xA3
Bit 7 and Bit 5
and Bit 1
SecondaryBase address 0xA4
Range 0xA4 and 0xA5
Bit 7 and Bit 5
and Bit 2

The primary address is the default supported by the current firmware and software. A second card can be added to the system with the secondary address selected. Support for a second card will depend on the software being used.

Powering the system

Fitting a jumper shunt to header JP2 connects the FTDI style 5-volt serial port’s power pin to the card’s 5-volt supply. This enables the retro computer system to be powered from the serial port, or a device connected to the serial port to be powered from this card.

Alternatively, power can be supplied from the Z50Bus.

Do not attempt to power the system from two different sources. If the system is not being powered from a serial port, then only fit a jumper shunt to JP2 if the serial device is being powered from the system.

Serial on the Z50Bus

The Z50Bus has two lines dedicated to a serial interface. This enables two cards to communicate via the bus rather than having to have additional cables between the cards.

Jumpers JP3 and JP4 enable the serial port to be connected to the Z50Bus RX and TX lines. Leave the jumper shunts off to isolate this card’s RX and TX signals from the Z50Bus.

Serial Port

The FTDI style 5-volt serial port is provided at P2 and S1.

The pin-out, below, describes signals with respect to the card, so output is a signal from the card to a computer or terminal.

6Clear To Send (CTS) input to the card
5Transmit Data (TxD) output from the card
4Receive Data (RxD) input to the card
3Vcc (5V)
2Request To Send (RTS) output from the card
1Ground (GND)

The serial port settings depend on the card’s jumpers and also the firmware or software being run. Typically these are:

Baud rate115200
Data bits8
Stop bits1
Flow controlHardware

As this card has its own non-programmable baud rate generator the baud rate settings in software, such as RomWBW and SCM, are ignored. Software should be set to 115200 baud regardless of the jumper shunt position on this card. See technical note below.

Technical note:
Software should set the ACIA’s internal divider to 64. This will generate the indicated baud rates from a 7.3728 MHz oscillator. It is best to set the software to assume 115200 baud, regardless of the jumper setting, as selecting a slow baud rate in software may result in the ACIA’s internal devider being set to a value other than 64.

Clock Source

This card has its own oscillator as a clock source for the ACIA, making the serial port function independent of the CPU’s main clock.

Software and Firmware Support

Z80 systems with the Small Computer Monitor in ROM:

Plug this card and a Z80 processor card, such as SC516, into a suitable backplane. Connect an FTDI style serial to USB adapter from serial port (S1 or P2) to a computer running a terminal emulation program. The terminal should be configured for 115200 baud, 8 data, 1 stop, no parity. Flow control can be either Off or hardware RTS/CTS.

Turn the power on. The terminal should show something similar to the illustration below.

Z180 systems with RomWBW support this card. Remember to configure RomWBW to assume 115200 baud, as described in the technical note above.

Homebrew 8-bit retro computing