SC521, v1.0, User Guide

The SC521, serial card houses a Z80 SIO (serial input/output) chip, which provides two TTL serial ports 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 0x80 (SIO occupies 0x80 to 0x83)
  • Both ports are set to 115200 baud
  • The clock source is X2 (7.3728 MHz)
  • Power is supplied by the Z50Bus (not SC521)
Default jumper shunt positions

Address Selection

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

CardAddressJumper shunt(s)
PrimaryBase address 0x80
Range 0x80 to 0x83
Bit 7
SecondaryBase address 0x84
Range 0x84 to 0x87
Bit 7 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 jumper shunts to header JP2 connect 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 enabled two cards to communicate via the bus rather than having to have additional cables between the cards.

Jumpers JP3 and JP4 enable one of the serial ports to be connected to the Z50Bus RX and TX lines. Each jumper has two positions: A and B. Position A links the port A signals to the bus, while position B is for the port B signals. Leave the jumper shunts off to isolate this card’s RX and TX signals from the Z50Bus.

Serial Ports

Two FTDI style 5-volt serial ports are provided at P2 and P3.

The pin-out, below, describes signals with respect to the card, so an 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 ports settings depend on the card’s jumpers and also the firmware or software being run. Typically these are:

SettingPort APort B
Baud rate115200115200
Data bits88
Stop bits11
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 positions on this card. See technical note below.

Current firmware assumes the use of serial port A as the default console device.

Technical note:
Software should set the SIO’s internal divider to 64. This will generate the indicated baud rates from a 7.3728 MHz oscillator. Some software provides baud rate control via a combination of a Z80 CTC and the Z80 SIO’s internal divider. It is best to set the software to assume 115200 baud, regardless of the jumper settings, as selecting a slow baud rate in software may result in the SIO’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 CTC, making the serial port function independent of the CPU’s main clock. There is a choice of two oscillators. One is a traditional crystal (X2, next to U4) the other is a half-sized can oscillator (X1). Either can be used. It is not necessary to include components for both. Select the desired clock source with JP7.

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 A (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