SC125, v1.0, User Guide

The SC125, serial and timer card houses a Z80 SIO chip, which provides two TTL serial ports for use with FTDI style serial adapter cables, and a Z80 CTC, which has four counter/timer channels.

Powering the system

Fitting jumper shunts to headers JP1 and/or JP2 connects the FTDI style 5 volt serial port’s power pin to SC125’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 SC125.

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 JP1 or 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 RXD and TXD signals from the Z50Bus.

Address Selection

Jumpers JP5 and JP6 allow the address ranges of the SIO and CTC chips to each be set to one of two options. Each chip occupies four I/O addreses. The addresses should be set to either:

Primary0x80 to 0x830x88 to 0x8B
Secondary0x84 to 0x870x8C to 0x8F

The primary addresses are the defaults supported by the current firmware. A second card can be added to the system with the secondary addresses selected, but there is not currently any software to support this.

The illustration below shows the default positions for the jumpers shunts on JP3 to JP6.

Default jumper shunt positions

Serial Ports

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

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

The serial ports settings depend on the firmware or software being run. Typically these are:

SettingPort APort B
Baud rate1152009600
Data bits88
Stop bits11
Flow controlNoneNone

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

Counter / Timer

The Z80 CTC is a four channel counter/timer interface. The channels are used as follows:

  • Channel 0: Serial port A baud rate clock
  • Channel 1: Serial port B baud rate clock
  • Channel 2 and 3: Clock tick generator/timer

The output of channel 2 is connected to the input of channel 3. This makes of a flexible clock tick generator or timer.

For full details search for Z84C30 datasheet or the Zilog Z80 Family CPU Peripherals User Manual (UM0081).

Clock Source

This card has its own oscillator as a clock source for the CTC, making the serial port and timer functions independent of the CPU’s main clock.

Homebrew 8-bit retro computing