RetroZ SBC

RetroZ SBC S80

A minimalist single board Z80 retro computer based on the S80 design specifications.

This board is developped using the S80 physical specifications. The specifications are not currently available online but I plan to fix that in a future revision.

Front of the board

Back of the board

You can find the board schematic as a PDF here.

BOM

You can download the BOM here. It is an Open Document Worksheet file and you should be able to open it with Open/Libre Office or MS Excel. If you have issue, contact me.

Development Note

2019-01-15

The review phase is done! I just sent the board to production. I should get the finished board next week.

2019-01-26

I recieved my boards (10 boards!) yesterday. I just finished creating the BOM and sorted through my parts to order what is missing. I expect to recieve my order in the coming week. I'll work on the NASCOM Basic once I got the computer assembled. Stay tuned!

2019-08-03

It's been a while since I updated that README file but as you can see, I'm committing regularily on the project. Simply put, I'm too busy playing with my working RetroZ SBC computer to update the front documentation. It is time to fix that, continue reading to get the full update.

What's Inside?

The RetroZ-SBC computer is a 8MHz Z80 CPU clocked at 7.3728 MHz. It supports up to 32kB of ROM and 32kB of RAM. An SIO/2 chip is connected to the USB Micro B socket using an FTDI serial converter. Rx and Tx leds let you know when the computer communicates over USB. The whole computer is powered through the same USB port and a power-on reset monitor resets the computer at startup. A Reset button allows warm computer restart. An On/Off DIP switch controls the power state of the computer and a power led indicator let you know that the beast is powered. All the Z80 processor pins are available through the 40 pins stacking header plus some other on the User Port A. User Port B stacking header is fully available (8 pins) for child boards future projects.

The Connection

The USB to serial conversion is made possible thanks to an FTDI chip behind the board and is clocked at 115200 bauds by default. This speed can be changed programmatically through the Z84C40 (Zilog SIO/2) registers. On a Windows machine the device can be accessed using COM5. For a Mac/Linux box it is /dev/ttyUSB0. Hardware flow control CTS/RTS is fully wired so feel free to flood the console with whatever you get, it should handle it.

Technical Reference

Power and commnunication over USB

Using a single USB Micro-B wire, it is possible to power-up and communicate with the computer. There is a DIP switch to set the computer on or off and a power indicator led indicates when the computer is powered on. There is also a power line monitor chip (IC1) that pull the /RESET line LOW for 240 ms after power on.

Serial Communication

Serial communication is achieved using a Z84C42 SIO/2 chip. The serial port A is connected to the USB jack through an FTDI UART to USB 2.0 converter. Rx and Tx leds indicate when communication occurs on that port. The serial port B is connected back into the User Port A (J2) PIN 2 (RxB) and Pin 3 (TxB) for slave cards serial communications.

The serial port A clock is directly tied to the clock and can be changed programmatically from the SIO/2 registers. In the NASCOM Basic provided, it is set to 115200 by default.

The serial port B clock The default speed is set to 115200 bauds as well. Again, that speed can be programmatically changed. The serial clock on port B goes through a dodecade counter and can be further adjusted using the Clock div. (J6) jumper that offers another set of 1,2,6 and 12 dividers. That gives a wide variety of standard baud rates down to 9600 bauds.

Onboard memory control

The PIN 1 of the User Port A is pulled low with a 2k resistor. This is the on-board memory enable input pin (/ME). To disable onboard memory, an expansion card can set that pin HIGH. This would be the case with a memory expansion board. It can be hardwired to 5V to disable the onboard memory completely or it can be tied to some logic to leverage the on-board memory as needed.

Interrupt Chaining

The RetroZ SBC design allows chaining interrupts of up to 4 slave boards. The onboard SIO/2 IEO pin is hardwired to the User Port A INT0 (PIN 4). Its Interrupt Enable In pin can be tied to any of the other INT pins using the IEI Select jumper (J4). Selecting the first position of that jumper (INT0) sets the on-board SIO/2 as the Master. On slave boards, the Interrupt Enable In will have 6 positions. Also, a 4 positions IEO jumper header should be provided to select the right User Port A IEO position (pins 5-8). More on that on the slave board design document.

WARNING User Port A pins 4-8 (interrupts) are not protected. If you connect a daughter board without checking jumper settings first, you could short an IO pin on your stack and damage a chip! Looking into a solution to that in a future revision.