The RC2014 Zed Mini is a combination of the core modules (CPU, ROM/RAM, Serial and Clock) from an RC2014 Zed II in a Mini form factor. This single board computer has 512k ROM, 512k RAM, a 7.3728MHz Z80 and 16C550 based serial running at 115200 baud. The Enhanced Bus connector gives it access to all modules in the RC2014 ecosystem.
Note that this page is for the RC2014 Zed Mini. Whilst this has a lot in common with both the RC2014 Mini II and the RC2014 Zed II/RC2014 Zed Pro II, those are different kits and you should refer to those paged directly for information on those products.
Click image to open PDF schematic
Hardware
The RC2014 Zed Mini is effectively the RC2014 CPU Module, 512k ROM 512k RAM Module, Dual Clock Module (Single speed) and a 16C550 Twin Serial Module (Single UART) all on one 2 layer PCB.
Z80 CPU
The Zilog Z80 is the heart of the RC2014 Zed Mini. This is clocked with a 7.3728MHz crystal circuit that is buffered with two inverters. The active low reset signal is held high with a 10k resistor that is shorted to ground when the manual reset button is pushed, but is also held low with the DS1233-5 reset manager integrated circuit. This keeps the Z80 in reset when power is first applied and then released when the power has stabalised, giving the Z80 a clean start. [Note, if you already have a Ds1233-5 fitted elsewhere (eg in a Front Panel I/O Module) then this should be omitted when you build the kit] The address bus, data bus and some of the control bus go to the ROM, RAM, MMU and Serial chips.
All signals are bought out to the RC2014 Standard Bus and RC2014 Enhanced Bus header for further expansion with any RC2014 compatable module.
512k ROM 512k RAM
The Z80 only has 16 address lines, which limit its address space to 64k. The Memory Management Unit (MMU) divides the 64k address space in to 4 16k banks. The 512k ROM and 512k RAM are also divided in to 32 banks of 16k each. This allows RomWBW to swap any memory bank in and out of any of 4 16k areas that the CPU can address. This ROM/RAM and MMU design is based on Sergey Kiselevs Zeta 2, and the subsequent RC2014 512k ROM 512k RAM Module. The key here is the two 74HC670 4 by 4 registers that control the high address pins on the ROM and RAM to select the required bank. The 4 memory bank registers are at hardware address 0x78 – 0x7B, and the value written to that address will select which bank of ROM or RAM the CPU can access.
Port Decoding
3 ports are used in the Zed Mini. 0x78 is the MMU, 0x80 is the base address for the UART and 0x88 is an alternative address for the UART (see below). This is acheived by feeding A4, A5, A6 and A7 through a 74HCT86 (XOR gates) and a diode based OR gate for IORQ to enable the ‘138. Then when M1 is high, A3 and A7 select the port.
UART
A single 16C550 is connected to port 0x80 (or 0x88), the data bus and A0,A1 and A2. The 7.3728MHz clock is divided by 4 in the 74HCT393 counter to give a 1.8432MHz clock. This results in a baudrate of 115200. The Tx, Rx, CTS and RTS are bought out to an FTDI compatable 6 pin header. The Tx and Rx signals also go to a 6 pin keyboard socket with the Tx and Rx pins swapped over.
Power
The Zed Mini can be powered from either 5v in to the barrel jack, 5v in to the FTDI header, or 5v in to the expansion bus – however, only one method should be used at any one time. If powering the Zed Mini from the FTDI connector you will need to fit a jumper on the pins labelled labelled “5v Direct”. The current draw even with a few expansion modules is pretty low, so an FTDI adapter can be used to power it. If you have a lot of expansion modules, or any power hungry ones, then either the barrel jack or expansion bus header should be used. In this configuration, remove the “5v Direct” jumper.
Alternative UART port
If you wish to move the UART to port 0x88, maybe because you have something else using 0x80 that cannot be changed, then there is a cuttable track under the 74HCT138. Once this is cut, solder a link between the center pad and the one marked 88. For most users it is unlikely that you will need to do this, so you can safely ignore this part.
Note that on revision v1.1 of the board the 80 and 88 text on the silkscreen is the wrong way around.
Storage and Expansion
Whilst the RC2014 Zed Mini can be used on its own without any storage or other expansion modules, the amount of use will be limited. At a minimum, some form of storage is recommended. Adding other I/O modules, such as the Digital I/O, Why Em-Ulator sound card, RP2040 VGA Serial Terminal or LCD display can really open up the options.
A single Enhanced Bus connector is fitted. This can use either Standard Bus (1×40 connector with most common signals present) or Enhanced Bus (1×40 + 1×20 containing extra signals) modules. Whilst this will only take one module, you can add a backplane to increase the number of modules you can add. A Backplane 5 with a stackable header in slot 1 will sit perfectly on top of the Zed Mini and give you 5 slots of possibility. Note that the Backplane 5 and Backplane 8 only contain Standard Bus slots, so if you need to use an Enhanced Bus module (eg Micro SD Storage) you will need to run a jumper cable between the module and the Zed Mini for any Enhanced Bus signals. The Backplane Pro does have Enhanced Bus slots, although this is much larger than the Zed Mini and the RC2014 Zed Pro II may be a better option.
For storage, any combination of compact flash, micro SD, CH375 USB or IDE hard drives are supported in RomWBW.
Software
The Zed Mini is supplied with RomWBW v3.6.0 preprogrammed on to ROM. This is a very powerful piece of software that lets you boot in to a system monitor, various CP/M operating systems, BASIC or Forth. There is a lot of bundled software that runs under CP/M including games, programming environments, utilities and productivity programs.
RomWBW takes care of all the memory management, as well as detecting and providing drivers for a large range of hardware. This includes the LCD Driver Module and the Front Panel Kit, or the Why Em-ulator sound card.
RomWBW has fantastic documentation which can be found here https://wwarthen.github.io/RomWBW/
Bill of materials
1 RC2014 Mini Zed PCB
1 40 pin wide DIL socket
2 32 pin wide DIL socket
1 PLCC44 socket
4 14 pin narrow DIL socket
4 16 pin narrow DIL socket
1 40 Way SIL Socket
1 20 Way SIL socket
1 6 Way SIL socket
1 6 pin RA header
1 2 pin RA header
1 jumper
1 2.1mm power jack
1 3mm green led
1 RA Tactile Switch
1 7.3728 MHz Xtal
2 22pf ceramic cap
12 100nf
5 1k resistor
2 10k resistor
1 1M resistor
1 100k resistor
1 1k 5 pin resistor pack
1 10k 5 pin resistor pack
1 4k7 5 pin resistor pack
2 1n4148
1 74HCT04
1 74HCT74
1 74HCT86
1 74HCT393
1 74HCT138
1 74HCT139
2 74HCT670
1 Z80 CPU
1 ST39SF040
1 ASC6C4008
1 16C550
1 DS1233-5+
1 USB Barrel Lead