SC721 is a 1 MB memory module designed for the 80-pin RCBus. It includes 512 kB RAM plus a choice of two 512 kB Flash ROMs. The memory is arranged as a simple linear memory (no paging or banking) so it requires a 20-bit address bus to accesses all of the memory. This makes it suitable for use with a Z180 CPU module, but unsuitable for use with a Z80 CPU module (unless it has a memory management circuit included).
- SC721 – Assembly guide
- SC721 – Compatibility
- SC721 – Firmware for Z180 CPU: RomWBW_SCZ180_SC700
- SC721 – Firmware for Z180 CPU: SCM S9
- SC721 – Firmware for Z80 CPU: RomWBW RCZ80_std
- SC721 – Firmware for Z80 CPU: SCM S7
- SC721 – Parts list
- SC721 – Printed circuit board
- SC721 – User guide
- SC700 series information
- SC700 series support
Downloads
- SC721, v1.0, Kit contents sheet (PDF)
- SC721, v1.0, Schematic (PDF)
- SC721, v1.0, PCB design files (OSHWLab)
- SC721, v1.0, Gerber files (ZIP)
Errata
- Nothing known
Suppliers
| Kits | Website | From | Currency |
| Small Computers Direct | SCDirect | UK | GBP |
| Stephen C Cousins | Tindie | UK | USD |
| Small Computer Central | Lectronz | UK | Euro/USD |
| PCBs | Website | From | Currency |
| Small Computers Direct | SCDirect | UK | GBP |
| Stephen C Cousins | Tindie | UK | USD |
| Small Computer Central | Lectronz | UK | Euro/USD |
| Assembled and Tested | Website | From | Currency |
| Not available | |||
| Components | |||
| See parts list |
Tindie does not collect VAT for EU countries
Lectronz does collect EU VAT for orders up to 150 EUR
Parts List
| Reference | Qty | Component |
| PCB | 1 | SC721, v1.0.x, PCB |
| C1 to C4 | 4 | Capacitor, ceramic, 100 nF |
| C5 | 1 | Capacitor, electrolytic, 100 µF |
| JP1 | 1 | Header, male, 1 row x 3 pin, angled |
| JP2 to JP4 | 3 | Header, male, 1 row x 3 pin, straight |
| JP5 | 1 | Header, male, 1 row x 2 pin, angled (not required if switch fitted) |
| Jumper | 5 | Jumper shunt |
| P1 | 1 | Header, male, 2 row x 40 pin, angled |
| R1 | 1 | Resistor, 10k, 0.25W |
| RN1 | 1 | Resistor network, 8x100k, SIL, 9-pin (optional) |
| SW1 | 1 | Switch, toggle, sub-miniature, SPDT |
| Screw (for spacer) | 1 | Machine screw, 6mm, M3 |
| Spacer | 1 | Spacer, 10mm, M3, nylon |
| U1 and U2 | 2 | FLASH 512k bytes SST39SF040 |
| U3 | 1 | RAM 512k bytes AS6C4008 |
| U4 | 1 | 74AHCT139 |
| IC socket 32-pin U1 to U3 | 3 | Socket, DIP, 32-pin |
| IC socket 16-pin U4 | 1 | Socket, DIP, 16-pin |
Printed Circuit Board
User Guide
SC721 is a 1 MB memory module designed for the 80-pin RCBus. It includes 512 kB RAM plus a choice of two 512 kB Flash ROMs. The memory is arranged as a simple linear memory (no paging or banking) so it requires a 20-bit address bus to accesses all of the memory. This makes it suitable for use with a Z180 CPU module, but unsuitable for use with a Z80 CPU module (unless it has a memory management circuit included).
The module includes two sockets for 512kB Flash ROMs, allowing two sets of firmware to be installed on the module. The required ROM is selected with switch SW1 (or JP5 if you fit that instead of the switch).
Jumpers JP2, JP3 and JP4 enabled the memory to be configured with either the RAM at the top of memory and the ROM at the bottom of memory, or the ROM at the top and the RAM at the bottom. Z80 family devices require the ROM at the bottom, while 6xxx family devices require the ROM at the top.
The Flash ROMs can, with appropriate software, be reprogrammed without removing them from the board. To do this the Write enable jumper (JP1) must be in the Write enable position. If it is in the Write protect position the ROM may not even be able to be correctly identified by the update software. The Flash ROMs require a very specific sequence to write new firmware to them so even if the jumper is left in the Write enable position it is very unlikely the firmware will be accidentally changed or corrupted.
Input/output port functions
| I/O Address | Read | Write |
| none | n/a | n/a |
Jumper options
| Jumper | Function |
| JP1 | FLASH write enable / protect JP 1.1-2, Write protect = Read only JP 1.2-3, Write enable = Read and write enabled The default is Write protect |
| JP2 | Selects if RAM (U3) is at top or bottom of memory JP 2.1-2, L = RAM at 0x00000 to 0x7FFFF (low) JP 2.2-3, H = RAM at 0x80000 to 0xFFFFF (high) The default for Z180 is RAM high (top of memory) |
| JP3 | Selects if ROM (U1) is at top or bottom of memory JP 3.1-2, L = ROM at 0x00000 to 0x7FFFF (low) JP 3.2-3, H = ROM at 0x80000 to 0xFFFFF (high) The default for Z180 is ROM low (bottom of memory) JP3 and JP4 should be fitted in the same position |
| JP4 | Selects if ROM (U2) is at top or bottom of memory JP 4.1-2, L = ROM at 0x00000 to 0x7FFFF (low) JP 4.2-3, H = ROM at 0x80000 to 0xFFFFF (high) The default for Z180 is ROM low (bottom of memory) JP3 and JP4 should be fitted in the same position |
| JP5 | If present, this selects the current ROM, either U1 or U2 Fit the jumper shunt to select U1 Remove the jumper shunt to select U2 This header will not be present if switch SW1 is fitted |
Assembly Guide
Below is the suggested order of assembly. A general guide to assembling circuit boards can be found here.
- Resistor R1
This can be fitted either way around - Decoupling capacitors C1 to C4
These can be fitted either way around - Header pins for jumper JP1
These are angled to allow access when the module is in use - Sockets for U1 to U4
Fit such that the notch in the socket matches the curve in the outline on the PCB silkscreen - Resistor networks RN1
This is optional. It is provided to pull up or down the high order address lines (A16 to A23). This component is not needed, but will do no harm, when this module is paired with a CPU module that provides all the required address lines (A16 to A19). If fitted with pin 1 in the marked position, address lines A16 to A23 are pulled up. If fitted with pin 1 at the other end of the row of holes, the address lines are pulled down. Note, there will be one spare hole in either position. The RCBus specification suggests the use of pull up resistors on these address lines, so the default should be to fit RN1 in the position indicated. - Bus header P1
Make sure the pins are parallel to the PCB so that the board is vertical when it is fitted into a backplane socket - Switch SW1 or JP5
Fit either switch SW1 or header pins for jumper JP5 as a means of selecting the required Flash ROM - Header pins for jumpers JP1, JP2 and JP3
- Capacitor C5
This must be fitted the correct way around, as described here - Insert the integrated circuits into their sockets
Make sure the notch in the component is at the end indicated by the notch in the socket and the curve on the PCB silkscreen
The kit includes pre-programmed Flash ROMs for U1 and U2. They can be fitted in either position. - Fit the nylon spacer in the mounting hole
Fit jumper shunts in the positions shown below in red.
Compatibility
This module conforms to the RCBus specification v1.0 for full 80-pin modules.
The RCBus specification includes RCBus-2014 (both RC2014 standard 40-pin bus and RC2014 enhanced 60-pin bus) and also the full 80-pin RCBus. The 80-pin RCBus provides support for advanced Z80 features, such as the interrupt daisy-chain, as well as support for other processor families.
Compatible with CPU modules SC722 and SC730, but not with CPU module SC706.
The table below indicates electrical compatibility with each backplane type (40, 60 and 80 pin)
| Backplane | ? | Compatibility notes |
| RCBus 80-pin |  | Fully supported |
| RCBus 60-pin (RC2014 enhanced) |  | A16 to A19 not supported |
| RCBus 40-pin (RC2014 standard) | | A16 to A19 not supported |
The following table indicates electrical compatibility with SC721. Check firmware and software details for any required software support.
| Product (80-pin) | ? | Compatibility notes |
| SC126 Z180 motherboard | | Function conflict |
| SC701 Backplane 6+1 | | Adds 6 module sockets RCBus 80-pin and PSU |
| SC702 Backplane 6+1 | | Adds 6 module sockets RCBus 80-pin |
| SC703 Power from 12v | | Adds power supply (8 to 12 volt input) and reset |
| SC704 I2C bus master | | Adds I2C bus master |
| SC705 Serial ACIA | | Adds one serial port Typically set to 0x40 |
| SC706 Z80 CPU | | SC721 needs memory management unit |
| SC707 Memory 128k | | Function conflict |
| SC708 Z80 SBC | | Function conflict |
| SC709 Backplane 12+1 | | Adds 12 module sockets RCBus 80-pin and PSU |
| SC710 Backplane 12+1 | | Adds 12 module sockets RCBus 80-pin |
| SC711 Prototyping | | Prototyping module |
| SC712 Power from 5v | | Adds power supply (5 volt input) and reset |
| SC713 RCBus tool | n/a | Simple reference tool No electrical function |
| SC714 Memory 512k | | Function conflict |
| SC715 Compact flash | | Adds Compact Flash storage |
| SC716 Z80 SIO/2 | | Adds two serial ports Typically set to 0x84 |
| SC717 Z80 PIO | | Adds parallel I/O using Z80 PIO |
| SC718 Z80 CTC | | Adds counter/timer using Z80 CTC |
| SC719 Digital I/O | | Adds digital I/O Eight in, eight out |
| SC720 Z80 motherboard | | Function conflict |
| SC721 Memory 512k | | Function conflict |
| SC722 Z180 CPU | | Adds Z180 CPU with two serial ports |
| SC723 Backplane 3 | | Adds 3 module sockets RCBus 80-pin |
| SC724 Breakout | | Prototyping breakout module |
| SC725 SIO+CTC | | Adds two serial ports and 4 counter/timers |
| SC726 Clock generator | | Generates CLK and CLK2 bus signals |
| SC727 Real time clock | | Adds real time clock for time and date |
| SC728 Module riser | | Simple module riser for RCBus 80-pin |
| SC729 Compact flash | | Adds Compact Flash storage |
| SC730 Z80 CPU + MMU | | Adds Z80 CPU, clock and MMU |
| SC731 Dual PWM | | Adds two PWM outputs |
| SC732 Dual servo | | Adds two servo outputs |
| SC733 Prototyping | | Prototyping module |
| SC791 Z80 system | | Function conflict |
| SC792 Z180 system | | Function conflict |
| SC794 Z80 system | | Function conflict |
| Product (40 pin) | ? | Compatibility notes |
| SC114 Z80 motherboard | | Function conflict |
| SC130 Z180 motherboard | | Function conflict |
| SC133 Backplane 11+1 | | SC721 needs an 80-pin bus |
| SC134 LED output port | | Adds eight LED outputs |
| SC135 Digital output port | | Adds eight digital outputs |
| SC136 Digital input port | | Adds eight digital inputs |
| SC137 I2C bus master | | Adds I2C bus master |
| SC139 Serial ACIA | | Adds one serial port Typically set to 0x40 |
| SC141 Backplane 12+1 | | SC721 needs an 80-pin bus |
| SC142 Power from 12v | | Adds power supply (8 to 12 volt input) and reset |
| SC143 Flash ROM | | Function conflict |
| SC145 Compact flash | | Adds Compact Flash storage |
| SC147 Backplane 6+1 | | SC721 needs an 80-pin bus |
| SC148 Power from 5v | | Adds power supply (5 volt input) and reset |
| SC149 Z80 CPU | | SC721 needs memory management unit |
| SC150 Paged RAM 128k | | Function conflict |
| SC151 Paged ROM 128k | | Function conflict |
| Other suppliers | ? | Compatibility notes |
| RC2014 Mini system | | Function conflict |
| RC2014 Pro system | | Function conflict |
| RC2014 Zed system | | Function conflict |
| RC2014 Micro module | | Function conflict |
| RC2014 Backplane 5 | | SC721 needs an 80-pin bus |
| RC2014 Backplane 8 | | SC721 needs an 80-pin bus |
| RC2014 Backplane Pro | | SC721 needs an 80-pin bus |
| RC2014 Z80 CPU | | SC721 needs memory management unit |
| RC2014 SIO/2 dual serial | | Adds two serial ports at 0x80 to 0x87 |
| RC2014 Compact flash | | Adds Compact Flash at 0x10 to 0x17 |
| RC2014 Dual clock | | Generates CLK and CLK2 bus signals |
| RC2014 Pageable ROM | | Function conflict |
| RC2014 64k RAM | | Function conflict |
| RC2014 Memory 512k | | Function conflict |
| RC2014 Digital I/O | | Adds eight LED outputs and eight push buttons |
| RC2014 Real time clock | | Adds real time clock for time and date |
| RC2014 RP2040 VGA terminal | | Connect VGA monitor and USB keyboard |
| RC2014 Pi Pico terminal | | Connect VGA monitor and USB keyboard |
| RC2014 YM2149 sound card | | Adds authentic retro sound |
| Weird electronics Flock v2 | | Adds floppy disk and real time clock |
Notes
- This product is designed for hobby use and is not suitable for industrial, commercial, or safety-critical applications.
- The product contains small parts and is not suitable for young children.
- RomWBW is copyright Wayne Warthen and has been provided free of charge with his permission.
Small Computer Central 