SC704 is an I2C bus master module for RCBus.
- SC704 – Assembly guide
- SC704 – Compatibility
- SC704 – Parts list
- SC704 – Printed circuit board
- SC704 – User guide
- SC700 series information
- SC700 series support
Downloads
- SC704, v1.0, Kit contents sheet (PDF)
- SC704, v1.0, Schematic (PDF)
- SC704, v1.0, PCB design files (OSHWLab)
- SC704, v1.0, Gerber files (ZIP)
Errata
PCB v1.0.0 has read and write address labels swapped so read should start at 0xA1 (not 0xA0) and write should start at 0xA0 (not 0xA1)
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 | SC704, v1.0, PCB |
| C1 to C5 | 5 | Capacitor, ceramic, 100 nF |
| JP1 | 1 | Header, male, 2 row x 8 pin, angled |
| JP2 to JP5 | 4 | Header, male, 1 row x 3 pin, straight |
| Jumper | 12 | Jumper shunt |
| LED1 and 2 | 2 | LED, green, 3mm, angled |
| P1 | 1 | Header, male, 2 row x 40 pin, angled |
| P2 and P3 | 2 | Header, male, 1 row x 6 pin, angled |
| R1 and R2 | 2 | Resistor, 10k, 0.25W |
| R3 and R4 | 2 | Resistor, 1k, 0.25W |
| RP1 | 1 | Resistor network, 8x100k, SIL, 9-pin |
| S1 and S2 | 2 | Header, female, 1 row x 6 pin, angled |
| Screw (for spacer) | 1 | Machine screw, 6mm, M3 |
| Spacer | 1 | Spacer, 10mm, M3, nylon |
| U1 | 1 | 74HCT688 |
| U2 | 1 | 74HCT74 |
| U3 | 1 | 74HCT125 |
| U4 | 1 | 74HCT32 |
| U5 | 1 | 24LC256 |
| IC socket 20-pin U1 | 1 | Socket, DIP, 20-pin |
| IC socket 14-pin U2, 3, 4 | 3 | Socket, DIP, 14-pin |
| IC socket 8-pin U5 | 1 | Socket, DIP, 8-pin |
Printed Circuit Board
Note: The PCB is designed to allow a DIP switch to be fitted instead of the header pins JP1.
User Guide
An I2C bus master is a device that can control I2C slave devices on an I2C bus. This makes the module useful for interfacing with external electronics. In this case, the module generates the I2C bus signals using big-bang code.
The RCBus I/O port address for this module is set in binary with JP1. Fit a jumper shunt to each bit position where the address bit should be a logic ‘1’ (high voltage).
If, for example, you wish to set the module to address 12 decimal, which is hexadecimal 0C or binary 00001100, then fit a jumper shunts to bits 2 and 3, as illustrated below.
The default address is decimal 12 (hexadecimal 0C, binary 00001100)
Jumpers JP2 to JP4 set the address of the EEPROM (U5) on the I2C bus (not the address on the processor bus) and also it’s write protect option.
The jumper positions shown below set the EEPROM to I2C address 0xA0 (write) and 0xA1 (read), plus it is set to write enabled. Each I2C address bit is set to zero. The higher address bits are fixed by the EEPROM to 1010 xxxx.
As bit zero of the I2C device address indicates the Read/Write request, there can be a little confusion with address bit numbers. I2C address bit A0 is actually bit 1 of the control byte, so is the address 7-bits shifted right with bit zero as R/W or is the address 8-bits where each device has two addresses, one for read and one for write. Here we use the latter scheme.
Note: PCB v1.0.0 shows the read and write addresses swapped. For example, 0xA0 should be write and 0xA1 should be read.
Input/output port functions
| RCBus I/O Address | Read | Write |
| Configurable *1 | Read SCL (bit 0) SDA (bit 7) | Write SCL (bit 0) SDA (bit 7) |
- The RCBus I/O address should be set to match the software you are using. Typically, this is 0x0C or 0x20.
Jumper options
| Jumper | Function |
| JP 1 | Set SC704’s RCBus I/O address |
| JP 2 to JP4 | Set one of eight possible I2C bus addresses for the I2C EEPROM (U5) |
| JP5 | Set I2C EEPROM write protection |
JP2 to JP4 set the I2C Bus address for the 25LC256 EEPROM (U5) as shown in the table below.
| JP4 A2 | JP3 A1 | JP2 A0 | I2C Write Address | I2C Read Address |
| 0 | 0 | 0 | 0xA0 | 0xA1 |
| 0 | 0 | 1 | 0xA2 | 0xA3 |
| 0 | 1 | 0 | 0xA4 | 0xA5 |
| 0 | 1 | 1 | 0xA6 | 0xA7 |
| 1 | 0 | 0 | 0xA8 | 0xA9 |
| 1 | 0 | 1 | 0xAA | 0xAB |
| 1 | 1 | 0 | 0xAC | 0xAD |
| 1 | 1 | 1 | 0xAE | 0xAF |
Assembly Guide
Below is the suggested order of assembly. A general guide to assembling circuit boards can be found here.
- Resistors R1 to R4
- Sockets S2 and S3.
- Decoupling capacitors C1 to C5
These can be fitted either way around - IC sockets for U1 to U5
Ensure the notch in the socket matches the marking on the PCB - Header pins P2 and P3
- Jumper pins JP1
- Bus header pins P1
- Resistor network RP1
- LED1 and LED2
The angled LEDs in the kit only fit one way around, but standard LEDs need to have the short lead in the hole marked with a flat line - Jumper pins JP2 to JP5
- Fit the IC’s in their sockets
Ensure the notch in the IC matches the PCB and IC socket - Fit jumper shunts in the default positions (see above)
- Fit the nylon spacer in the mounting hole
Fit jumper shunts in the positions shown below. All other jumpers are optional and should only be fitted if the feature is required.
Compatibility
This module conforms to the RCBus specification v1.0 for RCBus-2014 and RCBus-Z80.
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.
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) | | Fully supported |
| RCBus 40-pin (RC2014 standard) | | Fully supported |
The following table indicates electrical compatibility with SC704. Check firmware and software details for any required software support.
| Product (80-pin) | ? | Compatibility notes |
| SC126 Z180 motherboard | | Adds Z180 SBC/ motherboard |
| 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 | | Can add more than one I2C bus master |
| SC705 Serial ACIA | | Adds one serial port Typically set to 0x40 |
| SC706 Z80 CPU | | Adds Z80 CPU and main bus clock |
| SC707 Memory 128k | | Adds 128k RAM and 128k flash ROM |
| SC708 Z80 SBC | | Adds CPU, ROM, RAM clock and reset |
| 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 | | Adds 512k RAM and 512k flash ROM |
| 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 | | Adds Z80 SBC/ motherboard |
| SC721 Memory 512k | | Adds 512k RAM and 2 x 512k flash ROM |
| 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 |
| SC791 Z80 system | | Modular Z80 system |
| SC792 Z180 system | | Modular Z180 system |
| Product (40 pin) | ? | Compatibility notes |
| SC114 Z80 motherboard | | Adds Z80 SBC/ motherboard |
| SC130 Z180 motherboard | | Adds Z180 SBC/ motherboard |
| SC133 Backplane 11+1 | | Adds 11 module sockets RCBus 40-pin and PSU |
| 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 | | Can add more than one I2C bus master |
| SC139 Serial ACIA | | Adds one serial port Typically set to 0x40 |
| SC141 Backplane 12+1 | | Adds 12 module sockets RCBus 40-pin |
| SC142 Power from 12v | | Adds power supply (8 to 12 volt input) and reset |
| SC143 Flash ROM | | Adds 128k flash ROM |
| SC145 Compact flash | | Adds Compact Flash storage |
| SC147 Backplane 6+1 | | Adds 6 module sockets RCBus 40-pin |
| SC148 Power from 5v | | Adds power supply (5 volt input) and reset |
| SC149 Z80 CPU | | Adds Z80 CPU and main bus clock |
| SC150 Paged RAM 128k | | Adds 128k of paged RAM |
| SC151 Paged ROM 128k | | Adds 128k of paged ROM |
| Other suppliers | ? | Compatibility notes |
| RC2014 Mini system | | Adds Z80 SBC/ motherboard |
| RC2014 Pro system | | Modular Z80 system |
| RC2014 Zed system | | Modular Z80 system |
| RC2014 Micro module | | Adds CPU, ROM, RAM serial, clock and reset |
| RC2014 Backplane 5 | | Adds 5 module sockets RC2014 40-pin |
| RC2014 Backplane 8 | | Adds 12 module sockets RC2014 40-pin |
| RC2014 Backplane Pro | | Adds 12 module sockets RC2014 40-pin / 60-pin |
| RC2014 Z80 CPU | | Adds Z80 CPU |
| 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 | | Adds 64k paged ROM |
| RC2014 64k RAM | | Adds 64k paged RAM |
| RC2014 Memory 512k | | Adds 512k RAM and 512k flash ROM |
| 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.
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