SC705 is a serial port using the 68B50 ACIA with a fixed baud rate of 115200 baud.

• SC705 – Assembly guide
• SC705 – Compatibility
• SC705 – Parts list
• SC705 – Printed circuit board
• SC705 – User guide
• SC700 series information
• SC700 series support

Downloads

• SC705, v1.0, Kit contents sheet (PDF)
• SC705, v1.0, Schematic (PDF)
• SC705, v1.0, PCB design files (OSHWLab)
• SC705, 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

Parts List

Reference	Qty	Component
PCB	1	SC705, v1.0, PCB
C1 to C4	4	Capacitor, ceramic, 100 nF
C5	0	There is no C5
C6 and C7	2	Capacitor, ceramic, 22 pF
JP1	1	Header, male, 2 row x 8 pin, angled
JP2+3+7+8	1	Header, male, 2 row x 9 pin, angled
J4+5+6	1	Header, male, 1 row x 12 pin, angled
Jumper	18	Jumper shunt
LED1 and 2	2	LED, green, 3mm, angled
P1	1	Header, male, 2 row x 40 pin, angled
P2	1	Header, male, 1 row x 6 pin, angled
R1 to R4	4	Resistor, 2k2, 0.25W
R5 and R6	2	Resistor, 100k, 0.25W
R7	1	Resistor, 1M, 0.25W
R8 to R10	3	Resistor, 1k, 0.25W
RP1	1	Resistor network, 8x100k, SIL, 9-pin
S1	1	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	68B50 ACIA
U3	1	74HCT04
X1	1	Crystal, 7.3728 MHz
IC socket 20-pin U1	1	Socket, DIP, 20-pin
IC socket 24-pin U2	1	Socket, DIP, 24-pin
IC socket 14-pin U3	1	Socket, DIP, 14-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

The serial port provides bidirectional asynchronous serial communications usually at 115200 baud, 8 data bits, 1 stop bit, no parity. RTS/CTS handshaking provides hardware flow control in both directions. The interface is at 5-volt (TTL) levels.

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).

The default address is 128 decimal, which is 0x80 in hexadecimal and 1000000 in binary. To set this as the address requires a jumper shunt in bit position 8, as illustrated below.

The module occupies two addresses so bit zero of the address can be in either state. The address jumper bit zero is labelled ‘x’ to indicate this.

Jumper JP2 connects the module’s 5-volt supply to the serial port. A jumper should should be fitted here if the RCBus system is being powered from this serial port or if the device connected to this serial port needs to be powered from the RCBus system.

Jumper JP3 enables the ACIA’s interrupt output to be connected to the INT, INT1 or INT2 bus signals. The default position is INT.

Jumper JP4 enables selection of the clock source for the ACIA. This can either be the on-board 7.3728 MHz oscillator or the RCBus clock signal (CLK). Selecting the on-board clock means the serial port baud rate is independent of the processor speed.

JP5 and JP6 enable selection of the ACIA’s R/W and E inputs. Normally these are set to the IORQ and WR signals that are common to all implementations of the RCBus. Alternative, if the processor supports native R/W and E signals then these can be selected instead.

JP7 and JP8 enable the ACIA’s RXD and TXD signals to be connected to the RCBus RX and TX pins, and the ACIA’s CTS and RTS signals to be connected to the RCBus RX2 and TX2 pins. To connect all of these signals, fit jumper shunts in the positions illustrated below.

It is also possible to connect the ACIA’s RXD and TXD signals to the RCBus RX2 and TX2 pins. This is achieved by fitting one pair of jumper shunts in the middle positions, as illustrated below.

Input/output port functions

RCBus I/O Address	Read	Write
Configurable *1	0x80	0x80

1. The RCBus I/O address should be set to match the software you are using. Typically, this is 0x80 or 0x40.

Jumper options

Jumper	Function
JP 1	Set SC705’s RCBus I/O address
JP 2	Connects 5-volt power to serial port
JP 3	Selects which interrupt signal is generated JP 3.1 Interrupt goes to RCBus INT signal JP 3.2 Interrupt goes to RCBus INT1 signal JP 3.3 Interrupt goes to RCBus INT2 signal The default is INT
JP 4	Select the clock source JP 4.1-2 Clock source is the RCBus CLOCK signal JP 4.3-4 On-board 7.2728 MHz oscillator If jumper shunts are fitted in both positions the on-board oscillator supplies the RCBus CLOCK signal The default is the on-board oscillator
JP 5	Select the E signal source JP 5.1-2 RCBus pin 38 (for use with 68xx processors) JP 5.2-3 RCBus nIORQ (inverted) The default is the inverted nIORQ signal
JP 6	Selects to R/W signal source JP 6.1-2 RCBus pin 39 (for use with 68xx processors) JP 6.2-3 RCBus nWR signal The default is the nWR signal
JP 7	Connects the RXD input and the CTS input to the bus JP 7.1-2 Connects RXD to bus RX(1) signal JP 7.3-4 Connects CTS to bus RX2 signal
JP 8	Connects the TXD output and the RTS output to the bus JP 8.1-2 Connects TXD to bus TX(1) signal JP 8.3-4 Connects RTS to bus TX2 signal

Assembly Guide

Below is the suggested order of assembly. A general guide to assembling circuit boards can be found here.

1. Resistors R1 to R10
2. Socket S1
3. Decoupling capacitors C1 to C5
   These can be fitted either way around
4. Resistor network RP1
5. IC sockets for U1 to U3
   Ensure the notch in the socket matches the marking on the PCB
6. Capacitors C6 and C7
7. Header pins JP4, JP5 and JP6
   Fit as one strip with pins 4 and 8 removed
8. Header pins P2
9. Header pins JP1
   You may wish to remove the pins for address bit zero as the module occupies two I/O addresses and therefore bit zero is not used
10. Header pins JP2, JP3, JP7 and JP8
    Fit as one strip with pins 2 removed
11. Bus header pins P1
12. 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
13. Crystal X1
14. Fit the IC’s in their sockets
    Ensure the notch in the IC matches the PCB and IC socket
15. Fit jumper shunts in the default positions (see below)
16. Fit the nylon spacer in the mounting hole

Compatibility

This module conforms to the RCBus specification v1.0 for RCBus-2014, RCBus-Z80 and RCBus-68xx.

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.

Backplane	?	Compatibility notes
RCBus 80-pin		Fully supported
RCBus 60-pin (RC2014 enhanced)		Fully supported
RCBus 40-pin (RC2014 standard)		Limitations: Some optional signals are not available (TX2, RX2)

The following table indicates electrical compatibility with SC705. 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		Adds I2C bus master
SC705 Serial ACIA		Can add more than one Serial ACIA module
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		Adds I2C bus master
SC139 Serial ACIA		Can add more than one Serial ACIA module
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.