Add on board for multiple serial ports

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A background

For communications over long distances or in electrically noisy environments the ubiquitous RS232 serial is highly suited, using a point to point connection between the computer (often called DTE for data terminal equipment) and peripheral (often called DCE for data communications equipment), such as modem.

By cross wiring the point to point connection two DTEs can be connected together and used to transfer information directly between computers. Note it doesn't make sense to wire two DCEs together.

The BBC Microcomputer family is equipped (optional upgrade for the Electron and Master Compact) with an RS423 port in the form of a 5 pin domino layout DIN socket at the rear of the case. This is electrically compatible with modern equipment which usually has a more familiar 9 pin D sub connector.

However in, for example, control applications the motherboard serial port is often already occupied communicating with the DCE which makes it impractical to exchange data with a desktop PC which might present the system user interface. Similarly it opens the possibility of a serial network.

Original prototype

The original design was created in 2000 for an entirely unrelated project, and presented as build-it-yourself style project. Using a 6850 ACIA, the same serial controller used on the motherboard, it offered good compatibility and programmable baud rates from 75bps to 19200bps.

It occupied some spare test addresses &FC04-&FC07 in the memory map, which are no longer supported by the free driver software, as the updated schematic uses address &FC6x which was set aside for this application.

Second serial port board

To save the effort of working with a large piece of stripboard a ready made circuit board simplifies the design by using a small programmable logic device to clean up the 1MHz bus timing and decode the address bus into a pair of chip selects

The control latch replaces the serial ULA custom IC used on the motherboard, allowing

The spare output line is wired to pads for a panel mount 3mm LED, which in combination with address setting jumper links allows the possibility in the future for up to 4 extra external serial ports in the &FC6x region. Note this isn't currently supported in software.

Optional terminating resistors may be fitted if there are no other devices plugged into the 1MHz bus, or these can be omitted and the integral loop through connector used to "daisy chain" the other 1MHz bus devices together.

A 1.3mm power plug supplies the board with +5v, and serial interface is tracked for either a standard 9 pin D sub or the unusual 5 pin domino DIN socket used by the motherboard.

When assembled the extra serial port can be housed in a low profile plastic box or similar:

9 pin version 5 pin version
Second serial ports with 9 pin D sub or 5 pin domino DIN connectors

The ready made circuit board is designed using the same board outline as MiniB, the 2M/8M ram disc, and IDE interface.

How to solder together a BBC to PC serial link

As both the PC and the BBC are configured as DTE devices, a simple crossover cable is required.
Additionally, the physical connectors are both different as shown below:
BBC to PC serial lead


Orders can be placed online, or alternatively sent by post.

Two different serial port connectors are catered for, either a standard 9 pin D sub or 5 pin domino DIN:

Option: Price: Comments:
BBC to PC serial crossover cable
£8.90 inc inland UK postage
£11.40 inc airmail postage
Ready made 9 pin D sub female to 5 pin domino DIN male, 2m approx.
The PCB ready fitted with the programmed part
£29.00 inc inland UK postage
£31.50 inc airmail postage
You source the remaining components (Digikey/Farnell/etc...)
You build the board as per the instruction leaflet
Plug it in and away you go
Buy the finished board (D-Sub connector)
£54.00 inc inland UK postage
£57.50 inc airmail postage
Receive a built and tested board
Plug it in and away you go
Uses a 9 pin D sub connector
Buy the finished board (DIN connector)
£58.00 inc inland UK postage
£61.50 inc airmail postage
Receive a built and tested board
Plug it in and away you go
Uses a 5 pin domino DIN connector

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