FAQ

Short answers to the questions that come up most often with the bus board. The details are in Power and jumpers and Bus signals.

Which power supply should I use?#

Any of three, with one jumper cap to match:

• USB charger, 2 A or more, into J4, cap on JP4.
• Regulated 5 V DC, 2 A or more, into J3 or J2, cap on JP5.
• 9–12 V DC, into J3 or J2, cap on JP6: the on-board L7805 makes the 5 V.

For a main board with several cards, a regulated 5 V supply on JP5 is the best choice: nothing on the bus board limits its current, and nothing gets hot.

Can I power it from a 5 V supply?#

Yes. Plug a regulated 5 V supply into J3 or J2 and put the cap on JP5. The 5 V then goes straight to the bus, without the regulator.

It must be a regulated 5 V supply. JP5 has no protection: an adapter that gives 6 V or more, or a 9–12 V adapter left connected after moving the cap, puts that voltage on every chip in the system.

Can I power it from my computer's USB port?#

It may work for a main board alone, but often it does not. A USB 2.0 port gives 500 mA, and the main board alone needs about that much. The usual symptoms are random resets or a system that does not start. Use a USB charger rated 2 A or more instead.

Does the main board need its own power supply too?#

No. With a cap on JP4, JP5 or JP6, the bus board powers the main board and every card through pin 2 of the slots. Leave the main board's own power input unconnected.

The only time a card brings its own power is with no cap fitted (printed OFF – EXTERNAL): then the bus board supplies nothing, and the powered card feeds the bus. Never do both at once.

How many cards can I fit?#

Five slots: the main board and up to four cards. In practice two things limit you:

• Power. Add up what your cards draw. With JP6 the regulator runs out first; with a 5 V supply on JP5, the supply is the only limit.
• Load on the bus. The bus is not buffered, so every card adds load to the main board's outputs. Well-designed cards with CMOS inputs are fine in all five slots.

Which slot does the main board go in?#

The slots are identical, so any of them works. The board design recommends BUS5, the slot at the far right, furthest from the power section.

The regulator gets hot. Is that a problem?#

Warm is normal with JP6. Too hot to touch is not: the L7805 is shedding more heat than it can, and soon its thermal protection will switch the output off. The system then resets or turns off by itself after running for a while.

The heat is (input voltage − 5 V) × current. Use a 9 V adapter instead of 12 V, or move to a regulated 5 V supply on JP5, which does not use the regulator at all.

LED1 lights, but the system does not start. What now?#

LED1 only shows that VCC is present, not that it is right. Switch off, take out all cards, switch on and measure pin 2 to pin 6 of any slot: it must read 4.75 to 5.25 V.

• Voltage good: the bus board is doing its job. Fit the main board alone and check it with its own troubleshooting steps, then add the cards back one at a time.
• Voltage low: wrong cap for the supply, a weak supply, or (with JP6) a regulator fitted the wrong way round.
• Voltage above 5.25 V: switch off at once. A cap on JP5 with a supply above 5 V.

Can I plug cards in with the power on?#

No. Always switch SW1 off before fitting or removing a card. Plugging a card in live can damage it, the main board, or both.

Does the RAM expansion work in the bus board?#

Yes. The RAM expansion plugs into any slot, the same way it plugs onto the main board's expansion pins. JP4 on the main board must be shorted for it, wherever the card sits: see the RAM expansion address jumpers.

What are the 2×8 headers for?#

They carry 16 spare lines, EXT1 to EXT16, joined slot to slot and to nothing else. Cards can use them to talk to each other privately. If none of your cards use them, they simply stay unused.

What is J1 for?#

J1 brings out the data lines of the micro USB connector (ID, D−, D+). Nothing on the bus board or the bus uses them. Leave it unconnected unless a project of your own needs them.

Still stuck?#

Send clear photos of both sides of the board, the supply you use and where the jumper cap sits to Telegram or solderdemon.official@gmail.com.