Power and jumpers

The bus board does more than join slots: it is the power supply of the whole rosco_m68k system. Whatever feeds the bus board reaches pin 2 (VCC) of every slot, and from there the main board and every card.

This page explains where the power comes from, what each jumper position does, and how much a given supply can deliver.

IMPORTANT: one supply, one cap

The board has no fuse and no reverse-polarity protection. A wrong jumper or a supply above 5 V on the wrong input goes straight to every chip in the system. Read this page before the first power-on.

Power inputs#

J3 and J2 are the same input: use one of them, never both at once.

The power switch (SW1)#

SW1 is a double-pole switch. One pole switches the USB 5 V, the other switches the DC input from J3/J2. With SW1 off, neither input reaches the jumpers, so the bus is unpowered.

SW1 does not choose the source. The jumper does.

Power setting: JP4–JP6#

The three jumpers sit in one 2×3 block, printed on the board as POWER SETTING. Exactly one cap goes on, in the row that matches your supply.

JP4: USB#

The simplest option. Use a USB charger rated at least 2 A. A computer USB 2.0 port gives only 500 mA, and the rosco_m68k main board alone needs about that much: from a computer port the system may reset at random or not start at all.

JP5: regulated 5 V#

The most powerful option, because nothing on the bus board limits the current: the supply feeds VCC directly. Use a regulated 5 V supply of 2 A or more.

The supply must really be 5 V. JP5 bypasses the regulator, so anything above 5.25 V reaches every chip. Never fit JP5 with a 9 V or 12 V supply connected.

JP6: 9–12 V through the L7805#

Use this when you have a 9–12 V adapter. The L7805 turns it into 5 V. Stay within the 9–12 V printed on the board: a higher voltage only adds heat in the regulator.

The regulator turns the extra voltage into heat (see Current and heat below), so this is the weakest option for a large system.

No cap: power from a card#

With no cap fitted, the bus board does not power anything. VCC on the bus comes from whatever card is powered on its own, for example a main board with its own supply connected. LED1 still lights in this case, because it sits on VCC.

Do not fit a cap and power a card from its own supply at the same time: two supplies on one VCC line fight each other.

Never two caps

Two caps join two supplies. JP5 together with JP6 puts the full DC input on VCC. JP4 together with JP5 or JP6 joins the USB port to the DC supply and can push current back into the charger or the computer.

Current and heat#

The regulator drops the difference between the input and 5 V, and turns it into heat:

Heat (W) = (input voltage − 5 V) × current (A)

U1 lies flat on the board without a heatsink, so it can shed only about 1–2 W comfortably. Above that it runs too hot to touch, and the L7805's built-in thermal protection cuts the output: the system then resets or switches off by itself.

In practice:

• Main board only: JP6 with a 9 V adapter is fine. The regulator gets warm; that is normal.
• Main board and cards: use JP5 with a regulated 5 V supply, or USB from a 2 A charger.
• Regulator too hot with JP6: lower the input voltage to 9 V, or move to a 5 V supply. Bolting U1 to the board with an M3 screw and nut helps a little.

Checking the supply#

LED1 (PWR GOOD) is fed from VCC through R1. It only shows that VCC is present, not that it is correct: it lights at 4 V and at 9 V alike.

Measure before you fit any card:

1. Fit the cap for your supply, connect the supply, switch SW1 on.
2. Multimeter on DC volts at any slot: pin 2 (VCC) to pin 6 (GND).
3. Expect 4.75 to 5.25 V. Anything else: switch off and check the cap and the supply.

The build guide has the same check with board pictures and a short troubleshooting list.

Filtering#

C1 (470 µF) is the reservoir for the whole bus. C2–C6 (100 nF) decouple VCC along the slots, C7 (330 nF) sits at the regulator input and C8 (100 nF) at its output, as the L7805 datasheet asks.

J1: USB data lines#

J1 brings out the data lines of the micro USB connector J4: pin 1 ID, pin 2 D−, pin 3 D+, as printed next to it. Nothing on the bus board or the bus uses them: leave J1 unconnected unless a project of your own needs those lines.

Quick reference

If in doubt, use a regulated 5 V supply on JP5, and measure before fitting cards.