Mo & Terry Smedley

 

Options for powering the H8 with a new backplane:

With no changes required to any other board in the system:

1) OEM linear supplies for both +8v and +/-18v

2) Non-OEM switched supplies that provide similar voltages to the OEM supplies. 
Drop out for the linear regulators used on original cards (7805, 7812, or 7912) is about 2v under load.  So any supply that can provide at least 7v, or +/-14v when loaded could be used.  Note that these are absolute minimum voltages - targeting for 8v and +/-15v would provide a much better operating margin for linear on-board regulators.  Power supplies should be capable of sourcing up to 8A for the +8v supply, 500ma for +12v, and 150ma for -12v.  If on-board regulators have been replaced with switching devices, and modern static memory is used, the current draw of even a fully loaded H8 is considerably less:  I measure about 2.5A for the +8v supply, 250ma for +12v, and 50ma for -12v.

Substituting "step-up/step-down" regulators on each card to be used with the backplane:

3) A PC-ATX power supply can provide the H8 backplane with +5v and +/-12v.  
With step-up/step-down regulators installed on each board, you can use such a PC power supply.  The ATX connector on the backplane connects directly to a PC-ATX power supply.  Note that even small PC power supplies can source much more current than the H8 requires, which is why fast-acting overcurrent protection has been included on this board.  Step-up/Step-down regulators are readily available to replace the 7805s on individual boards, but similar replacements for the +/-12v regulators are much harder to find.  Some modern boards for the H8 have jumpers that allow you to bypass the on-board regulator and feed the card directly from the H8 bus.  This bypass would let you use the PC-ATX supply to directly provide the +/-12v that some cards require, or the +5v that all cards use.  Be aware that bypassing on-board regulation may result in digital noise artifacts that interfere with board operation - YMMV.

Options for connecting any power supply to the new backplane:

If you are using the on-board +/-18v rectifier, you must provide the board with center-tapped 25-30VAC from a transformer.  This must be connected to J102 which can be populated with a Molex KK396 header (polarization not required), or by soldering directly to the J102 pads if you must.

To provide DC to the board, from any source, you have several choices:

bulletA Molex KK396 pin header at J101 accepts +5V or +8V.
bullet+5v or +8v can be soldered directly to the board (as in the OEM Heath design).  The pads at J101 will accept up to 14ga stranded wire. 
bulletA Molex KK396 pin header at J102 can accept +/-12v or +/-18VDC if jumper wires are installed at D101, as shown on the silkscreen. 
bulletThe ATX connector at P21 can be used to connect +5V/+8VDC and +/-12V to +/-18VDC by fabricating a custom ATX plug.  Use the pins identified on the silkscreen to route individual wires from your supplies.  No jumpers at D101 are needed if all DC is sourced through P21.

On/Off control for PC-ATX power supply:

The backplane provides three ways to control the power on/off with a PC-ATX power supply:

  1. The AC switch provided on the PC-ATX case can be used as the primary ON/OFF switch for the computer.  With most PC-ATX supplies, you would need to ground the PWR_ON circuit.  You can do this by shorting the jumper at J103 on the backplane. 
  2. You can use a (low voltage, low current) toggle switch connected to J103 to control the power supply.  The AC switch on the PC-ATX case is left on at all times.  You will see the "STBY" LED light to indicate that the PC-ATX supply is receiving line AC even when the toggle switch is open and the H8 is "OFF".
  3. You can use a (low voltage, low current) momentary contact pushbutton switch at JP101 on the backplane, leaving the AC switch on at all times.  Push once, the power comes on.  Push again, the power goes out.  Note that unlike your PC, the H8 power supply is immediately turned off whenever the pushbutton is activated.  A PC interprets the pushbutton as a request to shut down, but the OS will typically wait until all pending disk and file operations are complete before shutting down.  With the H8, the shutdown is immediate and if done in the middle of file or disk I/O, there will likely be corruption.

Omitting unneeded components from the board:

bulletIf not using the on-board +/-18V rectifier and filter, you must omit bridge rectifier D101, and capacitors C1 and C2
bulletConnectors at J101, J102, P21, P2 and P3 are entirely at the builder's discretion - populate what you plan to use, and omit the rest.
bulletIf you don't plan on using the pushbutton ON/OFF feature for a PC-ATX supply, you can eliminate LM555N U101 and the associated resistors and capacitors.  These components are used only with the pushbutton - the toggle switch at J103 requires no components other than the switch itself.