SD-Systems SBC-200 Testing - Part 2

In the earlier post I wrote about testing an SD-Systems SBC-200 single board computer for the S-100 bus using my Northstar Horizon mainframe.

The end result of that testing was that the board seemed to work correctly but Philip had been unable to get it to operate in his s100computers.com S100 Bus motherboard.  

It happened that I had an unbuilt PCB for the same motherboard in my projects box... this was the kick I needed to get it assembled so see what was going on.  

Philip was kind enough to include a partial parts kit when he shipped up the SBC-200.  While most of the components were easily available from Jaycar or RS Components, the 270 ohm 9 resistor SIP resistor packs were proving difficult to locate and these were in the partial kit.  Thanks Philip.

My motherboard is labelled Revision 02 and differs slightly from Philip's in having an extra two slots and some rearrangement of the onboard components but is essentially the same design.  There were a couple of gotcha's documented on the web... the silk screen for Q1 is reversed and the fuse links are only fuses for the power status LED's (interesting) so can be replaced with links.

The motherboard undergoing smoke testing with my home built variable supply while waiting for the switching power supply modules to arrive.

Powering an S100 motherboard requires DC supplies of +8v, +16v and -16v.  The current thinking when using switching supplies is to run +7.5v, +15v and -15v if possible to have less heat dissipation in the onboard regulators.  Aliexpress came to the rescue with adjustable switching supply modules providing the right voltages at reasonable prices including shipping to New Zealand.  

 

S100 motherboard with switching supplies and SBC-200 installed.

 

S100 motherboard with switching supplies and SBC-200 installed.

With the motherboard assembled, the power supplies connected and the "known good" SBC-200 installed, I was surprised to find that the board didn't start properly.

Testing with a logic probe and a meter showed that the /RESET line did not go below 1.7v when the reset button was pressed which caused the SBC-200 to not reset correctly.  A clean /RESET is critical to the start address hardware on the SBC-200.  

The schematic showed R18, a 470 ohm resistor between the RESET button the GND.  I assume this was on the board to prevent a direct short to GND in the event that the other side of the reset button was tied directly to VCC rather than via a pullup resistor. The SBC-200 had a pullup resistor so the effect of R18 was to prevent /RESET going below 1.7V when the button was pressed.

Replacing R18 with a jumper solved the problem and a good reset was had with /RESET going to GND and the SBC-200 starting properly. The same solution worked for Philip's Revision 01 board.

 

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Contact Andrew Quinn

jaquinn@ihug.co.nz http://twitter.com/jaquinn