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Add reset button to Tandy 1000 (1000ex, 1000hx)

WimWalther

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The Tandy 1000 machines like the ex & hx models lack a hardware reset button. I'd really like to add one to save wear & tear on the power switch and PSU components. So is this as simple as perhaps...

Plus connector RESET pin -> 1K resistor -> 10uF cap to GND -> SPST Mom. from cap + to GND.

I may have the cap on the wrong side of the 1K.. not quite sure. Should it be connected directly to the RESET pin? It works pretty much the same either way - push the switch, which drains the cap leaving you with a 1K pulldown on RESET.

Thoughts?
 

alank2

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The cap is meant to provide stability to the reset pin side, which it may or may not already have.

The resistor is meant to limit the current that can be pulled from the reset pin side, which it may or may not need.
 

modem7

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So is it safe to use the RESET line that's exposed on the Plus connector?
Sometimes, to answer questions, one needs to be quite familiar with the computer. I am not familiar with Tandy computers.

However, a quick read online shows, "The EX and HX utilized a PLUS-style connector, which was electronically identical to an 8-bit XT ISA slot, but had a 62-pin Berg connector instead of a card edge, rendering it incompatible with ISA cards without an adapter."
The "electronically identical" informs me that the answer to your question is no. Refer to the IBM XT (IBM 5160) diagram at [here]. At the top-right are the ISA expansion slots. In ISA slots, the RESET signal is an output (output to ISA card), not an input.
 

Eudimorphodon

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In ISA slots, the RESET signal is an output (output to ISA card), not an input.

Took a look at the 1000EX schematics and, yeah, the reset signal that’s on the bus connector is the buffered output from a ‘244. So grounding it will only be shorting that output to ground, which probably won’t kill it if it’s through a 1k resistor, but also isn’t likely to do you much good.
 

Eudimorphodon

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… FWIW, comparing the schematic of the EX with the SX, which does have a reset switch, it looks like the only kosher way to add one would be to hack up the circuitry north of the “RSTINB” signal of the Light Blue timing chip to match the SX. Hacking a button in on the south side of Light Blue would have you fighting its output, and considering that it’s an irreplaceable custom ASIC that kinda sounds like a bad idea.
 

mogwaay

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I took a quick look at this, the original Tandy1000 schematics seem to have a reset switch, but for the Tandy1000EX I think you could mod the RC circuit that generates the intiial RST signal on power on with something to discharge the caps (switch the +'ve end to GND) and that might generate a new suitable reset signal for Light Blue chip... The caps in parallel here confuse my basic electrical brain a bit and I take NO RESPONSIBILITY FOR FRYING ANYONES OLD TANDY - This is just a suggestion... someone might want to jump in here and correct me...


1655981726571.png
 

Eudimorphodon

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This is just a suggestion... someone might want to jump in here and correct me...

Your suggestion is basically what I said, IE, make it like the SX. The SX’s circuit has all the same components but with a switch to ground… I just didn’t want to suggest which trace to cut because the schematic is drawn “twisted around” compared to the EX’s and my brain wasn’t awake enough to un-twist it without redrawing it.
 

WimWalther

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Interesting. I always thought that there was a CPU reset line on the ISA bus, so if a piece of hardware panicked / crashed leaving it in an undefined state it could force a system reset.

But mogwaay's suggestions seem to be the ticket. The 47K pullup / 10uF cap are standard RESET line stabilization parts. If I were to pull down the marked point with a 1K it would leave 100mV on the RESET pin which is well below the "low" threshold for any logic family on that board.

And no need to cut any traces, just tack wires to that point & GND.

As for the 0.1uF paralleling the 10uF, it's a high-frequency bypass, there to improve the (typically mediocre) high freq response of the electrolytic cap.
 

Eudimorphodon

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Interesting. I always thought that there was a CPU reset line on the ISA bus, so if a piece of hardware panicked / crashed leaving it in an undefined state it could force a system reset.

In a lot of older machines the reset line is held high by a pull-up on +5v "globally" and, yeah, it's safe for any piece of add-on hardware to just use an open-collector buffer to yank it down if you want to hand reset control off to something else. But apparently IBM deemed that on their bus RESET was a motherboard monarchy, not a democracy.
 
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