/RESET Mod for RC2014 Dual Clock and Reset Board

One of the modular components of the RC2014 Z80-based retro computer system is the Dual Clock and Reset Module. This board (as of version 2.1) actively drives the /RESET signal with a 74HCT04 inverter. This means that if any other component wishes to reset the system, it has to contend with that gate and short its high output to ground. For example, the Backplane Pro has a hard switch for /RESET, which literally shorts the signal to ground. While you can get away with this sort of thing most of the time, it’s theoretically possible to blow up the 74HCT04 this way.

I could have just disconnected the 74HCT04 from the /RESET signal and used the button on the Backplane Pro for manual resets. However, the button on the module is more conveniently placed. What’s more, the reset circuit on the module generates a power-on reset, which is very handy. I wanted to preserve these functions of the module. Here is what I came up with, showing the relevant part of the module’s schematic as modified.

Dual Clock mod schematic 01

We do disconnect the 74HCT04 U1 from /RESET, but we replace it with a 2N3904 NPN switching transistor. This acts as an inverter, just as U1B did, but it only drives actively when it’s pulling the signal low. For mechanical convenience, we pick up both the input and the output signals from P1, a 3-pin header intended for selection of either RESET or /RESET to connect to the RESET2 signal on the extended bus. I don’t use RESET2 and never installed that header. If you use this header, you’ll have to come up with a different mechanical arrangement.

Note that this assumes there is a pull-up resistor somewhere on the /RESET signal. All of the official RC2014 backplanes (the Backplane Pro, the Backplane-5, and the Backplane-8) provide such a pull-up.

Prepare a 1K resistor and a 2N3904 transistor as shown below. Solder one end of the resistor to the base (center lead) of the 2N3904. Bend the emitter lead out to one side.

Resistor connected to center pin of transistor

That’s a quarter-watt resistor, but if you have a smaller one handy, it would fit better. The value is not critical at all. Any NPN switching transistor would probably work fine, too, as long as you get the pin assignments right. This shows the pin assignment for a 2N3904:

2N3904 pinout

Now drop the assembly into the Dual Clock and Reset board, as shown below. The free lead of the resistor goes into pin 3 (RESET) of the P1 header, and the unbent collector lead of the 2N3904 goes into pin 1 (/RESET) of P1. The bent emitter lead picks up ground from pin 1 of the extended bus connector. Solder and trim both leads on the bottom of the board, and solder the emitter lead to the nearest pin on the extended bus connector and trim. Make sure the exposed base-resistor junction doesn’t touch any other exposed lead. Nothing is connected to pin 2 (RESET2) of P1.

mod assembly installed in module board

Finally, remove U1, the 74HCT04, from its socket, gently bend pin 4 up about 90 degrees, and reinstall U1 in its socket. Here’s what that looks like.

completed modification

That completes the modification. Put the board back into your system and test that reset still works. You shouldn’t notice any change in power-up behavior or behavior when you press either the reset button on the backplane or the one on the module. Rest easier knowing that resets don’t short out the 74HCT04 anymore.

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