Built open collector inverter to modify Documation M1000 to read cards with right side notches

 CHOSE SIMPLE BUT EFFECTIVE CIRCUIT

I needed an open collector inverter to tie to the OC inverter that drives the signal path for the row 12 data. No spare gates existed on the board and I wanted to avoid offboard implementations. Since the node where I will implement the modification is an OC inverter with a pull up resistor, all I need is a circuit that can sink enough current to pull the line down to logic LOW when the inverter is activated. 

Pulled up by a 1K resistor to VCC, the node would sink just under 5ma when either of the OC inverters is at low LOW output. Further, I don't want to load the CR81 signal with too much current draw to activate the inverter. I chose an N-channel MOSFET as the only component needed, as it will form a channel to ground for the node when the input to the MOSFET is high. Further, a FET has negligible impact on the input. 

It is important to ensure that the MOSFET doesn't conduct when the CR81 signal is at logic LOW, which can be a voltage above zero defined by the driving chip spec. I did some testing with a breadboard and assured myself that even with a logic low input of almost 0.7 volts, the inverter output stayed high, but any legit high input voltage the FET will pull the output down to almost zero. 

LOCATING THE COMPONENTS FOR INSTALLATION

The MOSFET can be mounted on the PCB near the 13 finger to the backplane which is the source of CR81.  That area has free space with no traces on either side, sufficient to drill two small holes to secure my one component in place. While I won't have copper traces on the board, I can use small jumpers over to ground, directly solder the gate to the 13 inger then run a jumper to the node where I am implementing the wired-OR. 

MOSFET IN PLACE

DRILLED AND MOUNTED IN PLACE, JUMPERS INSTALLED

As you can see, I drilled holes and put the transistor in place on the board. I soldered one jumper between the MOSFET source and ground. Another longer wire had to be run from the drain of the MOSFET over to the 7405N open collector inverter in position 8D on the PCB, to pin 2 which is the output where I am adding this circuit in a wired-OR.

Jumpers to ground and to the new wire-OR node