search

eduardocasino / kim-1-mtu-motherboard

Buffered expansion backplane for the KIM-1 that follows MTU's standard for cards like the K-1008 Visable Memory
Other
2 stars 1 forks source link

Data bus transceiver must be put in high impedance state when DECEN is low #2

Closed eduardocasino closed 9 months ago

eduardocasino commented 11 months ago

The 74LS245 data bus transceiver (U4 in the schematic) is enabled permanently (the OE line is tied to ground) This means that when the KIM-1 is handling the address decoding and the ~R/W line is low, both the KIM-1 and the transceiver are trying to drive the data bus, which renders the system unusable.

The solution is to disable the transceiver when DECEN is low, so the OE line must be driven by DECEN through an inverter.

Until this is fixed, the board is useless.

eduardocasino commented 11 months ago

The transceiver must be disabled also when K7 is low (interrupt vectors pointing to the kim-1 system ROM)