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I want to connect an old Intel 8085 CPU to an FPGA board. The basic issue is that the 8085 works at 5V while the FPGA will not accept any voltage avobe 3.3V so I decided to use the TXB0104 / TXB0108 bidirectional level shifters, however, I am unsure whether the bidirectional buses would work or would cause any trouble with either side of the shifter.

Also, the Intel 8085 is an NMOS circuit, not CMOS, so I am not really sure if that would cause any issues.

Edit: the H variant of 8085 I am using is "enhanced NMOS" HMOS

The circuit I am intending to use is as follows.

enter image description here

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To allow automatic bidirectional translation, the TXB has deliberately weak output drivers to allow other devices to override its own outputs.

This means that the other device must be able to drive the signals with at least 2 mA (see the section "Input Driver Requirement" of the TXB datasheet). However, the 8085 can drive a high signal only with 400 µA, and that at 2.4 V.

For this application, TXS shifters would be more appropriate. (The downside is that the pull-up resistors eat some current for each any signal that is low.)

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  • \$\begingroup\$ It seems like a fitting solution that is also pin-compatible, will try assembling the circuit with them and check if it works. Will mark as valid when i get some testing done. \$\endgroup\$ – NeonMan Nov 26 '18 at 0:52

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