This is a simple adapter for connecting a FPGA board to an E1/T1 interface via short distance twisted pair.

RX is done by a couple of comparators (KA319 or LM319) which detect, respectively, positive (RXP_N low) and negative (RXN_N low) pulses.

TX is done directly by the FPGA pins. Current flows either from TXP to TXN to produce a positive pulse, or from TXN to TXP to produce a negative pulse.

The FPGA design is one of the exercises of the Communications Technology class at UFSCar. Starter files and a guide for developing the project can be found here.

Schematics are available both in PDF and in Kicad format.

We don't have a proper PCB layout. Please feel free to contribute one.

Instead, please see below some photos of our milled by hand prototype board.

Please note that although one should ideally use 1:1 transformers, we only had at hand a couple of security camera baluns (designed to work as 75 ohm to 100 ohm impedance transformers), which worked just fine.