Nano Ethernet Shield
There are at least 3 versions of the Nano Ethernet shield in shops. Black version is by Robotdyn, red and blue version are from different manufacturers. The shields use the schematics recommended in the datasheet for a 5 V MCU. They use logic gates (74HCT08) instead of a Tri-State Digital Buffer (74HC125) to convert the MISO line to 5 V. This creates problems. SPI device not selected by CS pin should release the MISO line by setting the output pin to HI-Z state. The enc28j60 of course does this, but the logic gate can't. It stays HIGH, blocking the bus. No other device can be used on SPI with this shield attached. (Many 5 V SD card adapters have the same problem with then level conversion on the MISO line.)
The new 3.3 V Nano boards could be damaged by this shield, while without the 5 V conversion the shield would be OK for a 3.3 V Nano. This applies to any 3.3 V MCU with this shield wired as module.
The shields can be converted to work with a 3.3 V Nano.
To remove the interfering and unnecessary conversion to 5 V, remove the 74HCT08 chip from the shield and connect the contact with a trace to pin 12 to the contact to enc28j60 MISO pin. The 74HCT08 has 4 logical AND channels. The shield uses two of them as buffers by tying both inputs together (1 and 1 is 1, 0 and 0 is 0).
If you want to use the INT pin (EthernetENC doesn't use it), then connect the contact with the trace to pin 2 to the inputs of the channel. You can determine the right contact by following the trace or by measuring the continuity with a multimeter.
Some shields have a 5V pull-up on CS, INT and RESET pins (as is the case with the red shield on the photo bellow, where the 4 x 10 kOhm resistors IC between the 74HCT08 and ENC28J60 does this.) The shield by Robotdyn doesn't have this problem.
Modification of the Robotdyn version for a 3.3 V board:
Modification of the red version to avoid the MISO line problem:
Modification of the red version with the pull-up resistors IC removed too for use with a 3.3 V board (the solder bridge in yellow circle is for MISO, the bridge in the blue circle is for the INT line):
