Without going into a history lesson on ethernet, let's just say that a transceiver is a component that has long been required for transmitting and receiving on ethernet.
Ethernet used to commonly be used with a thick coax cable (thicknet or 10Base-5) or with a thinner coax cable (thinnet or 10Base-2). Devices usually had a standard interface called an AUI connector. You purchased a transceiver based on what network cable you were using (thick or thinnet). Go find some older ethernet devices like hubs or PC NIC cards and you will likely find this AUI connector.
As technologies developed and twisted pair (10Base-T) grew in popularity you found more and more devices with the transceiver built-in to the device. This was definitely more convenient but did commit you to one cable type. Today, thick and thin net have all but disappered and tranceivers with them. But older devices like the PLC-5 still require them. Even the first ControlLogix ethernet modules had an AUI connector along with the 10Base-T.
However, the 1785-ENET module does not require the transceiver as it has an RJ45 connector (10Base-T and 100Base-Tx) built-in .
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