Where a link of a communication network such as a powers multi-drop (PMD) trunk or a local area network (LAN), which are RS 485 communication networks, enters or exits a building or section of a building, it is desirable to electrically isolate the section of the link that leads into the building.  This is so that the equipment with which the communication link is connected is protected from transient overvoltage conditions that may develop along that link and cause damage to the equipment.  In addition, electrical isolation also provides signal level protection so that undesirable signal distortions or noises produced in the communication network are prevented from propagating into the link that enters the building. 
It is known to employ repeater apparatus having opto-isolators for providing electrical isolation and a data driven control circuit for providing data direction control.  However, on larger trunk systems, the conventional repeaters typically produce distortions which, when coupled with the distortions from the network, can cause communication errors.  In addition, conventional repeaters that have competitively priced components for a control circuit are normally only capable of operating at speeds from about 1200 to 57600 bits per second (bps).  The competitively priced control circuits also have a slow turnaround time when switching from a data transmit mode to a receive mode.  At higher speeds, these repeaters also are not capable of operating without producing significant errors.  The repeaters that operate at higher speeds and that have a fast turnaround time do exist, but they are disadvantageous from a cost perspective. 
Other conventional isolators that have overvoltage protection circuits do not meet the new Underwriter's Laboratory (UL) protection requirements for communication wiring that enters a building.  Another disadvantage of some of the conventional repeaters is that they are not adapted to conduct troubleshooting measures for themselves or for the communication lines to which they are connected to ensure that data are transferred through the communication lines without errors. 
Accordingly, one object of this invention is to provide an improved repeater apparatus for isolating a link of a communication network without producing unacceptable amount of distortions. 
Another object is to provide an improved repeater apparatus that utilizes relatively inexpensive components that are capable of transferring data at a high rate of at least 115.2K bps and have a fast turnaround time when switching to a data receive mode. 
Yet another object is to provide an improved repeater apparatus which meets the UL overvoltage protection requirements for protecting a communication link that enters into a building. 
Still another object is to provide an improved repeater apparatus having self-test capabilities for determining proper operation thereof. 
A further object is to provide an improved repeater apparatus for testing the communication line to which it is attached. 
Still further object is to provide an improved repeater apparatus incorporating all of the above-desired objects in a single unitary device.