10Base2 Interface
The 10Base2 media interface circuit is more complex than the 10Base-T circuitry. This circuit must not only be designed correctly, but must
also be physically laid out correctly to ensure correct impedance matching and minimal noise. In addition, suitable protection circuitry is
required to isolate the module from the Ethernet network. The circuitry shown below is only a guide and we cannot accept responsibility for
its correct operation, as the component layout is critical.
Note: All resistors used have a tolerance of 5% and a power rating of 0,1W unless otherwise noted.
In addition to transmit and receive signal pairs, there is also a carrier detect signal pair. As with the 10BaseT media interface circuit
(see 10BaseT Interface), these three signals must first pass through a small isolation transformer (such as a PE64103).
Then the isolated signals must be multiplexed onto a 50ohm transmission line via a device such as the DP8392. This device has three pairs of
inputs for each of the three signal pairs. It also has a single transmit data output and a receive data input for connection to the BNC
connector. These two signals must be isolated from one another via a signal diode (the circuit below has two diodes: D07 and SOTDIO).
The multiplexer requires a -9V isolated power supply. This is usually provided via a small DC/DC converter that steps up the +5V module
supply to +9V. Such a converter typically provides isolation potential from 1KV to 2KV. In the circuit below, two possible devices are
shown: either a UM251 or a HUS0509 - the UM251 is larger and requires an external resistor (100ohm 1%). There is also a 10uF 25V capacitor
across the 9V supply.
Note: The multiplexer's ground (BNC body) and the module's ground (chassis/GND) must be isolated. For protection purposes, there is
a 10nF 1KV capacitor across the BNC body and GND, another across the BNC body and the -9V supply rail (AGND) and a third across AGND and GND.
There is also a 10nF 1KV GAP75 spark gap and a 1MO 1/2W resistor across the BNC body and GND to protect against high-voltage spikes.
Each of the multiplexer's receive signals and carrier detect signals are pulled down to GND via 1.5Kohm 1% resistors, while there is a 37.5ohm
resistor across the transmit- signal pair (two 75ohm resistors). The multiplexer also requires an external 1Kohm 1% resistor.
Note: The various signal paths should all be of the same length and, most importantly, the leads from the multiplexer to the BNC
connector must be as short as possible.
10Base2 Media Interface Circuit
