My first 10BASE5 network segment

Thinwire (10base2) and 10base5 are electrically very similar. There is a difference (I forget what, but I seem to remember if you used the common 8392 transceiver IC there is a small passive component (resistor, perhaps diode) that's fitted for one and not the other), but it is small. You can often get away with using BNC-N adapters to connect one to the other. 51 Ohms is a prefered value and the nearest to 50 ohms you will easily get. 47Ohms should be fine too. (From the method used for collision detection it is generally better for the termination to be too low rather than too high.) Actually a 47 ohm at one end and a 56ohm at the other has almost the correct DC resistance (25.5 ohms, as against 25 ohms for a pair of 50 ohm terminators) and is probably the best you'll get with common parts. -tony