Terminal miscellany

 NEW 3179 TERMINAL ARRIVED

My first 3179 terminal is so extremely dim that I have to sit outside at night in the darkness to see the image. I hope to rejuvenate the CRT to increase its brightness, but I found another 3179 monitor which arrived yesterday. It is indeed brighter - both it and my best 3178 are hard to read in sunlight but in normal room light the 3179, like the 3178, should be suitable. 

KEYBOARD FOR 3179 ORDERED

My two 3179 terminals came without keyboards. The logic element checks for the keyboard and won't complete its initialization, therefore I can't work with the terminal until I have a keyboard attached. The part numbers listed in the manuals are very hard to find but a fellow hobbyist, Guy Sotomayor, has working 3179 terminals that he equipped with keyboards originally used with different terminal types.

Guy shared the part numbers of the keyboards he has successfully used, which allowed me to find one on eBay yesterday. I bought the keyboard and am awaiting its arrival so that I can bring up the terminal fully and test it out.

BALUNS TO REPLACE COAX WITH ETHERNET CABLES

The cabling between the terminals and the 3174 controller are coaxial with 93 ohm impedance. An alternative was developed to make use of twisted pair Ethernet cables between terminals and the controller. I was given several pairs and tested out one pair successfully. This will allow me to connect multiple terminals without having to buy 93 ohm coax cables.

The coaxial cable has an active center lead surrounding by an insulator and an outer cylinder of conducting shield. The shield is connected to ground while the signal travels on the one inner conductor. These are unbalanced transmission lines.

Twisted pair makes use of differential signal, which helps reject induced noise because the spurious signal injected on one wire of the pair is almost completely counterbalanced by a reverse direction signal on the other wire. Therefore twisted pair are balanced transmission lines. 

The devices I have are Baluns, an adapter that converts between unbalanced and balanced transmission lines. One side has a coax connector, which is installed on the terminal or 3174 controller. The other side is an RJ-45 jack where the ethernet cable is inserted.