by AltairClone » February 20th, 2014, 4:56 pm
On the B&B converter, you can actually use the same power supply for both of the loops and the RS232 logic, however, you have then lost isolation between the Teletype and the computer you are interfacing to.
Both the Altair SIO and 2SIO boards interfaced to the Teletype without isolation. The interface board I built also connects to the Teletype without isolation. The send side from the Teletype (keyboard, reader, answer-back) is a completely passive path not referenced to anything. Power and components in the receive side of the Teletype (UCC-6) are isolated from the A/C mains via a transformer and the minus side of the 20ma input is then referenced (I think) to chassis ground. If not, then the minus side of your circuit establishes the voltage reference of the Teletype's minus input. I've seen both in the drawings. I'll think now I'll check my Teletype and find out for sure!
Then again, if you have it working and there is no smoke, why mess with it!?
Mike