The goal is to replace the existing Inductosyn encoders with new Heidenhain RON806 incremental encoders. The simplest place to re-attach encoder information back to the existing analog ESSCO servo system is at the connector for analog error (position difference) signal. Thus the existing ESSCO position indicators and digital error calculators will not be used.
RON806 encoders require relatively short interface cables and for this reason one PC with a Heidenhain IK121 interface card is required at the antenna tower near the encoders. On the other hand, a digital display of antenna position is required in control room. The commanded antenna position is made available by MicroVAX, and ESSCO analog error input is also located in control room. These necessitate putting another PC in control room rack.
Connecting the two PCs will be most straightforward with a RS422 link using two twisted pairs. Running at 115kbaud it can convey about 1000 pairs of azimuth/elevation coordinates per second. (MicroVAX will generate coordinate changes less than 100 times per second.)