Communications Manual Rev 3.10  Software Versions 3.xx 
003R-618-310
  Page 9 
Long Ring Network: Multiple Instruments to PC (RXD, TXD, GND) 
The Long Ring Network layout can be used in situations where each leg of the 
cable  run  can  be  up  to  about  150 m  (500 ft)  at  9600  baud.    If  there  are 
communications errors, lower the baud rate to 4800 or 2400. 
 
Figure 5: RS-232 Long Cable Runs: Ring Network using COM Port (DB9)