Hello jim;

The requirements for a terminating resistor (terminator) goes beyond the particular equipment you are looking at. Most serial fieldbuses need them, either at one or both ends of a bus (depending on the network specifications used). Some PLCs need one after the last module is placed on the rack. And interface modules like the IM 360/361 require a special terminator at the end of the cable that joins them. And many other systems use them also.

Here is therefore a discussion that examines the general case where a terminator is used:

http://electronics.stackexchange.com/questions/10884/how-termination-resistors-work-what-happens-if-i-use-lower-values

Hope this helps,

Daniel Chartier