From Dedicated Micros USA Wiki

Protocol Translator

DM has many DVR's that only support Pelco "P" protocol and only 1 currently that supports Pelco "D" and that is the SD Range DVR. This device can be used to convert your telemetry code from Pelco "D" to Pelco "P" or Pelco "P" to Pelco "D" if you have a camera or DVR that supports one but not the other. 

Two companies that provide these translators are Pelco and Sennetech (recommended and shown below)

Sennetech provides information on there site for how their product works and is fairly easy for setup: http://www.sennetech.net/manuals_pdf/SC%2050/SC-50-Pelco-422.PDF