What is the maximum distance for a Real Port serial connection?

Problem: using current configuration, I can connect from a serial connection over a short span of cable but not over a long span of cable?
Current configuration - Digi PortServer TS2P MEI connecting to a Veriteq model SP–1000 data logger - serial connection using RealPort set to throughput.
Able to connect to the data logger over a 20’ CAT6 cable but cannot connect using the same setup over a 200’ CAT6 cable. The 200’ Ethernet cable has been tested working. The only difference in the cable configuration is the length of cable. The vendor said the serial port connection is capable of spans up to 800’ and promised Real Port would auto-configure the serial port settings with the greatest of ease. So far 20’ is the maximum obtainable distance.
Thanks in advance for any assistance you can provide!

For RS232: