The information on the web points in all directions. 
Yes, you need a null modem cable, but unfortunately, there are a lot of them
I. NULL MODEM CABLES
I started with a guide for serial Hi-pro’s on Windows 7:
According to this nice wiki about null modem cables this is a null modem cable with loop back handshaking (,albeit with a mistake in the picture–the text is correct.) I soldered the arrowed connections myself, because the one that I was supplied with—and worked well for a year— was a null modem cable without handshaking.
So apart from the obvious times the system was out-of-order, both worked for me with a connection from the Hi-Pro directly to the COM header on my PC motherboard. I’ve tested the loop back handshaking null modem cable connected to a Serial2USB device with a CH341 chip inside and it worked.
Depending on the connection your computer has to the Hi-Pro it seems that you have 4 (FOUR!) possibilities to try out:
Obviously you only have to de-/solder the loop back connections to switch between without handshaking and loop back: Both have 3 wires running from one DB9 connector to the other. The Partial and Full handshaking both have 7(!) wires running through the cable.
II. So what do you need?
As I was fixing a system that worked before and I was supplied with a cable that had three wires, I only tried the Partial handshaking. Do you need more? Looking at the wiki for serial ports:
Bit rates commonly supported include 75, 110, 300, 1200, 2400, 4800, 9600, 19200, 38400, 57600 and 115200 bit/s.
Comparing this to the technical specifications that state the baud rate as
1200 (default), 2400, 4800, 9600 or 19200 baud
, the Hi-Pro really has no high speed connection. So it is unlikely that the 7-wires connection from the Partial and Full handshaking is needed. Safest bet is Partial handshaking since it also allows for DTE/DCE compatibility.
However, for compatibility issues, especially those arising from USB2serial (USB2DB9) I would stop fidgeting with those settings and rather look at all four null modem cable options. (Mine only cost around 3€ and it works nicely with Partial handshaking.)
