tl;dr - Here's a program for interfacing with a Superconductor Technologies Superlink
There's a lot of data pumped out by the cryocooler while it's operating, including various temperatures, voltages, power consumption, et cetera. Unfortunately, I could only access that data through the manufacturer's software, which won't run on anything beyond Windows XP. I have a 10 year old laptop that will run it, but I mean c'mon, it's a ten year old computer. I'm not sure how much longer ol' Speed Turtle here is going to keep on staying critical-failure-free.
The System Status Portal software turns out to be a pretty face for the serial back-and-forth between the computer and the Superlink. The link runs at 19200 baud, 8 data bits, zero parity, no flow control. I ran a serial monitor program while the computer had the software open and talking to the Superlink, and it showed that the back and forth looked something like this:
Request: 3/13/2016 9:59:32 PM.57264 (+0.2567 seconds) 3C 53 59 20 4F 50 3D 22 4F 4B 22 2F 3E 0D 0A <SY OP="OK"/>.. Answer: 3/13/2016 9:59:32 PM.57264 (+0.0000 seconds) 3C 53 59 20 4F 50 3D 22 4F 4B 22 2F 3E 0A <SY OP="OK"/>. Request: 3/13/2016 9:59:32 PM.60364 (+0.0000 seconds) 3C 54 4D 20 4F 50 3D 22 47 54 22 20 4C 43 3D 22 <TM OP="GT" LC=" 43 52 22 2F 3E 0D 0A CR"/>.. Answer: 3/13/2016 9:59:32 PM.60364 (+0.0000 seconds) 0D 3C 54 4D 20 4F 50 3D 22 47 54 22 20 4C 43 3D .<TM OP="GT" LC= 22 43 52 22 3E 30 20 30 20 31 38 20 33 35 3C 2F "CR">0 0 18 35</ 54 4D 3E 0A TM>. Request: 3/13/2016 9:59:32 PM.63364 (+0.0000 seconds) 3C 54 4D 20 4F 50 3D 22 47 54 22 20 4C 43 3D 22 <TM OP="GT" LC=" 41 43 22 2F 3E 0D 0A AC"/>.. Answer: 3/13/2016 9:59:32 PM.63364 (+0.0000 seconds) 0D 3C 54 4D 20 4F 50 3D 22 47 54 22 20 4C 43 3D .<TM OP="GT" LC= 22 41 43 22 3E 41 43 42 20 33 20 41 43 39 20 31 "AC">ACB 3 AC9 1 30 3C 2F 54 4D 3E 0A 0D 0</TM>..
The interaction consists of query strings sent by the computer that are returned with the data requested. Pouring through the decompiled code for the System Status Portal (what can I say, I live a wild life), I managed to figure out which query strings returned the data I was interested in. Turns out, if you want to find the cold side temperature and cooler rejection temperature, you ask:
<TP OP="GT" LC="MS"/>
which is responded to with something like:
<TP OP="GT" LC="MS">F 6D80 3BE8 4D60 3C78</TP>
Each of those five hex numbers in the middle there correspond to five different measurements it's returning. Turns out the cold side wide range temperature is the second one (6D80) and the cooler rejection temperature is the third one (3BE8). That hex number is pretty meaningless, though. Pouring through more of the System Status Portal code, I found the formulas it used to convert those numbers to actual temperatures. The formula is:
Temp = (5 * (hex string converted to decimal)/32768 - Offset)/Gain
where for the cold wide range temperature, the offset is 5.814 and the gain is -0.01559. There's a different offset and gain for the rejection temperature and the other temperatures returned. Convert the two-letter parts of the query string ("TP", "OP", "LC", "MS") to their ASCII equivalent hex codes (TP=5450) and then convert that to a decimal number (hex 5450 = 21584), and search the System Status Portal code for that decimal number to work through the logic to find which data points are revealed by which query strings!
Similarly, the cooler power draw can be elucidated with the proper query string and formula, as can every other measurement that shows up in the System Status Portal. In addition to reading measurement values, the System Status Portal can also change the operating state of the cooler, switching it between manual mode where the cooler can be shut off and automatic mode, where the cooler will automatically ramp up power as the cold side temperature declines, keeping the cooler within its safe operating limits. This is accomplished with another query string,
<TP OP="ST" LC="SM">1 4</TP>
<TP OP="ST" LC="SM">0 4</TP>
will turn it back to automatic mode.
I needed an excuse to brush off my infantile Python skills, so I wrote a user interface that will allow you to see the cold side wide range temperature, cooler rejection temperature, cooler power, and status. You can also toggle between Manual Shutdown mode and Automatic mode, so you can shut down the cryocooler safely without the System Status Portal program. I've loaded the Python code to github - you'll need Python installed (I used version 3.5), along with the Tkinter and pySerial libraries to run it. I've only tested it on my Windows 10 computer, but it appears to work pretty well. Here it is on Github. There's a lot of room for improvement and expansion, so if you want to add functionality, go for it!
I next plan to replace the cold side temperature measurement with an external thermocouple, using an Arduino to simulate a resistance to send the temperature measurement to the Superlink. I'll then use the serial handshake to allow the Arduino to talk to the Superlink to read the temperature it's trying to simulate, allowing it to precisely control its output to make sure the Superlink is seeing the same temperature it is.