Cable

It is widely and incorrectly called a "straight through" (which it most definitely is NOT). So I don't use those labels, since the internet mostly seems to be getting them backwards. Instead, I always say to hold up the two connectors side-by-side, and ensure the wire colour order is the same on each, left to right.

It's called a "/R", because if you lay the cable down flat, one connector/tab is on the Reverse side of the cable compared to the other. This has the effect of swapping the wire order from end to end (twisted, not straight through!!), which is exactly what is needed here. The larger and better-known manufacturers of such cables label them as "/R", as does the Telecom industry which invented them.

Do NOT use the device with a 4-wire "/R" ("RJ11") cable, even if it appears to be working. It is missing the "BUSY" line that mitigates bus collisions, and could result in permanent electrical failures. The HBG3 is safe in this regard, but other Celestron gear is not.

Advised here

A 6P6C AUX cable with RJ12 plugs wired as /R ("reverse"). Best to buy crimpers and connectors and make your own. Commonly available ones tend to be VERY LONG. Eg. https://www.digikey.ca/en/products/detail/assmann-wsw-components/AT-S-26-6-6-B-7-R/1972588