66 Blocks are the classic old-school telecommunications wiring punch-down block.
These blocks have been and are used for:
- POTS (Plain Old Telephone Service) Lines
- Analog and digital DS0 data circuits. (2400 bps – 64Kbps)
- T1 lines. (4 wires, 24 channels [USA], 1.544 Mbps digital)
- Low-voltage alarm wiring for equipment alarms, door and window sensors, fire alarms, HVAC unit alarms, Emergency generator alarms, etc.
- Traditional PBX business office phone systems.
- CCTV Camera power leads.
Odds are high that if you run into an active 66-block in the field, it will be used for low-voltage alarm wiring. However, you can never assume that it’s not legacy services still running some of the above-mentioned uses. Railroads, for instance, still use analog DS0 circuits to control track switches and crossing signals. You do NOT want to be the technician who breaks one of those!
What follows are graphics and explanations of various facts regarding 66-blocks.
66-Block with wiring punched down. Left pin pair are electrically connected, right pair are, but left is insulated from right.
Pre-wired 25-pair 66-Block with standard telecommunications color code. Pre-wiring goes to the female Amphenol connector on the side of the block. (Shown with dust cap)
Female Amphenol connector
Male Amphenol connector.
Note the bridge clips being used to connect the left pin pair to the right. These are used to easily isolate one side of the wiring from the other, and just as quickly restore the connection. This is an extremely useful feature in troubleshooting wiring and equipment faults.
Punching down wiring on a 66-block. The underside of the tool cuts excess wire away to prevent shorts.
Watch for 66-Blocks with orange dust covers. ESPECIALLY if they have “RJ-21X” on the cover. An RJ-21X jack is a telecommunications provider demarcation point. The telecommunications provider owns one side of the block (Usually the side with the Amphenol pre-wire), and the customer may punch their equipment wiring down on the other side. This allows the telecom provider to isolate their service lines from the customer equipment by simply removing the bridge clips for a particular circuit during troubleshooting. The telecomm provider’s responsibility ends at their side of the RJ-21X.
Creating circuit loop-backs or extensions with 66-blocks. There are 66-block punch-down buts for punch-down tools that DO NOT have a cutting side. They can punch the wires down intact without cutting them. In the above picture, all the left-side pins of the same color are electrically connected because the wire is punched down, but not cut.
Standard 25-pair color code:
Pair colors: Blue, Orange, Green, Brown, Slate (Gray).
Tracer colors: White, Red, Black, Yellow, Violet.
More Color Code Charts:
Wikipedia on Telecomm wiring color codes:
Can you spot the Telco demarc in this picture?
Nice spaghetti bowl there…
66-Blocks offer a lot more wiring flexibility than 110-block RJ45 patch panels, but are rarely used for Ethernet. There’s too much chance for electrical or inductive interference with the sensitive data signal at Ethernet speeds. However if you see these blocks in the field, you’ll now have some idea what you may be looking at.
Thank you for reading through!