# What does S stand for in 75 S ohms?

From the manual for a Repeater unit (page 10 in pdf document).

Transmit LIU Waveshape (Build-out)
S2-4: ON          S2-5: ON        S2-6: OFF      75 S ohms w/ High Return Loss
S2-4: OFF         S2-5: ON        S2-6: OFF      125 S ohms w/ High Return Loss


• That table is oddly similar to the one in Cisco document 4030541_A.pdf which has no S in that position. – Ben Voigt Nov 14 '16 at 6:38
• I think it means someone messed up when typing. One of the "S" from "S2" go in there somehow, and nobody noticed. Oops. – Olin Lathrop Nov 14 '16 at 15:41

I vote for "typo". I also have designed E1 and T1 products, and have never seen an "S" suffix in this context. We do talk about 70 Ohm and 125 Ohm T1/E1 circuits, but there's no reason for the "S" other than someone making a mistake.

• E1 is the European standard for telephony and data of 2.048Mbps which came out after the T1 std. which is 32 time slots of 64kbps
• T1 (or DS1) is the North America standard data rate of 1.544 Mbps which contains 24 channels of 64kbps Data Stream or Telephony has historically been sent over Unshielded Twisted Pairs, UTP or shielded twisted pairs STP with a Manchester BiPhase baseband signal with a center bit clock edge.

There are many variables that affect signal integrity;

• Data edge rise time and over/undershoot
• 1T clock over/undershoot and plateau
• 2T clock over/undershoot and plateau
• clock edge lag/lead skew compensation
• data ISI precompensation and waveshape control
• Impedance attenuation pad for 50, 75, 125 Ohm
• cable impedance DM and CM
• differential mode, DM and single end termination ( CM) at Tx/Rx
• etc

It is somewhere in the last 3 items that this repeater can hard-select these options that depend on the type of cable and distance used.

So the S may refer to the type of transmission line and impedance matching options required. S for shielded or S for Single ended Source termination for each differential line. This requires training and consulting with tech support.

Side Notes

To give you an idea on the critical software controlled attributes of the Line Interface Unit (LIU), it has "Line Buildout" digital options (read ...transmission frequency and delay dependent parameters) with transmit signal control over many parameters of time and amplitude of each pulse. (i.e. it's not like RS485 which just sends out a simple pulse. )

Here are some pulse options by software controlled registers for the DAC output which are done after the attenutator impedance matching options in this question are set.

Each of these are designed to conjugate the impedance vs freq. and time mismatches in the transmission line and can extend the distance at an acceptable bit error rate(BER).

• That's a very long way to say "I don't know". – Olin Lathrop Nov 14 '16 at 15:38
• If you know "anything" thatmay be helpful Olin please stand up and say something. It is a notation exclusive to this brand. Snarky comments do not add any Intel. – Tony Stewart Sunnyskyguy EE75 Nov 14 '16 at 17:09
• At least I admit I don't know but I have designed and used T1 BER testers. It is related to proprietary internal design options for each impedance attenuation pads matching to specific transmission lines. That is about all I can say for sure, but there is obviously much more impact than classical lab circuits. – Tony Stewart Sunnyskyguy EE75 Nov 14 '16 at 17:17
• Clearly "S ohms" are Super Ohms--you know--to help compensate in those "High Return Loss" conditions... ;) – DIYser Feb 3 '17 at 2:08
• or Secret Splitter – Tony Stewart Sunnyskyguy EE75 Feb 3 '17 at 2:36