[pyr.syseng.comm] RS-232 vs. V.35

csg@pyramid.pyramid.com (Carl S. Gutekunst) (03/09/89)

Followups to comp.dcom.modems ONLY.

In article <1766@umbc3.UMBC.EDU> smith@umbc3.UMBC.EDU (Fred Smith) writes:
>I realize that the effective "maximum" distance recommended for RS-232
>under proper conditions, is 50 ft....

What RS-232-C actually says, is:

	Section 1.3
		This standard is applicable for use at data signalling
		rates in the range from zero to a nominal 20,000 bits
		per second.

	Section 3.1
		The use of short cables (each less than approximately
		50 feet or 15 meters) is recommended; however, longer
		cables are permissible, provided that the resulting
		load capacitance, measured at the interface point and
		including the signal terminator, does not exceed 2500

Typically you'll pick up anywhere from 300 to 600 picofarads (pf) before you
ever plug in the cable. Cable capacitance is in the neighborhood of 25pf per
foot, so that means something like 80 feet before you are exceeding the
RS-232-C standard. Not that it won't *work*; you'll just be outside what the
standard requires, and into the domain of implementation. You can also spend
more money, and buy cable with a lower capacitance. 

>I would like to know is if there exists an effective "maximum" or standard
>distance for running CCITT V.35?

In summary, it is exactly the same: 2500 picofarads. The "50 foot" suggestion
isn't made at all; just the limit due to capacitance.

Here's the long story:

V.35 is a mess. What Recommendation V.35 specifies is an encoding scheme at
48Kbps for leased lines. What has popularly become referred to as "V.35" is
just the "Interface" portions of V.35, contained in Section 10 and Appendix
II of the Recommendation. This in turn falls back to Recommendation V.28 for
everything that isn't specified in V.35. V.28 is only an electrical specifi-
cation; if you actually want to know what the circuits *do* you have to look
at V.24. And the big M34 connector typically used by V.35 interfaces isn't a
CCITT Recommendation at all. (Or at least, I've never found it.)

The V.35 Interface sections define a low-impedance balanced circuit, kinda
but not quite what us old Teletype hands called current loop. :-) The maxi-
mum length is "for further study," although clearly you could go hundreds
of feet without exceeding the standard's impedance requirements. (BTW, the
phrase "for further study" is a magic cookie that means the committee was
unable to agree on something, and let the issue drop for another four

Still with me?

The problem is that V.35 specifies *only* the Transmit Data, Receive Data,
and two clock circuits. The remaining circuits -- carrier detect, terminal
ready, request to send, clear to send, and so on, are covered by V.28.

And guess what V.28 is? Yup. The electrical portion of RS-232-C, hidden under
a CCITT label. 2500pf is all you get.