TOPIC -------------------------------------------------------
Is there a way to figure out the maximum length of RS-232 cables?
DISCUSSION --------------------------------------------------
The maximum length of RS-232 cables is related to the data rate. Thus, you
can calculate maximum cable length based on the data rate. The table below
provides the maximum length of RS-232 cables (average cable and special
cable) for the various data rates.
Data rate Maximum acceptable Maximum acceptable
(baud) Feet of average cable Feet of special cable
--------- ---------------------- ---------------------
19200 50 83
9600 100 163
4800 200 326
2400 400 652
1200 800 1304
600 1600 2608
300 3200 5216
Average cable is defined by its capacitance, 50 picofarads (pf) per foot.
Special cable is defined by a lower capacitance, 30 pf/foot, which makes
special cable more costly.
The top line of the table is defined by interface standard RS-232C. We have
made the remaining lines stay within the standard by taking the next
highest line and halving the baud rate to double the length.
How the Top Line Was Determined
-------------------------------
The RS-232C interface standard was set by and is available from the
Electronic Industries Association (EIA), Engineering Department, 2001 Eye
St., Washington, D.C., 20006. Several data reference manuals and booklets
also contain and explain the standard. Apple uses the EIA RS-232C interface
standard for all of its serial printer and communications interface
equipments (excluding any parallel interface or item on AppleTalk).
We have used this standard to determine maximum cable length by referring
to the specified amount of integrity of the transmission of data through
the cable. This amount of integrity is specified in terms of maximum signal
distortion at the maximum cable capacitance and at the maximum data rate.
For cables, the standard specifies the following maximum limits:
* Time Data Signal Distortion: 1 millisecond or 4 percent, whichever is
greater.
* Cable Capacitance: 2500 picofarads (pf)
* Data Rate: 20,000 bits/sec.
Because the highest supported data rate is 20Khz (only the Macintosh can go
as fast as this), there is some "fudging" allowed for cable distance. As no
maximum distance between units is called out, all the cable has to do is
provide a connection that meets the distortion and capacitance limits while
supporting the data rate.
Most data cable has an average capacitance of 50 picofarads per foot. 50
feet of such cable would have a capacitance of 2500 picofarads, the maximum
allowable capacitance.
Special, more costly, low-capacitance cable with less than 30 pf/foot would
have a capacitance of 2500 pf or less in a length of around 83 feet.
Depending on the actual capacitance, the low capacitance cable could be a
lot longer, as long as its total capacitance stayed under 2500 pf.