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Conclusions:

For any practical directly connected RS232 system the combination of the RS232 line driver and the communication line can be approximated as a lumped resistance-capacitance low pass filter. Any transmission line reflected energy effect is negligible. This low pass filter has some bandwith that determines the maximum baud rate. There is nothing that would cause a resonance effect at 9600 baud.

Because the line driver is of a fixed source impedance the bandwidth will be inversely proportional to cable length. Total cable capactance is capactance/foot times cable length. Thus, maximum baud rate is inversely proportional to cable length.

If you stay adequately within the bandwidth limits of the communication link (driver, cable, and receiver), then the only major problems you have are related to noise. This noise can be common mode ground path currents, capacitively and magnetically coupled signals, and radiated energy.

The most likely major source of noise is in the common ground path.


Notes:

The following photographs show the waveforms at the destination (the receiving end) of a CAT-5E cable.

The tests were run with two different cable lengths, 150 feet of an import of unknown manufacturer, and 1000 feet of Belden 1583A. These appear to have slightly different propogation velocities, an observation, but this is of no importance.

Three different drivers were used. These all had a source voltage of a rectangular pulse of approximately +/-12 v open circuit. They differed only in source resistance.

The first source was a Tektronix CFG250 Function Generator with only its internal source resistance of about 25 ohms.

The second source was the same except that a 250 ohm resistor was inserted in series with the output.

The third source was the same generator driving the input of an MC1488 (circa 1978). The 1488 is a standard RS232 driver with a 300 ohm output resistor preceeded by a +/-10 ma current limiter supplied with +/-12 vdc. Thus the output is an approximately constant current source until the 300 ohm resistor becomes dominate. This causes the leading edge to be more linear than for a straight exponential RC curve. The initial current thru a 300 ohm resistor charging a capacitor from a 24 v source would be 80 ma. This is 8 times the current limit and thus the reason for the slow rate of rise when the 1488 is the source. The initial slope of the curve is 1/8 of what it would be if the current limit was not present. Compare photo P7 with P9.

Any of these combinations will probably work at 9600 baud. This does not mean that they should be used. There are no resonance effects except at the leading edge of the pulse. For the 150 foot cable this dies out within 1.6 microseconds. For the 1000 foot cable it dies out in 8 microseconds. After this transient period at the leading edge the signal is steady state.

A typical RS232 receiver has an input impedance of 3000 to 7000 ohms. This would have little effect in comparison with our 10 megohm scope input. Our goal here was to create a worst case condition which is represented by an open circuit transmission line.

The tests were run with unshielded CAT-5E cable because this is low capacitance cable with about a 100 ohm characteristic impedance. The capacitance is about 13.6 pfd/foot. Belden 8723 which contains two twisted pairs each individually shielded is about 66 pfd/foot. Roughly speaking an 8723 cable will fail to work at about 20% of the length of CAT-5E cable with other parameters the same.

Within useable cable lengths there is nothing in the characteristics of normal RS232 drivers, receivers, and cable to cause any resonance at 9600 baud.

Most CNC RS232 communication will be asynchronous. This means that the UART (Universal Asynchronous Receiver Transmitter) has an internal precision clock programmed for the baud rate of the data being transmitted and received. Today virtually all communication within a given path is at the same rate for transmission and reception, and the same number of data and stop bits, and parity in each direction. In the past send and received might have been at at different baud rates. Parity must always match in one direction. Transmit stop bits can any number greater than or equal to the setting for the receiver. There is always exactly one start bit, and at least one stop bit.

Bit, byte, and word need to be defined. A bit is a single binary digit that can have one of two states, 0 or 1. A binary word is a group of binary digits. Some examples are 4 bit, 8 bit, 16 bit, 32 bit, etc. A one bit word can represent 2 items, a 4 bit 16 items, 8 bit 256 items, etc. Generally a 4 bit word is called a nibble, an 8 bit word a byte, and after this usually 16 bit word, 32 word, etc. However, some of these may be referred to as double word. But that useage becomes specific to a particular computer or language. So double word might mean 32 or 64 bits, etc. Sometimes byte is associated with character, and sometimes characters are represented by less than 8 bits. In fact CNC machines are usually communicated to via RS232 with 7 bits per character. 7 bits allows 128 entities.

The UART looks for the leading edge of a start bit. This is identified by a change from binary 1 (the rest state) to binary 0. An internal counter is started and counts to the midpoint of the start bit and checks that it is still binary 0. If so, then the remainder of the incoming data word is processed with testing of each bit position in the middle of its time period. At the end of the word there is a stop bit which is binary 1 and therefore equal to the rest state. Now the UART waits for the next start bit. Thus the UART resynchronizes on each received word. If you have 7 data, 1 parity, 1 start, and 1 stop bits, then the UART word is 10 bits long. What distortion exists on the leading edge of a bit position does not matter so long as it does not continue to the center of the bit position, or cause sampling of bit centers to be incorrectly located. Some peculiar distortion on the leading edge of the start bit might be of concern, but the following waveforms do not illustriate such a problem.

Copyright©   2005, 2006   Gordon A. Roberts    All rights reserved.     060728-1933