| PCCOM - Solution Provider for RS232/422/485
Serial Communication Multi Port Serial Card Serial - TCP/IP Communication RS232/422/485 Converter and Repeater Data Acquisition and Instrument Control Real - Time Data Collection form any Serial Device |
| EIA-232
Cable Not for High Speed & Long Distance |
| The
conventional EIA-232 serial port is inherently low speed and is
severely limited in distance. Ads often read "high speed" but it can
only work at "high speed" over very short distances such as to a modem
located right next to the computer. Compared to a network card, even
this "high speed" is low speed. All of the EIA-232 serial cable wires
use a common ground return wire so that twisted-pair technology (needed
for high speeds) can't be used without additional hardware. More modern
interfaces for serial ports exist but they are not standard on PC's
like the EIA-232 is. See Successors to EIA-232. Some multiport serial
cards support them. It is somewhat tragic that the RS-232 standard from 1969 did not use twisted pair technology, which could operate about a hundred times faster. Twisted pairs have been used in telephone cables since the late 1800's. In 1888 (over 110 years ago) the "Cable Conference" reported its support of twisted-pair (for telephone systems) and pointed out its advantages. But over 80 years after this approval by the "Cable Conference", RS-232 failed to utilize it. Since RS-232 was originally designed for connecting a terminal to a low speed modem located nearby, the need for high speed and longer distance transmission was apparently not recognized. |
| Inefficient Interface to the Computer |
| To
communicate with the computer, any I/O device needs to have an address
so that the computer can write to it and read from it. For this purpose
many I/O devices (such as serial ports) use a special type of address
known as an I/O addresses (sometimes called an I/O port). It's actually
a range of addresses and the lower address in this range is the base
address. If someone only says (or writes) "address" it likely really
means, "Base address" Instead of using I/O addresses, some I/O devices read and write directly from/to main memory. This provides more bandwidth since the conventional serial I/O system only moves a byte at a time. There are various ways to read/write directly to main memory. One-way is called shared memory I/O (where the shared memory is usually on the same card as the I/O device). Other methods are DMA (direct memory access) on the ISA bus and what is about the same as DMA (only much faster): "bus mastering" on the PCI bus. These methods are a lot faster than those used for the serial port. Thus the conventional serial port with its interrupt driven (every 14 bytes) interface and single bytes transfers on a bus which could accommodate 4 (or 8) bytes at a time is not suited for very high speed I/O. |
| Why Use RS232? |
| RS232 can build in very light CPU and low cost of circuit design for safety communication. |