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RS-232 (Recommended Standard 232)
RS-232 (Recommended Standard 232)
D.E.V.I.C.E. is an encyclopedia of terms used by manufacturers of test and measurement equipment. T&M Atlantic created this service to better explain the functionality of instruments it offers, and to highlight the latest developments in the world of measurement equipment. We are using such tools as animation to bring words and pictures to life and to create not just an understanding but also an appreciation for technology that goes into the design of every instrument.
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RS-232 standard is used to transfer data at a rate of several thousands of bits per second. Bipolar levels from ± 5 to ± 15V are defined by this standard. Besides that this standard describes interface control signals, data transfer and connectors types. There are asynchronous and synchronous modes of exchange provided by the standard but COM-ports support only asynchronous mode. An interface does not provide galvanic isolation of devices and requires a safety ground for connected devices if both of them are supplied by AC network and have network filters. This standard application allows using multi-core cables without any screening as the maximum voltage change rate of formers to minimize crosstalk is intentionally limited to 30 V/ms.
In the standard modem connection cables with DB25P, DB9P plugs and DB25S, DB9S sockets are connected in accordance with the picture below:
If devices are the same (2 computers) there are null-modem cables used because their sockets are the same:
| DB9/DB9 |
|---|
| Receive Data | 2 | 3 | Transmit Data |
| Transmit Data | 3 | 2 | Receive Data |
| Data Terminal Ready | 4 | 6+1 | Data Set Ready + Carrier Detect |
| System Ground | 5 | 5 | System Ground |
| Data Set Ready + Carrier Detect | 6+1 | 4 | Data Terminal Ready |
| Request to Send | 7 | 8 | Clear to Send |
| Clear to Send | 8 | 7 | Request to Send |
| DB25/DB25 |
|---|
| Receive Data | 3 | 2 | Transmit Data |
| Transmit Data | 2 | 3 | Receive Data |
| Data Terminal Ready | 20 | 6+8 | Data Set Ready + Carrier Detect |
| System Ground | 7 | 7 | System Ground |
| Data Set Ready + Carrier Detect | 6+8 | 20 | Data Terminal Ready |
| Request to Send | 4 | 5 | Clear to Send |
| Clear to Send | 5 | 4 | Request to Send |
| DB9/DB25 |
|---|
| Receive Data | 2 | 2 | Transmit Data |
| Transmit Data | 3 | 3 | Receive Data |
| Data Terminal Ready | 4 | 6+8 | Data Set Ready + Carrier Detect |
| System Ground | 5 | 7 | System Ground |
| Data Set Ready + Carrier Detect | 6+1 | 20 | Data Terminal Ready |
| Request to Send | 7 | 5 | Clear to Send |
| Clear to Send | 8 | 4 | Request to Send |
The animated picture below demonstrates a signal waveform in the data transfer loop sending 16 bytes of data with 00h, 11h, 22h,…FFh symbols without P parity bit
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