One of the earliest standards for interfacing digital devices and modems is the EIA RS-232 standard, called "Interface between Data Terminal Equipment and Data Circuit- Terminating Equipment Employing Serial Binary Interface." RS-232C is the latest version (CCITT V.24 is virtually identical). It lists the electrical and mechanical interface characteristics, describes the function of signals, and lists subsets of signals for specific interface types. A computer, printer, etc., is Data Terminal Equipment (DTE), a modem or data set is Data Circuit-Terminating (or Communication) Equipment (DCE). As its name indicates the standard is intended for DTE-DCE connections.
Table 12-1 shows the most commonly used RS-232C signals and their pin numbers on the standard 25-pin D-shell connector. Signal names are given with DTE as reference. A male D-shell connector is used on DTE, a female one on DCE; a straight female-male cable connects DTE to DCE. The standard defines a total of 21 signals, including "secondary" signals and signals that allow data rate selection. Most applications use a subset of these signals; some use a 9-pin D-shell connector instead of the 25-pin connector.
Table 12-2 shows the RS-232C electrical specifications. The voltage levels specified for the RS-232C driver outputs provide zero crossing and better noise immunity than the levels used in standard TTL or MOS technologies but require either power supply voltages (usually +12v/-12v) that are not available, or needed, in the rest of the DTE circuitry, or the use of chips that derive a negative supply voltage on-chip from the standard +5v supply, e.g., the MAXIM MAX232, a 3 driver/3 receiver chip.
Table 12-1. Most Commonly Used RS-232C Signals
| 25-Pin # [9-Pin] | Signal Name | Source |
|---|---|---|
| 1 | Protective (Earth) Ground | |
| 7 [5] | Signal Ground | |
| 2 [3] | Transmitted Data (TxD) | DTE |
| 4 [7] | Request to Send (RTS) | |
| 20 [4] | Data Terminal Ready (DTR) | |
| 3 [2] | Received Data (RxD) | DCE |
| 5 [8] | Clear to Send (CTS) | |
| 6 [6] | Data Set Ready (DSR) | |
| 22 [9] | Ring Indicator (RI) | |
| 8 [1] | Received Line Signal Detect / Carrier Detect (RLSD/CD) |
Table 12-2. RS-232C Electrical Specifications
| Mode of operation | single-ended (unbalanced) |
| Cable length | 50 feet max. |
| Data rate | 20 kb/s max. |
| Driver output | +5v to +15v for "0", -5v to -15v for "1" |
| Voltage applied to driver output | ±25v max. |
| Driver load | 3 kΩ to 7 kΩ |
| Output slew rate | 30 v/µs max. |
| Receiver input range | ±15v |
| Receiver sensitivity | ±3v |
| Receiver input resistance | 3 kΩ to 7 kΩ |
Neither the 50' maximum cable length nor the maximum data rate of the RS-232C standard should be a serious limitation in the DTE-DCE application for which the standard is written. The DTE and the modem are usually located near each other, and reliable communication over the switched telephone network at more than about 2400 bit/sec is very difficult at present. Surprisingly, the RS-232C specifications do not recognize the usefulness of such a standard in applications other than local DTE-DCE connections, nor do they specify any distance vs. rate tradeoffs; in practice, twisted pair cable can be used successfully up to 3000' at rates up to 1200 bits/sec, and up to 250' at up to 9600 bits/sec.
The major difficulty with using RS-232C over long distances is that the Signal Ground is usually connected to earth at both ends so that ground current through the cable causes offsets in the voltages sensed. Newer standards for single-ended systems, e.g. EIA RS-423A, specify a common return path for all signals and ground the return path only at the transmitter; this standard also relates maximum data rate and maximum distance: 100 kbit/sec at 30', 10 kbit/sec at 300', or 1 kbit/sec at 4000'. Even higher data rates over longer distances are possible with systems using differential signal transmission: e.g., EIA RS-422A specifies 10 Mbit/sec at 40', 1 Mbit/sec at 400', or 100 kbit/sec at 4000'.