Chapter 12. RS-232D Serial Interface Standard
The RS-232D Serial Interface Standard added the mechanical characteristics to the RS-232C Standard. The RS-232D Standard defines:
The mechanical characteristics of the interface
The electrical characteristics of the interface
The function of each signal
Subsets of the signals for certain applications
The European version of RS-232D is defined in:
V.24 - mechanical standard
V.28 - electrical standard
Mechanical Characteristics of the RS-232D
The connector is a DB25 connector. DB9 is not universally accepted.
The connector gender is male at the DTE and female at the DCE.
The assignments of signals to pins.
The maximum cable length is 50 ft.
The maximum cable capacitance = 2500 pF. Typical cable has 50 pF/foot capacitance.
Electrical Characteristics of the RS-232D
The transmitter side generates a voltage between +5 and +25 volts for a space (digital 0 or low) and generates a voltage between -5 and -25 volts for a mark (digital 1 or high).
The receiving side recognizes a space as any voltage between +3 and +25V and a mark as any voltage between -3 and -25V. The standard allows for a voltage loss through the cable and noise immunity by reducing the receive requirements to +/-3 volts.
Function of Each Signal
| Pin | Name | Description | EIA Circuit |
| 1 | GND | Chassis Ground | AA |
| 2 | TXD | Transmit Data (TXD) | BA |
| 3 | RXD | Receive Data (RXD) | BB |
| 4 | RTS | Ready to Send | CA |
| 5 | CTS | Clear to Send | CB |
| 6 | DSR | Data Set Ready (DCE Ready) | CC |
| 7 | SGND | Signal ground | AB |
| 8 | DCD | Carrier Detect (CD or RLSD)(RLSD - Received Line Signal Detector) | CF |
| 9 | n/u | ||
| 10 | n/u | ||
| 11 | n/u | ||
| 12 | DCD2 | Secondary Carrier Detect (SRLSD) | SCF |
| 13 | CTS2 | Secondary Clear to Send | SCB |
| 14 | TXD2 | Secondary Transmit Data | SBA |
| 15 | TxSigC | Transmitter Signal Element Timing - DCE | DB |
| 16 | RXD2 | Secondary Receive Data | SBB |
| 17 | RxSig | Receive Signal Element Timing - DCE | DD |
| 18 | LL | Local Loopback | |
| 19 | RTS2 | Secondary Ready to Send | SCA |
| 20 | DTR | Data Terminal Ready (DTE Ready) | CD |
| 21 | SQ/RL | Signal Quality/Remote Loopback | CG |
| 22 | RI | Ring Indicator | CE |
| 23 | DSRS | Data Signal Rate Selector | CH/CI |
| 24 | TxSigT | Transmitter Signal Element Timing - DTE | DA |
| 25 | TM | Test Mode |
The signals in bold/italic are required for a basic asynchronous modem connection.
Subsets of Signals for Certain Applications
Data Signals
Control Signals
| 4 | RTS | Ready to Send | DTE wishes to transmit |
| 5 | CTS | Clear to Send | DCE ready to receive |
| 6 | DSR | Data Set Ready (DCE Ready) | DCE powered on & ready to go |
| 20 | DTR | Data Terminal Ready (DTE Ready) | DTE powered on & ready to go |
| 22 | RI | Ring Indicator | Phones ringing |
Test Modes
| 18 | LL | Local Loopback | Initiate Local Loopback Self-Test |
| 21 | SQ/RL | Signal Quality/Remote Loopback | Initiate Remote Loopback Self-Test |
| 25 | TM | Test Mode | Initiate Test Mode |
Synchronous Control Signals
| 21 | SQ/RL | Signal Quality/Remote Loopback | Error in received data! |
| 23 | DSRS | Data Signal Rate Selector | DTE can dynamically select 1 of 2 data rates |
Synchronous Timing Signals
| 15 | TxSigC | Transmitter Signal Element Timing - DCE | DCE generated |
| 17 | RxSig | Receive Signal Element Timing - DCE | DCE generated |
| 24 | TxSigT | Transmitter Signal Element Timing - DTE | DTE generated |
Ground/Shield
| 1 | GND | Chassis ground | Shield DTE side only for noise protection.Do NOT connect to signal ground! |
| 7 | SGND | Signal ground | Signal return path |
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