RS232 Wiring Stuff
RS232 standards are defined by EIA/TIA (Electronic Industries Alliance /Telecommunications Industry Association). RS232 defines both the physical and electrical characteristics of the interface. RS232 is practically identical to ITU V.24 (signal description and names) and V.28 (electrical). RS232 is an Active LOW voltage driven interface and operates at +12V to -12V where:
Signal = 0 (LOW) > +3.0V
Signal = 1 (HIGH) < -3.0V
Signal voltages in the range >-3.0V to +3.0V is regarded as the 'dead area' and allows for absorption of noise.
Watch the terms DTE (Data Terminal Equipment - e.g. a terminal or PC) and DCE (Data communications Equipment - e.g. a modem) the meaning and use of certain pins may differ. All the diagrams below define the interface from the DTE perspective. An RS232 interface has a female and male connector, the male connector has got the bits sticking out!!
NOTE: The terms Data Carrier Detect (DCD) and Received Line Signal Detect (RLSD) are one and the same. We use DCD throughout 'cos we think its more common.
Signal/pin primer
| Pin No. | Name | Notes/Description |
| 1 | - | Protective/shielded ground |
| 2 | TD | Transmit Data (a.k.a TxD, Tx) |
| 3 | RD | Receive Data (a.k.a RxD, Rx) |
| 4 | RTS | Request To Send |
| 5 | CTS | Clear To Send |
| 6 | DSR | Data Set Ready |
| 7 | SGND | Signal Ground |
| 8 | CD | Carrier Detect (a.k.a DCD) |
| 9 | - | Reserved for data set testing |
| 10 | - | Reserved for data set testing |
| 11 | - | Unassigned |
| 12 | SDCD | Secondary Carrier Detect |
| 13 | SCTS | Secondary Clear to send |
| 14 | STD | Secondary Transmit Data |
| 15 | DB | Transmit Clock (a.k.a TCLK, TxCLK) |
| 16 | SRD | Secondary Receive Data |
| 17 | DD | Receive Clock (a.k.a. RCLK) |
| 18 | LL | Local Loopback |
| 19 | SRTS | Secondary Request to Send |
| 20 | DTR | Data Terminal Ready |
| 21 | RL/SQ | Signal Quality Detector/Remote loopback |
| 22 | RI | Ring Indicator (DCE raises when incoming call detected used for auto answer applications) |
| 23 | CH/CI | Signal Rate selector |
| 24 | DA | Auxiliary Clock (a.k.a. ACLK) |
| 25 | - | Unassigned |
Signal/pin primer
| Pin No. | Name | Notes/Description |
| 1 | DCD | Data Carrier Detect |
| 2 | RD | Receive Data (a.k.a RxD, Rx) |
| 3 | TD | Transmit Data (a.k.a TxD, Tx) |
| 4 | DTR | Data Terminal Ready |
| 5 | SGND | Ground |
| 6 | DSR | Data Set Ready |
| 7 | RTS | Request To Send |
| 8 | CTS | Clear To Send |
| 9 | RI | Ring Indicator |
More properly EIA/TIA - 561. Use when connecting to or from a serial port with a 8 position Modular Jack (RJ45). If you are cross-connecting from a DB9 or a DB25 use the signal names to cross connect the appropriate connections.
Signal/pin primer
| Pin No. | Name | Notes/Description |
| 1 | DSR/RI | Data set Ready/ring indicator |
| 2 | DCD | Data Carrier Detect |
| 3 | DTR | Data Terminal Ready |
| 4 | SGND | Signal Ground |
| 5 | RD | Receive Data |
| 6 | TD | Transmit Data |
| 7 | CTS | Clear to Send |
| 8 | RTS | Request to Send |
Use when connecting two systems (e.g. PCs) via their DB25 interfaces without a modem (i.e. back-to-back). See the full signal names in the DB25 sections.
Signal/pin primer
| DB25 | Signal | DB25 | Signal |
| 3 | RD | 2 | TD |
| 2 | TD | 3 | RD |
| 20 | DTR | 6,8 | DSR, DCD |
| 6,8 | DSR, DCD | 20 | DTR |
| 4 | RTS | 5 | CTS |
| 5 | CTS | 4 | RTS< |
| 7 | SGND | 7 | SGND |
| 22 | RI | 22 | RI |
NOTE: Leave all pins not specified above unconnected.
Use when connecting two systems (e.g. PCs) via their DB9 interfaces without a modem (i.e. back-to-back). See the full signal names in the DB9 section.
Signal/pin primer
| DB9 | Signal | DB9 | Signal |
| 2 | RD | 3 | TD |
| 3 | TD | 2 | RD |
| 4 | DTR | 6,1 | DSR, DCD |
| 6,1 | DSR, DCD | 4 | DTR |
| 7 | RTS | 8 | CTS |
| 8 | CTS | 7 | RTS |
| 5 | SGND | 5 | SGND |
| 9 | RI | 9 | RI |
NOTE: Leave all pins not specified above unconnected.
Use when connecting a DB9 (e.g. a PC) to a DB25 (e.g. a modem) interface. See the full signal names in the DB9 and DB25 section.
Signal/pin primer
| DB9 | Signal | DB25 |
| 1 | DCD | 8 |
| 2 | RD | 3 |
| 3 | TD | 2 |
| 4 | DTR | 20 |
| 5 | SGND | 7 |
| 6 | DSR | 6 |
| 7 | RTS | 4 |
| 8 | CTS | 5 |
| 9 | RI | 22 |
RS232 DB9 to DB25 NULL Modem Pinout
Use when connecting two systems (e.g. PCs) when one has a DB9 interface and the other a DB25 interface without a modem (i.e. back-to-back). See the full signal names in the DB9 and DB25 sections.
Signal/pin primer
| DB9 | Signal | DB25 | Signal |
| 2 | RD | 2 | TD |
| 3 | TD | 3 | RD |
| 4 | DTR | 6,8 | DSR, DCD |
| 6,1 | DSR, DCD | 20 | DTR |
| 7 | RTS | 5 | CTS |
| 8 | CTS | 4 | RTS |
| 5 | SGND | 7 | SGND |
| 9 | RI | 22 | RI |
The A and B below refer to each signal pair used in balanced serial interfaces. BEWARE: RS-530 is wired differently. This is the 530-A pinout spec.
Signal/pin primer
| Pin No. | Name | Notes/Description |
| 1 | Shield | - |
| 2 | Transmit Data (A) | - |
| 3 | Received Data (A) | - |
| 4 | RTS (A) | Request To Send |
| 5 | CTS (A) | Clear To Send |
| 6 | DCE Ready | Data Communications Equipment Ready (modem/CSU) |
| 7 | Signal Common | - |
| 8 | DCD (A) | Data Carrier Detect (a.k.a CD or RLSD) |
| 9 | Receiver Signal Element Timing (B) | RX Clock |
| 10 | DCD (B) | Data Carrier Detect (a.k.a CD or RLSD) |
| 11 | Ext Transmit Clock (B) | - |
| 12 | Tramsmit Signal Element Timing (B) | TX CLOCK |
| 13 | CTS (B) | Clear to Send |
| 14 | Transmitted Data (B) | Secondary Transmit Data |
| 15 | Transmit Signal element Timing (A) | TX CLOCK |
| 16 | Received Data (B) | RX Data |
| 17 | Receiver Signal Element Timing (A) | RX CLOCK |
| 18 | Local Loopback | - |
| 19 | RTS (B) | Request to Send |
| 20 | DTE Ready | Host end |
| 21 | Remote Loopback | - |
| 22 | RI | Ring Indicator |
| 23 | Signal Common | - |
| 24 | Ext TX Clock (A) | - |
| 25 | Test Mode | - |
No one remembers Centronics (well I do) but their connector is still used to this day (is this a legacy or what!).
An exotic 36 pin connector invented (I guess) by the old Centronics company who made printers (nah!!).
| Connector | Name | Function/Notes | Connector | Name | Function/Notes |
| 1 | Data Strobe | Active LOW | 19 | Data Strobe | RETURN |
| 2 | Data Bit 1 | LSB | 20 | Data Bit 1 | RETURN |
| 3 | Data Bit 2 | - | 21 | Data Bit 2 | RETURN |
| 4 | Data Bit 3 | - | 22 | Data Bit 3 | RETURN |
| 5 | Data Bit 4 | - | 23 | Data Bit 4 | RETURN |
| 6 | Data Bit 5 | - | 24 | Data Bit 5 | RETURN |
| 7 | Data Bit 6 | - | 25 | Data Bit 6 | RETURN |
| 8 | Data Bit 7 | - | 26 | Data Bit 7 | RETURN |
| 9 | Data Bit 8 | MSB | 27 | Data Bit 8 | RETURN |
| 10 | Acknowledge | Active LOW | 28 | Acknowledge | RETURN |
| 11 | Busy | Active HIGH | 29 | Busy | RETURN |
| 12 | Paper End | Active HIGH | 30 | Input Prime | RETURN |
| 13 | Select | Active HIGH | 31 | Input Prime | Active LOW |
| 14 | Supply Ground | - | 32 | Fault | Active LOW |
| 15 | Oscillator Tx | - | 33 | Undefined | - |
| 16 | Logic Ground | - | 34 | Undefined | - |
| 17 | Chassis Ground | - | 35 | Undefined | - |
| 18 | +5V | Power | 36 | Undefined | - |