RS232 Serial Communications
Glossary of Terms used in RS232 Serial Communications
General Notes on Serial Communications
Serial ports on PC's come in 2 sizes, older PC's had one 25 way and one 9 way, more modern computers tend to have two 9 way ports.
The pin assignments for the two types are detailed here, only the relevant pins are shown.
9 way
| 1 | DCD | Data Carrier Detect | In |
| 2 | RX | Receive | In |
| 3 | TX | Transmit | Out |
| 4 | DTR | Data Terminal Ready | Out |
| 5 | SG | Signal Ground | |
| 6 | DSR | Data Set Ready | In |
| 7 | RTS | Ready To Send | Out |
| 8 | CTS | Clear To Send | In |
25 way
| 1 | FG | Frame Ground | |
| 2 | TX | Transmit | Out |
| 3 | RX | Receive | In |
| 4 | RTS | Ready To Send | Out |
| 5 | CTS | Clear To Send | In |
| 6 | DSR | Data Set Ready | In |
| 7 | SG | Signal Ground | |
| 8 | DCD | Data Carrier Detect | In |
| 20 | DTR | Data Terminal Ready | Out |
Pin assignments on CNC machines vary depending on how the machine builder was feeling on the day, read the appropriate machine documentation to discover their secrets.
This should be connected to the cable shield at one end. (Also see the note on frame ground)
The signal ground is the common between the two devices and must always be connected together.
The transmit and receive lines are the actual data lines, the transmit should be connected to the receive and the receive should be connected to the transmit. (Also see the note on line length)
RTS & CTS are a pair of flow control wires, when the receiving machine wants to stop the flow of data it changes the RTS line, when it is again able to receive data it reverts the RTS line back to its original state.
Sometimes the RTS and CTS lines are connected together to effectively switch off this handshaking.