Serial communication signals
- RTS = Request To Send
- DTR = Data Terminal Ready
- TD = Transmit Data
- GND = ground
- RD = Receive Data (DTE input)
- DSR = Data Set Ready (DTE input)
- CTS = Clear To Send (DTE input)
- DCD = Data Carrier Detection (DTE input)
- RI = Ring Indicator (DTE input)
RS232 Handshaking
- DTE = Data Terminal Equipment
- usually a PC
- equipment has a male DB9 (DB9M)
- DCE = Data Communications Equipment
- usually a Modem
- equipment has a female DB9 (DB9F)
- Null-modem cable
- connect DTE-to-DTE
- cable has two females DB9
- RTS, DTR = controlled by DTE
- CTS = controlled by DCE
Hardware flow control (RTS/CTS flow control)
pinout
| DeviceA (DB9M) |
Device B (DB9M) |
| TD |
RD |
| RD |
TD |
| CTS |
RTS |
| RTS |
CTS |
| GND |
GND |
- each device use
- RTS (output) to signaling it is ready to accept new data
- CTS (input) to know if it is allowed to send data
- so… a fast device can use its RTS pin to control other things as long as the slow device has it CTS pin tied to asserted logic
Legacy hardware flow control
pinout
| DTE (DB9M) |
DCE (DB9F) |
| TD |
RD |
| RD |
TD |
| CTS |
CTS |
| RTS |
RTS |
| GND |
GND |
- when DTE wants to transmit, it asserts RTS
- only DTE controls RTS signal
- if DCE can accept, it responds asserting CTS
- transmission will occur until DCE deasserts CTS
- deasserting CTS means pause transmission (by DCE)
- reasserting CTS means resume transmission (by DCE)
- when DTE has finished, it will deasserts RTS
REF: https://www.silabs.com/documents/public/application-notes/an0059.0-uart-flow-control.pdf
- [DTE] RTS put in ON, requesting permission to send data
- [DCE] CTS goes into ON, acknowledging the request
- [DTE] respond with DTR line into the ON state
- [DTE] while transmitting, DTR remains ON
- (at EOT) DTR e RTS goes to the OFF state, then DCE pulls CTS line to OFF
Flow Control
Pinout Console/Aux port
| pin |
signal |
direction |
| 1 |
RTS |
output |
| 2 |
DTR |
output |
| 3 |
TD |
output |
| 4 |
GND |
|
| 5 |
GND |
|
| 6 |
RD |
input |
| 7 |
DSR |
input |
| 8 |
CTS |
input |
Rollover Cable
| 8P8C |
8P8C |
| 1 |
8 |
| 2 |
7 |
| 3 |
6 |
| 4 |
5 |
| 5 |
4 |
| 6 |
3 |
| 7 |
2 |
| 8 |
1 |
Adapter jack 8P8C / DB9 female
| 8P8C |
DB9 |
| 1 |
8 |
| 2 |
6 |
| 3 |
2 |
| 4 |
5 |
| 5 |
5 |
| 6 |
3 |
| 7 |
4 |
| 8 |
7 |
Console/Aux port <–> Console device
| signal |
pin |
rollover |
adapter |
signal |
| RTS |
1 |
8 |
8 |
CTS |
| DTR |
2 |
7 |
6 |
DSR |
| TD |
3 |
6 |
2 |
RD |
| GND |
4 |
5 |
5 |
GND |
| GND |
5 |
4 |
5 |
GND |
| RD |
6 |
3 |
3 |
TD |
| DSR |
7 |
2 |
4 |
DTR |
| CTS |
8 |
1 |
7 |
RTS |
References