1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
|
# modbus_rtu_set_rts
## Name
modbus_rtu_set_rts - set the RTS mode in RTU
## Synopsis
```c
int modbus_rtu_set_rts(modbus_t *ctx, int mode)
```
## Description
The *modbus_rtu_set_rts()* function shall set the Request To Send mode to
communicate on a RS-485 serial bus. By default, the mode is set to
`MODBUS_RTU_RTS_NONE` and no signal is issued before writing data on the wire.
To enable the RTS mode, the values `MODBUS_RTU_RTS_UP` or `MODBUS_RTU_RTS_DOWN`
must be used, these modes enable the RTS mode and set the polarity at the same
time. When `MODBUS_RTU_RTS_UP` is used, an ioctl call is made with RTS flag
enabled then data is written on the bus after a delay of 1 ms, then another
ioctl call is made with the RTS flag disabled and again a delay of 1 ms occurs.
The `MODBUS_RTU_RTS_DOWN` mode applies the same procedure but with an inverted
RTS flag.
This function can only be used with a context using a RTU backend.
## Return value
The function shall return 0 if successful. Otherwise it shall return -1 and set
errno to one of the values defined below.
## Errors
- *EINVAL*, the libmodbus backend isn't RTU or the mode given in argument is invalid.
## Example
Enable the RTS mode with positive polarity:
```c
modbus_t *ctx;
uint16_t tab_reg[10];
ctx = modbus_new_rtu("/dev/ttyS0", 115200, 'N', 8, 1);
modbus_set_slave(ctx, 1);
modbus_rtu_set_serial_mode(ctx, MODBUS_RTU_RS485);
modbus_rtu_set_rts(ctx, MODBUS_RTU_RTS_UP);
if (modbus_connect(ctx) == -1) {
fprintf(stderr, "Connection failed: %s\n", modbus_strerror(errno));
modbus_free(ctx);
return -1;
}
rc = modbus_read_registers(ctx, 0, 7, tab_reg);
if (rc == -1) {
fprintf(stderr, "%s\n", modbus_strerror(errno));
return -1;
}
modbus_close(ctx);
modbus_free(ctx);
```
## See also
- [modbus_rtu_get_rts](modbus_rtu_get_rts.md)
|
