Re: Modbus asyn issue with RS485 connection (maybe hardware related)

["Heinz Junkes \(FHI\) via Tech-talk" <tech-talk@aps.anl.gov>]

Hi,

as so often when you have written something here ;-)

The hardware board offers two modes via jumpers : RS485-RX and RS485-TX.

In Mode RS485-TX (in our case without jumper)

asynSetOption("DISCM",0,"rs485_enable", "Y”)

works as the sole option.

We had previously set the RS485-RX mode with a jumper and then
SER_RS485_RTS_AFTER_SEND must be set
(and SER_RS485_RTS_ON_SEND must not be set).

Viele Grüße
Heinz


> On 13. Aug 2025, at 16:19, Heinz Junkes (FHI) via Tech-talk <tech-talk@aps.anl.gov> wrote:
>
> Hi,
> we want to operate a motor (https://www.omc-stepperonline.com/de/icl-serie-nema-17-integrierter-rs485-schrittmotor-mit-geschlossenem-regelkreis-0-4nm-56-64oz-in-20-36vdc-mit-14-bit-encoder-icl42-rs06) directly via RS485 (https://jetwaycomputer.com/spec/expansion/ADPIE1COM4.pdf) with modbus.
>
> In order to be able to operate the motor with our hardware setup, we need to set the serial interface to
> SER_RS485_ENABLED and SER_RS485_RTS_AFTER_SEND.
>
> We do this via asynSetOption():
>
> asynSetOption("DISCM",0,"rs485_enable", "Y")
> asynSetOption("DISCM",0,"rs485_rts_after_send", "Y”)
>
> This can be found in the Linux kernel:
>
> +/* We should do proper H/W transceiver setting before change to RS485 mode */
> +static int f81504_rs485_config(struct uart_port *port,
> +       struct serial_rs485 *rs485)
> +{
> + u8 setting;
> + u8 *index = (u8 *)port->private_data;
> + struct pci_dev *pci_dev = container_of(port->dev, struct pci_dev, dev);
> +
> + pci_read_config_byte(pci_dev, UART_START_ADDR + UART_OFFSET * *index +
> + UART_MODE_OFFSET, &setting);
> +
> + if (!rs485)
> + rs485 = &port->rs485;
> + else if (rs485->flags & SER_RS485_ENABLED)
> + memset(rs485->padding, 0, sizeof(rs485->padding));
> + else
> + memset(rs485, 0, sizeof(*rs485));
> +
> + /* F81504/508/512 not support RTS delay before or after send */
> + rs485->flags &= SER_RS485_ENABLED | SER_RS485_RTS_ON_SEND;
> +
> + if (rs485->flags & SER_RS485_ENABLED) {
> + /* Enable RTS H/W control mode */
> + setting |= RTS_CONTROL_BY_HW;
> +
> + if (rs485->flags & SER_RS485_RTS_ON_SEND) {
> + /* RTS driving high on TX */
> + setting &= ~RTS_INVERT;
> + } else {
> + /* RTS driving low on TX */
> + setting |= RTS_INVERT;
> + }
> +
>
> This means that when SER_RS485_ENABLED (0x01) is set, SER_RS485_RTS_ON_SEND (0x02) is also set automatically.
>
> However, in order to be able to communicate with the motor via Modbus (with our hardware), we need the flag 0x05
> (SER_RS485_ENABLED | SER_RS485_RTS_AFTER_SEND) // SER_RS485_RTS_AFTER_SEND == 0x04
>
> We have implemented this as a workaround. If this is of general interest, I will make a pull-request to asyn.
>
> epics@epics-gate14:~/EPICS/epics-support/asyn$ git diff
> diff --git a/asyn/drvAsynSerial/drvAsynSerialPort.c b/asyn/drvAsynSerial/drvAsynSerialPort.c
> index 1ed6ab47..97078a5d 100644
> --- a/asyn/drvAsynSerial/drvAsynSerialPort.c
> +++ b/asyn/drvAsynSerial/drvAsynSerialPort.c
> @@ -529,6 +529,7 @@ setOption(void *drvPvt, asynUser *pasynUser, const char *key, const char *val)
>     else if (epicsStrCaseCmp(key, "rs485_rts_after_send") == 0) {
>         if (epicsStrCaseCmp(val, "Y") == 0) {
>            tty->rs485.flags |= SER_RS485_RTS_AFTER_SEND;
> +           tty->rs485.flags &= ~(SER_RS485_RTS_ON_SEND);
>         }
>         else if (epicsStrCaseCmp(val, "N") == 0) {
>            tty->rs485.flags &= ~(SER_RS485_RTS_AFTER_SEND);
>
>
>
> Heinz
>
> ------------------------------------------------------------------------------
> Fritz-Haber-Institut    | Phone:         (+49 30) 8413-4270
> Heinz Junkes             | Fax (G3+G4):   (+49 30) 8413-5900
> Faradayweg 4-6        | VC: https://zoom.fhi.berlin/junkes
> D - 14195 Berlin        | E-Mail:        junkes@fhi-berlin.mpg.de
> ------------------------------------------------------------------------------
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> All sort of things cropping up at the last moment. Uh, how are we for time?”
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>

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