Modbus asyn issue with RS485 connection (maybe hardware related)

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

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

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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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