modbus_proxy

Name

modbus_proxy - forward a request to a backend device and relay the response

Synopsis

int modbus_proxy(modbus_t *frontend_ctx, modbus_t *backend_ctx, const uint8_t *req, int req_length);

Description

The modbus_proxy() function shall forward a Modbus request received on frontend_ctx to a device connected through backend_ctx, and relay the response back to the original requester on frontend_ctx.

This function is intended for building Modbus gateways or proxies that bridge two different backends. A typical use case is a TCP-to-RTU gateway: a client sends a TCP request which is forwarded to an RTU device on a serial bus, and the RTU response is relayed back as a TCP response.

The request req of length req_length should be a raw request as returned by modbus_receive. The slave address is extracted from req and set on backend_ctx before forwarding.

The function handles protocol translation between backends automatically:

the PDU (Protocol Data Unit) is extracted from the frontend framing
it is re-framed for the backend protocol
the backend response PDU is extracted and re-framed for the frontend protocol
the transaction identifier from the original request is preserved

On communication errors, an appropriate Modbus gateway exception response is sent back to the frontend client:

MODBUS_EXCEPTION_GATEWAY_PATH (0x0A) if the request cannot be forwarded or if a non-timeout communication error occurs
MODBUS_EXCEPTION_GATEWAY_TARGET (0x0B) if the backend device does not respond (timeout) or sends an invalid response

Return value

The function shall return the length of the response sent on frontend_ctx if successful. Otherwise it shall return -1 and set errno.

Errors

EINVAL, frontend_ctx or backend_ctx is NULL, or req is NULL, or req_length is less than 1.
EMBBADDATA, the PDU extracted from the request is invalid (too short or too long).

Example

modbus_t *tcp_ctx;
modbus_t *rtu_ctx;
uint8_t query[MODBUS_TCP_MAX_ADU_LENGTH];
int rc;

tcp_ctx = modbus_new_tcp("0.0.0.0", 1502);
rtu_ctx = modbus_new_rtu("/dev/ttyUSB0", 38400, 'E', 8, 1);

int s = modbus_tcp_listen(tcp_ctx, 1);
modbus_connect(rtu_ctx);

for (;;) {
    modbus_tcp_accept(tcp_ctx, &s);

    for (;;) {
        rc = modbus_receive(tcp_ctx, query);
        if (rc < 0)
            break;
        if (rc == 0)
            continue;

        /* Forward the TCP request to the RTU device and relay the response */
        modbus_proxy(tcp_ctx, rtu_ctx, query, rc);
    }

    modbus_close(tcp_ctx);
}

modbus_close(rtu_ctx);
modbus_free(tcp_ctx);
modbus_free(rtu_ctx);