Sending a simple USB BulkTransfer and receiving a response

Question

I am trying to get a weight value from a set of weighing scales over USB. It should be quite simple, according to their doc, I need to send two bytes the letter "W" and the carriage return byte. It then responds with 16 bytes of data representing the current weight on the device.

The device has 1 interface, 2 endpoints with a max packet size of 64. I believe I must use the bulkTransfer function as the endpoint types are USB_ENDPOINT_XFER_BULK.

Here is the doc graphic:

How exactly should I send this request and receive the response? My attempt is below, and the response is simply a Start of Heading symbol, then a back quote symbol "`" and a load of zeroes. I have tried to run the code on a polling loop or just a single request but get the same result.

    val connection = usbManager.openDevice(scales)
    val intf: UsbInterface = scales.getInterface(0)
    connection.claimInterface(intf, true)

    val endpointReadIn = intf.getEndpoint(0)
    val endpointWriteOut = intf.getEndpoint(1)

    val bytes = byteArrayOf(0x57.toByte(), 0x0D.toByte())

    thread {
        val request = connection.bulkTransfer(endpointWriteOut, bytes, bytes.size, 0)
        Log.d(TAG, "Was request to write successful? $request")
        val buffer = ByteArray(16)
        val response = connection.bulkTransfer(endpointReadIn, buffer, buffer.size, 0)
        Log.d(TAG, "Was response from read successful? $response")
        val responseString = StringBuilder()
        for (i in 0..15) {
            responseString.append(buffer[i])
        }
        Log.d(TAG, "Response: $responseString")

        val hex = toHexString(buffer)
        Log.d(TAG, "Hex: $hex")

        connection.close()
    }


    fun fromHexString(hexString: String): ByteArray {
        val len = hexString.length / 2
        val bytes = ByteArray(len)
        for (i in 0 until len) bytes[i] = hexString.substring(2 * i, 2 * i + 2).toInt(16).toByte()
        return bytes
    }

Output:

Was request to write successful? 2
Was response from read successful? 2
Response: 19600000000000000
Hex:  01 60 00 00 00 00 00 00 00 00 00 00 00 00 00 00

score 1 · Answer 1 · answered Mar 06 '20 at 13:41

There was a couple of missing pieces here. I think the big one was not specifying the baud rate, data bit, stop bit and parity which are all achieved via the controlTransfer function.

In the end I couldn't get it to work myself despite getting successful responses while setting these. Then I found this lovely library which is compatible with this RS232 device and it works well. I just need to specify the vid / pid to get a custom driver using the FtdiSerialDriver class.

Sending a simple USB BulkTransfer and receiving a response

1 Answers1