fix: Fix USB reading request's TransferSize field value.

ScpiNet/ScpiConnectionExtensions.cs

@@ -41,10 +41,13 @@ public static async Task<string> ReadString(this IScpiConnection conn, int speci
 		{
 			ReadResult chunk;
 			StringBuilder response = new();
-			byte[] buffer = new byte[1024];
+
+			// Some devices (such as the Keysight multimeters) do not like when we require reading longer than some specific constraint.
+			// Therefore we will use only 128-bytes long buffer for generic string reads.
+			byte[] buffer = new byte[128];
 
 			do {
-				chunk = await conn.Read(buffer, -1, specialTimeout, cancellationToken);
+				chunk = await conn.Read(buffer, buffer.Length, specialTimeout, cancellationToken);
 				response.Append(Encoding.ASCII.GetString(chunk.Data, 0, chunk.Length));
 			} while (!chunk.Eof && chunk.Length > 0);
 

ScpiNet/UsbScpiConnection.cs

@@ -655,11 +655,12 @@ protected async Task<int> ReadUsb(byte[] buffer, int timeout, CancellationToken
 		/// <param name="msgId">Message ID.</param>
 		/// <param name="eom">True to set EOM flag (end of message).</param>
 		/// <param name="data">Data to add after the header.</param>
+		/// <param name="length">Length of data to be transferred. If zero, the length is inferred from the data array length.</param>
 		/// <returns>Byte array containing the USB TMC request.</returns>
-		private byte[] CreateTmcRequest(UsbTmcMsgId msgId, bool eom, byte[] data)
+		private byte[] CreateTmcRequest(UsbTmcMsgId msgId, bool eom, byte[] data, uint length = 0)
 		{
-			uint len = 0;
-			if (data != null) {
+			uint len = length;
+			if (data != null && length == 0) {
 				len = (uint)data.Length;
 			}
 
@@ -762,7 +763,7 @@ public async Task<ReadResult> Read(byte[] buffer, int readLength = -1, int speci
 			await EnforceCommPause(cancellationToken);
 
 			// Write reading request to the device:
-			await WriteUsb(CreateTmcRequest(UsbTmcMsgId.DevDepMsgIn, false, null), cancellationToken);
+			await WriteUsb(CreateTmcRequest(UsbTmcMsgId.DevDepMsgIn, false, null, (uint)readLength), cancellationToken);
 
 			// Receive the answer:
 			byte[] receptionBuffer = new byte[readLength + headerSize];