동적배열의 문제 같거든요
GetBlock의 파라메터가 어떤식인지 몰라서 제가 정확히 답변을 못드리지만

var BtDatas : array[0..1024] of byte;
begin
  asPort.GetBlock(btDatas, Count);
end;

이런식으로 바꾸어 보세요 아니면
var btDatas : array of byte;
begin
  SetLength(btDatas,Count);
  asPort.GetBlock(@btDatas[0], Count);
  // GetBlock의 파라메터를 몰라서.. 또는
  asPort.GetBlock(btDatas[0], Count);

end;

도움말에 나와있는 GetBlock의 함수설명입니다.
지금 보니깐 Char배열이네요..쩝.. 근데 Char로 해도 안되네요..ㅋㅋ

procedure GetBlock(var Block; Len : Word);

Returns a block of received characters and removes them from the dispatcher buffer.

This routine makes a request to return the next Len received bytes. The data is moved into the buffer referenced by Block. If Block is not large enough to hold Len bytes this will result in a memory overwrite. If fewer than Len bytes are available, none are returned and an EBufferIsEmpty exception is generated. The returned bytes are removed from the Async Professional dispatcher buffer.
To determine if line errors occurred while the communications driver was receiving this data, check the LineError property after calling GetBlock.

The following example calls GetBlock to remove the next 128 bytes from the dispatcher buffer, and handles the various possible outcomes:


var
  Block : array[0..127] of Char;
...
try
  ApdComPort.GetBlock(Block, 128);
except
  on E : EAPDException do
    if E is EBufferIsEmpty then begin
      ...protocol error, 128 bytes expected
      raise;
    end;

end;

See also: CharReady, GetChar, InBuffUsed

var btDatas : array of byte;
begin
  SetLength(btDatas,Count);
  asPort.GetBlock(btDatas[0], Count);
end;
이런식으로 해보세요
될거에요
^^