Specifies the length (in bytes) of the blocks to be sent
 during loopback testing.  The length must be a decimal
 integer in the range of 1 through 4096.  Note that the
 LENGTH parameter must be less than 50 for the DMC-11 circuit
 operating in either controller loopback mode or cable
 loopback mode (a DMC-11 with an attached loopback cable).
 If the parameter is omitted, a block length of 4096 bytes is used.