Ametek 7280 Instruction Manual page 142

Chapter 6, COMPUTER OPERATION
6-8
command and a slow computer were being used then the instrument may actually
clear and then reset bit 0 (i.e. actually complete the command) before the first
serial poll operation were executed. Hence the loop must include the provision to
timeout under these conditions; a value of 10 ms should be satisfactory in most
cases.
3) If bit 0 clears, perform repeated serial polls testing both bit 0 and bit 7 (data
available) and, if bit 7 is asserted then perform a read operation. This cycle (i.e.
test bit 0 (command complete) and test bit 7 (data available)) should then
continue until the lock-in amplifier asserts bit 0 to indicate that the command-
response sequence is complete, so that the instrument will then be ready for the
next command.
This procedure, although apparently complex, deals successfully with compound
commands and responses to the curve transfer commands.
In RS232 communications, comparatively rapid access to the status byte is provided
by the prompt character which is sent by the lock-in amplifier at the same time as bit
0 becomes asserted in the status byte. This character is sent out by the lock-in
amplifier after each command response (whether or not the response includes a
transmission over the interface) to indicate that the response is finished and the
instrument is ready for a new command. The prompt takes one of two forms. If the
command contained an error, either in syntax or by a command parameter being out
of range, or alternatively if an overload or reference unlock is currently being
reported by the front panel indicators, the prompt is ? (ASCII 63). Otherwise the
prompt is * (ASCII 42).
These error conditions correspond to the assertion of bits 1, 2, 3 or 4 in the status
byte. When the ? prompt is received by the computer, the ST command may be
issued in order to discover which type of fault exists and to take appropriate action.
The prompts are a rapid way of checking on the instrument status and enable a
convenient keyboard control system to be set up simply by attaching a standard
terminal, or a simple computer-based terminal emulator, to the RS232 port. Where
the prompt is not required it can be suppressed by setting the noprompt bit, bit 4 in
the RS232 parameter byte. The default (power-up) state of this bit is zero.
Because of the limited number of bits in the status byte, it can indicate that an
overload exists but cannot give more detail. An auxiliary byte, the overload byte
returned by the N command, gives details of the location of the overload.
A summary of the bit assignments in the status byte and the overload byte is given
below.