RS232 Character Set
Because of the need for XON/XOFF handshake it is possible to send ASCII coded data only;
binary blocks are not allowed. Bit 7 of ASCII codes is ignored, i.e. assumed to be low. No
distinction is made between upper and lower case characters in command mnemonics and they
may be freely mixed. The ASCII codes below 20H (space) are reserved for addressable RS232
interface control. In this manual 20H, etc. means 20 in hexadecimal
Addressable RS232 (ARC) Interface Control Codes
All instruments intended for use on the ARC bus use the following set of interface control codes.
Codes between 00H and 1FH which are not listed here as having a particular meaning are
reserved for future use and will be ignored. Mixing interface control codes inside instrument
commands is not allowed except as stated below for CR and LF codes and XON and XOFF
codes.
When an instrument is first powered on it will automatically enter the Non- Addressable mode. In
this mode the instrument is not addressable and will not respond to any address commands. This
allows the instrument to function as a normal RS232 controllable device. This mode may be
locked by sending the Lock Non-Addressable mode control code, 04H. The controller and
instrument can now freely use all 8 bit codes and binary blocks but all interface control codes are
ignored. To return to addressable mode the instrument must be powered off.
To enable addressable mode after an instrument has been powered on the Set Addressable
Mode control code, 02H, must be sent. This will then enable all instruments connected to the
ARC bus to respond to all interface control codes. To return to Non-Addressable mode the Lock
Non-Addressable mode control code must be sent which will disable addressable mode until the
instruments are powered off.
Before an instrument is sent a command it must be addressed to listen by sending the Listen
Address control code, 12H, followed by a single character which has the lower 5 bits
corresponding to the unique address of the required instrument, e.g. the codes A-Z or a-z give the
addresses 1-26 inclusive while @ is address 0 and so on. Once addressed to listen the
instrument will read and act upon any commands sent until the listen mode is cancelled.
Because of the asynchronous nature of the interface it is necessary for the controller to be
informed that an instrument has accepted the listen address sequence and is ready to receive
commands. The controller will therefore wait for Acknowledge code, 06H, before sending any
commands, The addressed instrument will provide this Acknowledge. The controller should time-
out and try again if no Acknowledge is received within 5 seconds.
Listen mode will be cancelled by any of the following interface control codes being received:
12H
Listen Address followed by an address not belonging to this instrument.
14H
Talk Address for any instrument.
03H
Universal Unaddress control code.
04H
Lock Non-Addressable mode control code.
18H
Universal Device Clear.
Before a response can be read from an instrument it must be addressed to talk by sending the
Talk Address control code,14H, followed by a single character which has the lower 5 bits
corresponding to the unique address of the required instrument, as for the listen address control
code above. Once addressed to talk the instrument will send the response message it has
available, if any, and then exit the talk addressed state. Only one response message will be sent
each time the instrument is addressed to talk.
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