INFICON RQCM - Quartz Crystal Microbalance Research System - Manual (Page 80)

RQCM – RESEARCH QUARTZ CRYSTAL MICROBALANCE
COMPUTER INTERFACE 7-4
Table 7-2 IEEE-488 Pin Assignments
7.8 PROTOCOL
All communications between the computer and the RQCM are in the form of messages with the
format:
* Two byte header - (FFh, FEh i.e. Chr$(255), Chr$(254)) The header indicates the beginning of
a message.
*One byte device address - (1 to 32) The device address byte defines the bus address of the
instrument that sent or should receive the message. The device address will range from 1 to 32.
A message sent to a device address of zero will be received by all RQCMs except in the case of
the IEEE-488 interface. With this interface, only the addressed device will receive the message.
*One byte instruction code - (0 to 6) Defines the code number of the message.
*One byte message length - (0 to 249) Indicates the number of data bytes contained in the
message.
* One byte checksum - (0 to 255) The checksum byte is used for transmission error detection. If
the RQCM receives a message with an incorrect checksum, it will disregard the message. The
checksum is the compliment of the one-byte sum of all bytes from, and including, the instruction
code to the end of the message. If the one-byte sum of all these bytes is added to the checksum,
the result should equal 255.
If the sum of all bytes occupies more than one byte, a single byte checksum can be generated
using the expression: checksum=! (Sum MOD 256), i.e. the checksum is the complement of the
remainder byte, which results from dividing the sum of all bytes by 256.
7.9 DATA TYPES
There are three data types stored in the RQCM: One byte, two byte, and three byte parameters.
All data types are stored as integers in binary format with the most significant byte first. The one-
byte data types are ASCII characters, numeric values (0-255), or 8 bit registers. Some of the
multiple byte data types are decimal values stored as integers. To convert these values to their
decimal equivalent, use the following equation:
Decimal Value = (Integer Value)/(10*DP)
Where:
DP
= the value’s decimal point position.
The decimal point positions for all the parameters are constant and are given in tables along with
the parameters’ range.
7.10 MESSAGE RECEIVED STATUS
Following the receipt of each message, the RQCM will send a one-byte ‘received status’
message, indicating how the message was received, with the following format:
Header
Address
Inst=253