INFICON MDC-260 Thin Film Deposition Controller - Manual (Page 145)

MDC-260 DEPOSITION CONTROLLER
COMPUTER INTERFACE 11-7
11.3 PROTOCOL
Once you have established a connection to the MDC-260, the next step is to
actually exchange data with it. All communications between the host and the
MDC-260 are in the form of message character strings with the following format
(shown in decimal and hexadecimal):
Header Interface
Address
Instruction
Code
Length Message Data
(0 - 249 bytes) Checksum
Dec 255 254 1...32 253 0...249 xx, xx, xx, etc. 0...255
Hex FF FE 01...20 FD 00...F9 xx, xx, xx, etc. 00...FF
• Header (2 bytes): Indicates the beginning of a new message.
• Interface Address (1 byte): Defines which MDC-260 should receive and
respond to the message, or which MDC-260 the message originated from.
The USB Interface Address may range from 0 to 32 (set via Edit System
Setup, Edit Utility Setup, USB Interface Address). An address of zero
will be received by all controllers present on the bus
• Instruction Code (1 byte): Indicates what type of message is being sent
or responded to.
• Message Length (1 byte): Indicates the amount of data being transmitted
in the message. This does not include the header, interface address,
instruction code, the length byte itself or the checksum.
• Message Data (0 – 249 bytes): The data, if any, being sent to or from the
controller. Because of the upper limit of 249 bytes, it is possible that
certain instructions will take multiple messages to complete their task.
• Checksum (1 byte): To ensure the integrity of the data sent, a checksum
is calculated and attached to every message sent to or from the controller.
It only applies to the instruction code byte, message length byte and the
message bytes. 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.
11.4 DATA TYPES
There are three data types stored in the MDC-260: 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 either ASCII characters,
numeric values (0-255), or 8 bit registers. Some of the multiple byte data types