Page 17 of 28
Gamma Vacuum DIGITEL MPCq Users Manual
Document 900034, Rev B
14. SERIAL COMMUNICATION
Following serial protocols are supported:
Gamma Protocol
Modbus ASCII Protocol
Modbus RTU Protocol
NOTE:
Modbus register map is available on Gamma Vacuum website under download section.
14.1 Gamma Procotol Over Serial Connection
Gamma Protocol Command Packet Structure Over Serial Connection
The command packet is made up of at least five fields. The minimum command packet (single command with no data) is 11 bytes long. No
new commands should be sent to the controller before the response from the previously sent command has been received.
Table 4. Command Packet Structure Over Serial Connection
START
space
ADDRESS
space
COMMAND
space
DATA
space
CHECKSUM
TERMINATOR
1 byte
1 byte
2 bytes
1 byte
2 bytes
1 byte
(variable)
1 byte
2 bytes
1 byte
See table below for more detail on packet structure.
4.1 Field
4.2 Size
4.3 Comment
1. START
1 ASCII character
ASCII character is ‘~’ (TILDA)
Start is the first byte in the command packet and tells remote controllers to start decoding a message.
2. ADDRESS
2 ASCII hex characters
Range 00 through FF
This field should be filled in with the hexadecimal representation of the integer address of the controller. The range provides 255 unique
addresses. Only 32 devices may reside on the same serial port due to hardware loading limitations.
NOTE:
Must be supplied, even when running RS 232.
3. COMMAND CODE
2 ASCII hex characters
Range 00 through FF
See “Table 9. Gamma Protocol Commands” on page 20 for list of available command codes. The command code must be two hex
digits, even if the first is a zero.
4. DATA field(s)
As needed
ACSII printable characters only
Data field(s) are for any commands that have a data value. Not all commands require data field. If command has more than one data value
associated with it, such as setting an X and a Y value, the command field could be followed by two data fields (X and Y) separated by a
comma and space between them. All data must be sent in ASCII printable format (no binary or “control” characters).
5. CHECKSUM
2 ASCII hex characters
Computed checksum of packet
The calculated checksum must have its value in ASCII hexadecimal notation. It is calculated by adding the decimal value of all characters in
the packet (excluding start, checksum, and terminator), and then dividing the result by 256. The integer remainder converted to two ASCII
hex digits is the checksum. When a remote device receives a packet, the passed checksum is compared with a computed checksum and if
they do not match, the device discards the packet.
NOTE:
Checksum field can be bypassed by specifying “00”.
6. TERMINATOR
1 ASCII hex character
ASCII carriage return
ASCII value of carriage return character placed at the end of a command packet. There is not a space between the checksum and
terminator field.
