2. Character Description
2.1. Character Structure
The data communication is a serial, bit synchronous, character asynchronous format as defined in ISO1177, utilising one start bit, seven data bits, one even parity bit and two stop bits.
 | I.E. Standard UART serial port data 7N2; Gordon |
| There's a drawing of a character to go here, but I'll not bother right now; Gordon |
2.2. Character Set
The character set conforms to the International Alphabet Number 5 (CCITT V3 and ISO646, see Appendix A) referred to in the text as IA5.
| The differences from ASCII, probably the most familaiar character set, are all minor and probably of little consequence. Primarily, a few characters change (tilde becomes overline, dollar becomes a generic currency indicator and the control codes are grouped under TC (Transmission Control?), FE (Feed or Form something?), DC (Data Control? Xon, Xoff etc.) and IS (Item Separator?). |
2.3. Control Characters
2.3.1. Transmission Control Characters
The following control characters have special meaning in the protocol and will be referred to as Transmission Control Characters
- TC1
SOH Start Of Header. Start of a header of a message.
- TC2
STX Start Of Text. Precedes a text field and terminates a header.
- TC3
ETX End Of Text. Terminates a text field
- TC4
EOT End Of Transmission. Terminates a transaction of one or more texts. Returns control to the Control Station
- TC5
ENQ Enquiry. Requests a response from a remote station, may include station identity.
(Probably should include station identity; Gordon)
- TC6
ACK Acknowledge. Transmitted by the receiving station as a positive response to the sender.
- TC7
DLE Data Link Escape. Not used.
- TC8
NAK Negative Acknowledge. Transmitted by the receiving station as a negative response to the sender.
- TC9
SYN Synchronous Idle. Not used.
- TC10
ETB End of Transmission Block. Not Used.
2.3.2. Information Separator Characters
Control Characters used to separate and qualify data logically.
- IS1
US Unit Separator.
- IS2
RS Record Separator.
- IS3
GS Group Separator. Not used.
- IS4
FS File Separator. Not used.
2.4. Transmission Control Prefixes
A number of these characters may be prefixed to control information at various stages during the protocol:
- ENQ
May be prefixed with the device address for polling and selecting.
(Probably should be prefixed as stated; Gordon)
- NAK
May be prefixed to (with; Gordon) an error identifier if known. The presently defined errors are:
- '1'
Transmission error. Corrupt character(s) or corrupt BCC received by the station. (BCC: Block Check Character, ISO1155)
- '2'
Busy. Unable to accept a transaction, e.g., queue full etc.
- '3'
Invalid message. The type or content of the message was not recognised by this station.
ACK and EOT are not prefixed within the protocol.
Note! Character within ' ' means character according to IA5.
| I.E., this is a character 1, 2 or 3, not a binary value 0x01, 0x02 or 0x03; Gordon |