NVT (Network Virtual Terminal) descriptionCommunication with the TCP/IP device over the Ethernet network can be extended to more functions using NVT (Network Virtual Terminal) commands. It can be used according to the RFC2217 standard to change baudrate on remote Virtual Serial Port for example. The simple NVT control commands are included in the data stream with the character "FF" used as the command prefix. If the "FF" character occurs within the normal data stream, it is simply doubled.
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| Dec | HEX | Shortcut | Description |
| 240 | F0 | SE | End of sub negotiation parameters |
| 241 | F1 | NOP | No Operation |
| 246 | F6 | AYT | Are You There |
| 250 | FA | SB | Indicates that what follows is sub negotiation of the indicated option. |
| 255 | FF | IAC | Data Byte 255 |
Com Port Control Client to Access Server constants
CAS_SIGNATURE, 0
CAS_SET_BAUDRATE, 1
CAS_SET_DATASIZE, 2
CAS_SET_PARITY, 3
CAS_SET_STOPSIZE, 4
CAS_SET_CONTROL, 5
CAS_NOTIFY_LINESTATE, 6
CAS_NOTIFY_MODEMSTATE, 7
CAS_FLOWCONTROL_SUSPEND,8
CAS_FLOWCONTROL_RESUME, 9
CAS_SET_LINESTATE_MASK, 10
CAS_SET_MODEMSTATE_MASK,11
CAS_PURGE_DATA, 12
CAS_OPT_GPIO, 50
CAS_SET_GPIO, 51
Com Port Control Access Server to Client constants
ASC_SIGNATURE, 100
ASC_SET_BAUDRATE, 101
ASC_SET_DATASIZE, 102
ASC_SET_PARITY, 103
ASC_SET_STOPSIZE, 104
ASC_SET_CONTROL, 105
ASC_NOTIFY_LINESTATE, 106
ASC_NOTIFY_MODEMSTATE, 107
ASC_FLOWCONTROL_SUSPEND,108
ASC_FLOWCONTROL_RESUME, 109
ASC_SET_LINESTATE_MASK, 110
ASC_SET_MODEMSTATE_MASK,111
ASC_PURGE_DATA, 112
ASC_OPT_GPIO, 150
ASC_SET_GPIO, 151
Behind the sequence IAC SB, there might also be the enlargement COM-PORT-OPTION command (the command is ended by the IAC SE sequence of course) in RFC2217 standard. We are only describing some of the sub-commands. The whole description is in the RFC2217 standard.
The values up to 100 dec are valid in the Client >> Server mode.
The values higher than 100 dec are valid in Server >> Client mode
Dec HEX Description 0 00 CAS_SIGNATURE 1 01 CAS_SET_BAUDRATE 2 02 CAS_SET_DATASIZE 3 03 CAS_SET_PARITY 4 04 CAS_SET_STOPSIZE 5 05 CAS_SET_CONTROL 6 06 CAS_NOTIFY_LINESTATE 7 07 CAS_NOTIFY_MODEMSTATE 8 08 CAS_FLOWCONTROL_SUSPEND 9 09 CAS_FLOWCONTROL_RESUME 10 0A CAS_SET_LINESTATE_MASK 11 0B CAS_SET_MODEMSTATE_MASK 12 0C CAS_PURGE_DATA 50 32 CAS_OPT_GPIO 51 33 CAS_SET_GPIO : : +100 +64 ASC_ 150 96 ASC_OPT_GPIO 151 97 ASC_SET_GPIO
Supported NVT commandsASC_ is the device response to the CAS_ command. It means the PC will send a CAS_SET_PARITY command and the TCP/IP device will reply with the parity value confirmation ASC_SET_PARITY.
For direct I/O pin control we use the double byte command GPIO-50 or 51 (which is behind the COM-PORT-OPTION 44 command) followed by the sub option sequence.
Sub option 50 (32 hex)
- 0 (00 hex) – request for input state reading - the answer contains the value of the input port (The CPU’s pins or input shift register)
- 16 .. 23 (10 .. 17 hex) – set the output bit 0..7 as 1
- 32 .. 39 (20 .. 27 hex) – set the output bit 0..7 as 0
- 48 (30 hex) – request for output state reading - tha answer contains the value of the output portSub option 51 (33 hex)
- sets the sent XX value to the output port, see example. It sends the same answer back, because it reads it from the inner pseudoregister.We practically control the GPIO port by sending the "FF FA 2C 32 XX FF F0“ sequence with XX port value. (the XX value is sent to the output port). For example the 0x11 (0x11 = 11 hex) value sets P1.1 to 1, other P1 pins stay unchanged.
Sub option 52 (34 hex)
Sends the outputs value, if there has been a change in any state, or if the device was powered on. This command does not expect any answer, therefore there is no value for 152 (98 HEX) in the table (a single report is basically an unrequested answer).
This command can be preceded with a "FF FA 2C 32 00 FF F0", sequence. This command is used to synchronize binary inputs and outputs of two devices connected to each other.
If we receive the "FF FA 2C 34 XX FF F0" in the data flow, we know that an input port has been changed.
Why two sequencies ?
In the appropriate communication between two I/O controller devices, the first device sends the "FF FA 2C 32 00 FF F0" sequence only once, because the other I/O Controller responds with "FF FA 2C 97 XX FF F0" (where XX is its inputs state). From now on, both sides send only the "FF FA 2C 34 XX FF F0" if there has been an input state change.Note: To activate the function that informs about the input changes, you need to set the monitored input range with the "#T: Trigger AND mask"command.
- To transmit all the inputs set: #T=255
- To turn off inputs transmitting set #T=0
The following command sets the output GPIO port to hex value AA (10101010 bin)
<IAC><SB><COM_PORT_OPTION><CAS_SET_GPIO><byte to output)><IAC><SE>
FF FA 2C 33 AA FF F0
This sequence is the answer from the device :
<IAC><SB><COM_PORT_OPTION><ACS_SET_GPIO><byte to output)><IAC><SE>
FF FA 2C 97 AA FF F0
You can see that the answer from the device contains +100 DEC (+64 HEX because 0x33 + 0x64 = 0x97) value for the NVT command confirmation.
By sending the FF FA 2C 32 00 FF F0 sequence >> the embedded server responds with: FF FA 2C 96 XX FF F0
- where XX is the actual value of the input pins.
By sending the FF FA 2C 32 30 FF F0 sequence >> the embedded server responds with : FF FA 2C 97 XX FF F0
- where XX is the actual value of the register, which is used to set the outputs.
Note: The output value here is the output REGISTER value. This can be very useful if you have to combine bit and byte oriented commands.
By sending the FF FA 2C 32 25 FF F0 sequence >> the embedded server responds with : FF FA 2C 97 DF FF F0
- where DF is the actual value of the output port (it also depends on the previous port state). The embedded server only changes one bit, but returns the whole port value.
If you do not send ani request from your PC and the input state changes, you receive the "FF FA 2C 32 00 FF F0 " and the "FF FA 2C 34 XX FF F0" sequence
Where the XX is the actual value of the binary input.
The function works similar to the incoming data on the serial port. If your device works in the TCP Client/Server mode and has NVT enabled, then when you receive 1 byte from the serial port (or there is any binary input change), teh device tries to establish a TCP connection and send the data. When the TCP connection is established, only the actual inputs status is sent, not the whole history of changes.
Note: To activate the function that informs about the input changes, you need to set the monitored input range with the "#T: Trigger AND mask"command.
- To transmit all the inputs set: #T=255
- To turn off inputs transmitting set #T=0
You can check the actual Port speed by sending the 00 00 00 00 sequence. If you send any other value, the Port speed will be changed by the server. The dec value corresponds with the port speed in Bd.
By sending the FF FA 2C 01 00 00 00 00 FF F0 sequence >> The server returns : FF FA 2C 65 00 00 25 80 FF F0
After the 00 00 25 80 HEX->DEC conversion, we can read the port speed directly, which is 9600Bd.
If data is not transferred, the server terminates the connection. To keep the connection active all the time, use the “K: Keep connection” command in the setup mode of the Embedded device.
The connection is active for an unlimited time, because the NOP command sends (FF F1 sequence) from the Embedded device side every 5seconds. The TCP connection will close if the TCP layer finds it can't deliver packets for too long a time.Note : Keep connection function works only if NVT support is switched on!
The “space mark” NVT feature has been implemented from version 2.3. This feature can be used for the 9th bit settings, which was commonly used by older applications during 90's.
Note, the parity change is asynchronous, it is not buffered, but it is completed after the character is received. The synchronous functions are available as our proprietary solution by sending the 0xFE’P’ sequence, which reserves the parity. You have to enable the "Variable Parity" support in the SETUP.
If you would like to check whether the device is available via the network, there is a special “Are you there” command in telnet application. Usually, the response from a standard Unix device is “Yes”. We have extended this response as follows: If you send the FF F6 sequence, our Embedded server will answer the response in the following format:
<WEB51 HW 4.5 SW 2.3 SN 01A03B #01>
Which means :
<WEB51 HW XXX SW XXX SN 1035EE #0F *OvErr *ParErr *FlErr>There is the device name, HW version, firmware version and “S/N”, which is the last 3 Bytes of the MAC address. The sequence behind the * character is just for the status response and is not necessary.
The Telnet protocol is often thought of as simply providing a facility for remote logins to a computer via the Internet. This was its original purpose although it can be used for many other functions.
It is best understood in the context of a user with a simple terminal using the local telnet program (known as the client program) to run a login session on a remote computer where his communication needs are handled by a telnet server program. It should be emphasised that the telnet server can pass on the data it has received from the client to many other types of process including a remote login server. It is described in the RFC854 standard, first published in 1983.
The NVT uses 7 bit codes for characters, the display device, referred to as a printer in the RFC, is only required to display the "standard" printing ASCII characters represented by 7 bit codes and to recognise and process certain control codes. The 7 bit characters are transmitted as 8 bit bytes with most significant bit set to zero. An end-of-line is transmitted as the character sequence CR (carriage return) followed by LF (line feed). If it is desired to transmit an actual carriage return this is transmitted as a carriage return followed by a NUL (all bits zero) character.
NVT ASCII is used by many other Internet protocols.
The following control codes are required to be understood by the Network Virtual Terminal.
| Name | code | Decimal Value | Function |
|---|---|---|---|
| NULL | NUL | 0 | No operation |
| Line Feed | LF | 10 | Moves the printer to the next print line, keeping the same horizontal position. |
| Carriage Return | CR | 13 | Moves the printer to the left margin of the current line. |
The following control codes are optional but should have the indicated effect on the display.
| Name | code | Decimal Value | Function |
|---|---|---|---|
| BELL | BEL | 7 | Produces an audible or visible signal (which does NOT move the print head. |
| Back Space | BS | 8 | Moves the print head one character position towards the left margin. [On a printing devices this mechanism was commonly used to form composite characters by printing two basic characters on top of each other.] |
| Horizontal Tab | HT | 9 | Moves the printer to the next horizontal tab stop. It remains unspecified how either party determines or establishes where such tab stops are located. |
| Vertical Tab | VT | 11 | Moves the printer to the next vertical tab stop. It remains unspecified how either party determines or establishes where such tab stops are located. |
| Form Feed | FF | 12 | Moves the printer to the top of the next page, keeping the same horizontal position. [On visual displays this commonly clears the screen and moves the cursor to the top left corner.] |
The NVT keyboard is specified as being capable of generating all 128 ASCII codes by using keys, key combinations or key sequences.
| Name | Decimal Code | Meaning |
|---|---|---|
| SE | 240 | End of subnegotiation parameters. |
| NOP | 241 | No operation |
| DM | 242 | Data mark. Indicates the position of a Synch event within the data stream. This should always be accompanied by a TCP urgent notification. |
| BRK | 243 | Break. Indicates that the "break" or "attention" key was hit. |
| IP | 244 | Suspend, interrupt or abort the process to which the NVT is connected. |
| AO | 245 | Abort output. Allows the current process to run to completion but do not send its output to the user. |
| AYT | 246 | Are you there. Send back to the NVT some visible evidence that the AYT was received. |
| EC | 247 | Erase character. The receiver should delete the last preceding undeleted character from the data stream. |
| EL | 248 | Erase line. Delete characters from the data stream back to but not including the previous CRLF. |
| GA | 249 | Go ahead. Used, under certain circumstances, to tell the other end that it can transmit. |
| SB | 250 | Subnegotiation of the indicated option follows. |
| WILL | 251 | Indicates the desire to begin performing, or confirmation that you are now performing, the indicated option. |
| WONT | 252 | Indicates the refusal to perform, or continue performing, the indicated option. |
| DO | 253 | Indicates the request that the other party perform, or confirmation that you are expecting the other party to perform, the indicated option. |
| DONT | 254 | Indicates the demand that the other party stop performing, or confirmation that you are no longer expecting the other party to perform, the indicated option. |
| IAC | 255 | Interpret as command |
There are a variety of options that can be negotiated between a telnet client and server using commands at any stage during the connection. They are described in detail in separate RFCs. The following are the most important.
| Decimal code | Name | RFC |
|---|---|---|
| 1 | echo | 857 |
| 3 | suppress go ahead | 858 |
| 5 | status | 859 |
| 6 | timing mark | 860 |
| 24 | terminal type | 1091 |
| 31 | window size | 1073 |
| 32 | terminal speed | 1079 |
| 33 | remote flow control | 1372 |
| 34 | linemode | 1184 |
| 36 | environment variables | 1408 |
Options are agreed by a process of negotiation which results in the client and server having a common view of various extra capabilities that affect the interchange and the operation of applications.
Either end of a telnet dialogue can enable or disable an option either locally or remotely. The initiator sends a 3 byte command of the form
IAC,<type of operation>,<option>
The response is of the same form.
Operation is one of
| Description | Decimal Code | Action |
|---|---|---|
| WILL | 251 | Sender wants to do something. |
| DO | 252 | Sender wants the other end to do something. |
| WONT | 253 | Sender doesn't want to do something. |
| DONT | 254 | Sender doesn't want the other end to do anything. |
Associated with each of the these there are various possible responses
| Sender Sent | Receiver Responds | Implication |
|---|---|---|
| WILL | DO | The sender would like to use a certain facility if the receiver can handle it. Option is now in effect |
| WILL | DONT | Receiver says it cannot support the option. Option is not in effect. |
| DO | WILL | The sender says it can handle traffic from the sender if the sender wishes to use a certain option. Option is now in effect. |
| DO | WONT | Receiver says it cannot support the option. Option is not in effect. |
| WONT | DONT | Option disabled. DONT is only valid response. |
| DONT | WONT | Option disabled. WONT is only valid response. |
For example if the sender wants the other end to suppress go-ahead it would send the byte sequence
255(IAC),251(WILL),3
The final byte of the three byte sequence identifies the required action.
For some of the negotiable options values need to be communicated once support of the option has been agreed. This is done using sub-option negotiation. Values are communicated via an exchange of value query commands and responses in the following form.
IAC,SB,<option code number>,1,IAC,SE
and
IAC,SB,<option code>,0,<value>,IAC,SE
For example if the client wishes to identify the terminal type to the server the following exchange might take place
Client 255(IAC),251(WILL),24
Server 255(IAC),253(DO),24
Server 255(IAC),250(SB),24,1,255(IAC),240(SE)
Client 255(IAC),250(SB),24,0,'V','T','2','2','0',255(IAC),240(SE)
The first exchange establishes that the terminal type (option number 24) will be handled, the server then enquires of the client what value it wishes to associate with the terminal type. The sequence SB,24,1 implies sub-option negotiation for option type 24, value required (1). The IAC,SE sequence indicates the end of this request. The response IAC,SB,24,0,'V'... implies sub-option negotiation for option type 24, value supplied (0), the IAC,SE sequence indicates the end of the response (and the supplied value).
The encoding of the value is specific to the option but a sequence of characters, as shown above, is common.
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