AEA PAKRATT PK-232, Technical Reference Manual

Page: 72 / 101

PK-232 TECHNICAL MANUAL APPENDIX B – KISS TNC Specification
PK232TM Rev. A 5/87 B-3 Page 72
rameters, namely the transmitter keyup delay, the transmitter persistence variables and any spe-
cial hardware that a particular TNC may have.
To distinguish between command and data frames on the host/TNC link, the first byte of each
asynchronous frame between host and TNC is a "type" indicator. This type indicator byte is broken
into two 4-bit nibbles so that the low-order nibble indicates the command number (given in the ta-
ble below) and the high-order nibble indicates the port number for that particular command. In
systems with only one HDLC port, it is by definition Port 0. In multi-port TNCs, the upper 4 bits of
the type indicator byte can specify one of up to sixteen ports. The following commands are defined
in frames to the TNC (the "Command" field is in hexadecimal):
Command Function Comments
0 Data frame The rest of the frame is data to be sent on the HDLC channel.
1 TXDELAY The next byte is the transmitter keyup delay in 10 ms units. The
default start-up value is 50 (i.e., 500 ms).
2 P The next byte is the persistence parameter, p, scaled to the range
0 - 255 with the following formula:
P = p × 256 - 1
The default value is P = 63 (i.e., p = 0.25).
3 SlotTime The next byte is the slot interval in 10 ms units. The default is 10 (i.e.,
100 ms).
4 TXtail The next byte is the time to hold up the TX after the FCS has been
sent, in 10 ms units. This command is obsolete, and is included here
only for compatibility with some existing implementations.
5 FullDuplex The next byte is 0 for half duplex, nonzero for full duplex. The default
is 0 (i.e., half duplex).
6 SetHardware Specific for each TNC. In the TNC-1, this command sets the modem
speed. Other implementations may use this function for other hard-
ware-specific functions.
FF Return Exit KISS and return control to a higher-level program. This is useful
only when KISS is incorporated into the TNC along with other applica-
tions.
The following types are defined in frames to the host:
Command Function Comments
0 Data frame Rest of frame is data from the HDLC channel.
No other types are defined; in particular, there is no provision for acknowledging data or command
frames sent to the TNC. KISS implementations must ignore any unsupported command types. All
KISS implementations must implement commands 0, 1, 2, 3 and 5; the others are optional.
5. Buffer and Packet Size Limits
One of the things that makes the KISS TNC simple is the deliberate lack of TNC/host flow control.
The host computers run a higher level protocol (typically TCP, but AX.25 in the connected mode al-
so qualifies) that handles flow control on an end-to-end basis. Ideally, the TNC would always have
more buffer memory than the sum of all the flow control windows of all of the logical connections
using it at that moment. This would allow for the worst case (i.e., all users sending simultane-
ously). In practice, however, many (if not most) user connections are idle for long periods of time,