DataPage MK2 (USA) Desktop UHF Radio Paging System
DPUSA 02/00
20
Issue 2
Appendix A
TAP - (T)elocator (A)lphanumeric (P)rotocol:
TAP is commonly used in the wide-area paging industry, generally between remote transmitting stations via a
modem. Its' advantage lies in the two-way nature of the communications link, and using checksums, the ability to
request retransmissions in the event that an error is detected by the receiving station.
There are two optional parameters: the
password
and a
sign-off text message
, and either or both of these can
be included.
Scope TAP implementation adheres strictly to the Version 1.8 specification, and will allow both of these
parameters to be defined or left out. If a password is not programmed into the system, it will not expect one, the
same goes for the sign-off string, if the entry is blank it will not appear.
A typical TAP transaction is as follows:
1
The caller makes sure the receiving station is connected by requesting a response.
2
The receiving station responds with a request for the identity of the caller, in this case the enquiry
consists of the text string 'ID='.
3
The caller then sends a string to identify the protocol it intends to use, and a password if this is required.
4
If the receiving station accepts the response and password it acknowledges the fact and then requests
the sender deliver the message it is holding for transmission.
5
The sender transmits the pager number and text message to deliver, and attaches a checksum based
on the contents of the data transmitted. (see example below).
6
The receiving station computes the checksum from the received data and if it matches the appended
checksum of the received data, acknowledges the fact or requests a re-transmission of the data if this is
not the case.
7
The sending station will then either request another message block is transmitted, or indicate to the
remote station that the transaction is complete.
8
Once all the data has been sent, and the sending station indicates this fact, the receiving station will
optionally send a sign-off message, and instruct the sender to terminate the link. If no sign-off message
is included, only the link termination command is issued.
On a modem-connected link, this would be the point at which the line is dropped by both sides. On a permanent
link, the sender and receiving station return to their normal handshake exchanges to verify the link, or cease
communication until another transaction is required.
These steps are normally limited to a set number of attempts, due mainly to the requirement that a telephone or
modem link should not be maintained indefinitely. For this reason there are also time constraints which specify
how long to wait for a given response.
These 'retry' numbers and time limits are the other 'flexible' options in the TAP protocol. The Scope system
allows the installer to specify these values for themselves to suit the application to which the protocol is applied.