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AN93
258 Rev. 1.3
Another consideration for EPOS applications is the method of error detection and error correction. Early EPOS
terminals adopted the Zilog 85C30 Serial Communications Controller (SCC) in conjunction with a synchronous
modem to implement an HDLC/SDLC-based data link layer. The complexities of the HDLC handling is done by the
SCC, while the modem performs strict data pump function. However, given the broad availability of UARTs, HDLC
handling can be performed by the modem also. To facilitate this system partitioning, the V.80 protocol is used. V.80
allows the multiplexing of data, control, and status information so that the host processor can specifically control
what frames are sent to, or received from, the modem across the asynchronous UART (DTE) interface. The host
performs much of the other layers of the protocol stack beyond this Data Link Layer. A discussion of host software
is beyond the scope of this appendix.
Recommendation V.80
The goal of V.80 is the concept of “abstracting hardware circuits”. This is achieved by the addition of a control and
status channel alongside the main data channel. The main data channel is effectively the information transfer
across the UART TX and RX lines. The control/status channel that runs alongside the main data channel is
signaled by the use of
EM shielding. This means a “special character” is chosen to signify the beginning of the
control/status channel. In its simplest form, this “special character,” in conjunction with the “next” character, is taken
together as a single nugget of information denoting a special control message or a special status.
V.80 uses <0x19> as a special control character. The next question becomes how to send the character 0x19 as
data? This is accommodated by the concept of
transparency, in which the host is required to send a special
sequence to signify its desire to send 0x19 as data rather than a control character.
The concept could have been very simple, but there are additional complications:
The desire to support 7-bit data and 1-bit parity asynchronous protocols
The desire to support XON and XOFF handshaking
The desire to limit bandwidth usage
The desire to support 7 data bits and 1 parity bit creates the possibility that the host would be sending <0x99>
when the intention is to be sending the <0x19> special character. The <EM> character is really a shortcut for
saying <0x19> or <0x99>.
The XON and XOFF characters are <0x11> and <0x13>, respectively. These characters are treated in a special
way by many UARTs, and, therefore, V.80 must ensure that
neither <0x11> or <0x13> occur in the data stream so
that a lower protocol layer will not need to be rewritten. Hence, the final “special character set” for V.80 includes
<0x19>, <0x99>, <0x11>, and <0x13>.
What happens if the data file being sent is a constant stream of <0x19> bytes? By the single-transparency rules,
one would then argue that the number of bytes sent across the DTE would effectively be doubled. In order to
ensure that the throughput does not become bloated by the EM Shielding, provisions for all combinations of two
special character combinations are created. This adds yet another sixteen EM Shielding cases since there are 4 x
4 matrix of combinations of these special characters.
At this point, the transparency cases for EM Shielding can thus guarantee the ability to send “anything” over the
DTE with the special considerations of 7 vs 8 data bits, XON and XOFF characters and throughput considerations.
However, once the data channel has been architected, the rest of the unused EM codes can be used for the
primary purpose of V.80, which is the concept of
hardware abstraction.
In EPOS applications, there is little value in abstracting pins such as RI
or RTS. The value comes in abstracting the
TXCLK and RXCLK of the Synchronous UART. The Synchronous UART is the primary method of connecting to the
Zilog 85C30 SCC. V.80 allows the interface between the host and the modem to be a simple asynchronous DTE,
while allowing for synchronous operation performed by the modem itself.
The purpose of going through this explanation is to allow the easier reading of the V.80 standard and to provide the
proper framing of the use of V.80 in an EPOS application. It is important to note that the
usage of V.80 for HDLC
function does
not use many of the other aspects of V.80.
For example, the data transferred across the UART is assumed to be 8 bits, even though V.80 also provides the
ability to transfer 7-bit ASCII data. Also, it is rare for XON/XOFF handshaking to be used in an EPOS application,
but the transparency rules of EM Shielding are burdened with these extra EM codes in any case.