Silicon Laboratories SI2493/57/34/15/04, SI2494/39 manual Recommendation

AN93

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.

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