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D.C. Hayes --  80-103 Modem

This was the board that launched the D.C. Hayes company. It used the Bel 103 standard frequencies for modems at 110 or 300 BPS.

Hayes 103 Modem

The 80-103A was an S-100 compatible card which when properly installed, performed the function of interfacing the S-100 computer bus to a tele© (telephone company) supplied data access arrangement (DAA). The computer, using Hayes supplied software, controled the activity of the DCA by moving bytes into data and control registers and by sensing the status of the interface and accepting data from the receiving register.

The address decode and control signals select the board and register and determine whether the processor is performing a write or read operation on the selected register. If it was determined that the current operation was a write, the Data Out Bus (DOO through D07) was gated to the selected register and saved in that register. If the decoding logic determines that the operation was a read, the selected register was gated onto the Date In Bus (DIO through D17) with the proper timing so that the processor can accept the data.

Once the processor has setup the connection to be established and the DCA was ready to transfer data, the software determined whether to transmit or receive characters on the line. If transmit was chosen, then the status register was checked to see if the transmit holding register (TRE) was empty. The character to be sent was then written into the transmit holding register. After this operation was complete, the transmit register was checked automatically to see if it was empty. When it was found to be empty, the transmit holding register was loaded into the transmit register; the transmit holding register was marked empty allowing the next write. Following this transfer the transmit register and associated logic sends a start bit. When the start bit was finished, the least significant bit was transmitted followed by the succeeding bits of the character.

If parity was set in the control register, the appropriate parity was generated followed by the indicated number of stop bits. Each of the transmitted bits passes through the modulator where the bit was converted into the appropriate  frequency using a digital sine wove generator. The originate and answer modes use different sets of frequencies allowing the ability to transmit full duplex (in both directions at the same time).

Each set of frequencies consists of two frequencies with one frequency corresponding to each state of the line, i.e. "1" or "0". In all modes the signal was passed through to the telephone line at the OT arid DR pins of the telco Interface. Normally (except for the self test), the filter prevents the transmitter signal from feeding back into the receiver and prevents noise or unwanted signals from interfering with the data reception. The received signal comes through the telco interface, passes through the precision filter, and was demodulated by the digital demodulation circuit. Start and stop bits are checked for framing, then stripped off, and the character was assembled
   in  the  receive register,  and if parity was called for, the parity was checked creating a parity error if it was incorrect. When the character was completely assembled, the logic checks to see if the receive holding register was empty. If it was empty, the data was transferred from the receive register and the status was set to indicate that the receive holding register was full (RRF). If the logic was unable to transfer the received character to the receive holding register because the previous data has not been read by the processor, then the overflow error flag was set in the status register.

The self test mode was a variation where the filter was switched to the same set of frequencies as the transmitter so that the receiver gets each character sent by the transmitter thus checking the modulator and demodulator and all parts of the circuit except the transformer coupling the circuit to the line and the connection to the DAA.

Back in the bad old days when Bell controlled all telephone systems it was against the law to hook up an "unapproved device" to the phone line. You had to use a Bell System 1001D CBT coupler or Data Access Arrangement (DAA) from them (for a rip-off monthly fee).   The Hayes 80-103A plugged into this DAA. That was the way things were before the days of deregulation! 

The board itself could have its I/O ports (4 in all), located at any multiple of 4H up to 7FH.  It could also be controlled by memory mapping though this was seldom used.  The board could generate interrupts on ring detect, data ready etc. though the supplied software did not utilize this feature.

The manual was well written with examples and details of how to use the board.
The manual for this card can be obtained here.

Note on Bell 103 Protocol.
Bell 103 - Asynchronous data transmission, full-duplex operation over 2-wire dialup or leased lines; 300-bps data rate. Ideal for the "low demand" user who exchanges files infrequently with another PC user or an on-line bulletin board. Comparable to ITU V.21. This modulation is well suited for bad phone lines – as the communication guys use to say "103 modems will work on barb wire".


Other D.C. Hayes S-100 Boards
80-103a    Micromodem-100


This page was last modified on 10/25/2013