WO1996002885A1

WO1996002885A1 - Modem adapter

Info

Publication number: WO1996002885A1
Application number: PCT/GB1995/001671
Authority: WO
Inventors: Trevor Burke
Original assignee: Phonelink Plc
Priority date: 1994-07-14
Filing date: 1995-07-14
Publication date: 1996-02-01
Also published as: GB9414277D0; AU2932995A

Abstract

A communications adapter for connection to a modem and a computer terminal between which data is transmitted. The adapter has a first port for connection to the modem and a second port for connection to the terminal. A buffer memory is provided for storing data received at either of the ports. Data is transmitted between the modem and the adapter at a first rate and data is transmitted between the terminal and the adapter at a second, higher rate. The flow of data between the adapter and the terminal is controlled, whereas the flow of data between the modem and the adapter is not controlled. Accordingly data arriving from the modem is initially stored and then transmitted onto the terminal in dependence upon a voltage appearing on a terminal port pin indicating that the terminal is ready to receive data. Data received from the terminal is stored and then transmitted to the modem, a voltage applied to a pin of the terminal port indicating to the terminal when the adapter is ready to receive further data.

Description

MODEM ADAPTER

The present invention relates to on-line communications, and in particular to an adapter for connection between a modem and a computer terminal between which data is transmitted in both directions.

On-line data communication systems are widely used to transfer data between, for example, a central computer and a series of system terminals. For example, in on¬ line searching systems, users key into terminals details of searches to be conducted and as a result search request data is generated and transmitted to a central database. Search results data is extracted from the central database in accordance with the received search request data, and the search results data is transmitted to the users terminal. Data is transmitted between the users terminal and the central database over conventional communications link that generally include telephone lines, the users terminal and the central database being connected to those lines through respective modems. The transmission rate and transmission reliability is fundamental to the economics of such systems. Conventional telephone lines are of limited band width, and therefore a relatively low data rate must be used. Current modems typically have a data rate of 19200 Baud. This data rate is satisfactory however for many commercial purposes, as is indicated by the rapid expansion of on-line systems worldwide.

Modern personal computers (PC's) have the capacity to perform many tasks, and the designers of on-line systems have taken advantage of this capacity to enhance their systems by adding more facilities. When working with such enhanced systems, the communications capability of the system tends to degrade as the programs in the user terminals become larger, the degradation manifesting itself in loss of sections of transmitted data. Such degradation can be the result of programming errors, but assuming such errors have not been made the problem can often be traced to the way in which data is transferred between the modem and the PC. Particular problems are encountered when the PC is operating multi-tasking software such as "Windows" which is marketed by Microsoft. For example, when data is transmitted to a PC terminal from a central database, the receiving modem transfers a stream of characters to the serial port of the PC. The appearance of a character at the serial port generates an interrupt which indicates to the PC that it should stop whatever it is currently occupied with and deal with that input. The PC will, however, often generate interrupts unrelated to activity at the serial port and occasionally these further interrupts will be of a higher priority than the interrupts generated as a result of receipt of data at the serial port. If such a higher level interrupt is generated whilst data is being transmitted to the serial port of the PC the incoming data will be lost. Another source of character loss is the way that Windows actually processes interrupts. Due to the multitasking nature of Windows, various interrupts (including communications) are virtualised. which means that instead of the interrupt passing directly to the part of the program that should handle it, Windows intercepts the interrupt, acknowledges it. and then passes it on to a process. This extra level of processing allows many tasks within Windows to treat the computer hardware as if it was being used by them exclusively, when, in fact, they are all sharing the same hardware. Virtualisation also has the effect of acknowledging the physical interrupt (thus resetting the physical interrupt condition) before the actual driver software sees it, which is a source of potential character loss on a heavily loaded machine.

This problem can be addressed by incorporating a known high performance universal asynchronous receiver/transmitter (UART) into the PC, such a UART incorporating a buffer and overriding the normal processing of interrupts to ensure that data is not lost. Unfortunately, many users who wish to become subscribers to on-line systems already have PC's that are not equipped with such a specialised UART. Furthermore, retrofitting such a UART into an existing PC is a difficult task, requiring both hardware and software modifications which can only be implemented on the basis of a full understanding of how the PC was originally designed to operate. Accordingly in many circumstances upgrading an existing PC with a high specification UART is not commercially feasible.

It is an object of the present invention to obviate or mitigate the problems outlined above.

According to the present invention, there is provided a communications adapter for connection to a modem and a computer terminal between which data is transmitted. comprising a modem port for connection to the modem, a terminal port for connection to the terminal, means for storing data received at the modem port and data received at the terminal port, means for reading out to the terminal port stored data received at the modem port, means for reading out to the modem port stored data received at the terminal port, and means for controlling the flow of data between the storing means and the terminal port in dependence upon signals applied to the terminal port, the data transmission rate between the storing means and the terminal port being higher than the data transmission rate between the storing means and the modem port.

The invention enables the aforementioned problem to be addressed by imposing flow control on the transfer of data to and from the PC, whilst continuing to operate in a normal manner without flow control on the transfer of data through the modem. The invention can be implemented without any modifications to the PC hardware, the only requirement being simple programming adjustments to ensure that the terminal is set up to communicate with the adapter at an appropriate rate in response to flow control signals generated in the adapter.

The data rate between the adapter and the modem may be for example 19200 Baud, and the data rate between the adapter and the terminal may be for example 115,200 Baud.

The terminal port may comprise one pin the voltage on which indicates whether or not the adapter is ready to receive data from the terminal, and another pin the voltage on which indicates whether or not the terminal is ready to receive data from the adapter.

Thus full flow control can be readily achieved. Preferably the data is transmitted to and from the adapter in blocks of characters.

The adapter may be provided with a power supply which is energised by signals appearing on one or both of the ports.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:-

Fig. 1 is a schematic illustration of adapter in accordance with the present invention connected to a modem and a series port of a PC; and

SUBSTITUTE SHEET RULE 2P) Fig. 2 illustrates the cable connection arrangement of a prototype adapter in accordance with the invention.

Referring to Fig. 1. the schematically illustrated arrangement comprises a PC 1. the serial port 2 of which is connected to an adapter 3. The adapter 3 is in turn connected to a modem 4 linked to a telephone line 5. Data is transmitted on the line 5 in conventional modulated carrier form, the data is demodulated from the carrier in a conventional manner by the modem 4 and the demodulated data is applied to an RS 232 port connected to cable 6. The cable 6 is connected to an RS 232 interface 7 which is a part of the adapter 3.

As will be described in greater detail with reference to Fig. 2 below, some of the signals applied to the interface 7 are applied to a microcontroller 8, others being connected directly through the adapter to a further interface 9. The microcontroller 8 controls the writing of data to and the reading of data from a random access memory 10.

The adapter is provided with a power supply 1 1 which is energised from the signals applied to the RS 232 interfaces 7 and 9. The supply 1 1 provides all the power required by the adapter and accordingly no external power supply is required.

The interface 9 is connected by a cable 1 1 to the serial port 2 of the PC. In a conventional system, the adapter 3 would be omitted and thus the cable 6 would be simply connected directly to the cable 11. In such circumstances the flow of data between the modem and the serial port would be uncontrolled and thus some mechanism would have to be provided to ensure that the PC 1 was always able to respond to the appearance of data at the serial port 2. As explained above, if the PC was responding to a high level interrupt at the time that data arrived, the data would be lost. In order to avoid this possibility, the adapter 3 provides a buffer memory to store data received by it from either the modem or the serial port of the PC and exercises control over the flow of data between the interface 9 and the serial port 2. To enable the adapter to keep up with the rate at which data is supplied to it from the modem even if the supply of data from the adapter to the PC is intermittent the rate at which data is transferred on the cable 1 1 must be significantly higher than the rate at which data is transmitted on the cable 6. A device of the type shown in Fig. 1 has been tested with a data rate on the telephone line 5 and cable 6 of 19.200 Baud and a data rate on the cable 1 1 of 115,200 Baud. Thus a stream of data received on the telephone line 5 is converted into a serial stream of bits defining a serial stream of bytes each representative of a single character. The microcontroller 8 stores the received bytes of data in the RAM 10. As soon as eight bytes of data have been stored in the RAM those eight bytes of data are ready for transmission to the PC. To enable it to do this it takes control of the transfer of data to the PC by in effect generating its own interrupt which causes the PC to suspend other operations pending receipt of the block of eight bytes which represent eight characters. The block of data is transmitted from the interface 9 to the serial port 2 and subsequently processed in an appropriate manner by the PC. On the other hand, when data is to be transmitted from the PC to the telephone line 5, initially the data is transmitted at the high data rate to the interface 9 and stored by the microcontroller in the RAM 10. That data is subsequently read out at the relatively lower data rate through the interface 7 to the modem 4. Thus the adapter 3 exercises flow control over the transfer of data in both directions between the interface 9 and the serial port 2, whereas there is no flow control over the transfer of data between the modem 4 and the interface 7.

Referring now to Fig. 2, this illustrates the cabling requirements of the adapter 3 of Fig. 1. The drawing schematically represents the RS 232 port 12 of the modem, the RS 232 serial port 13 of the PC. the port 14 of the interface 7 of Fig. 1 and the port 15 of the interface 9 of Fig. 1. Conventional letter codes have been used to identify the signals appearing on the RS 232 pins as follows:-

TXD transmitted data

RXD received data

RTS request to send

CTS clear to send

DSR data set ready

GND ground

DCD data carrier detected

DTR data terminal readv

The data rate on the conductive paths linking the ports 12 and 14 is 19,200 Baud, whereas the rate on the conductive paths linking the ports 13 and 15 is 1 15,200 Baud. Transmission of data from the PC serial port 13 is performed with the DTR pin on the adapter connected to the DSR pin on the PC serial port. The DTR pin is set to logic 1 to indicate that the adapter can receive characters from the PC. The PC then transmits data to the adapter and that data is loaded into the adapter RAM. If the adapter RAM is nearly full and therefore can only accept a limited amount of further data, the pin DTR on the adapter is set to logic 0 to indicate that no more characters should be transmitted from the PC. The pin DTR will not be re-asserted to logic 1 until the RAM has been emptied by transmitting the previously stored data to the modem port.

When data is received from the modem, the data representing eight successive characters will be stored in the adapter RAM before those characters are transmitted as a block to the PC. If the data received represents less than 8 bytes, for example 5 bytes, no data will be transmitted to the PC until a predetermined time delay has elapsed, for example 2 cycles of the adapter system clock. Once that delay has occurred the stored characters will be transmitted to the PC as a reduced size block.

During transmission of data from the adapter to the PC, the pin RTS on the PC is set to logic 1 (asserted) to indicate that the PC is ready to receive data. When the adapter detects that pin CTS (corresponding to pin RTS on the PC) is asserted, the adapter asserts its pin RTS and transmits a single character to the PC. The adapter then waits until adapter pin CTS is cleared and then transmits the rest of the block of data, that is up to a maximum of seven further bytes. After that block of data has been transmitted pin RTS falls to logic 0 to indicate that no more characters are to follow as part of that transmission operation.

It will be noted that the DCD. DTR and RI signals bypass the active components of the adapter and are routed directly from the modem to the PC serial port. Thus the simple actions triggered by signals appearing on these pins are completely unaffected by the presence of the adapter. As described above, the adapter buffers data in both directions of transmission. Data from the PC is controlled using hardware flow control. A minimum of sixteen bytes of RAM should be provided to buffer characters received from the PC. By applying flow control when eight characters have been received this allows characters currently being transmitted to be received before flow control takes effect.

It may be assumed that the PC will always be ready to receive characters so a minimum of sixteen bytes is required to store data transmitted from the modem. Eight bytes of data may be in the process of transmission to the PC, the remaining buffer space being provided for storing data while sending an earlier block of data to the PC. It will be appreciated, however, that these buffer capacities are minimum values and generally a larger RAM capacity will be provided.

Tests have shown that the provision of an adapter in accordance with the present invention solves the communications problems which have been encountered in online searching systems. The problems have been solved in a manner which does not require any hardware modifications to conventional PC's and only minor easily effected modifications to the software. The invention thus makes a significant commercial contribution to the further expansion of online communications systems.

Claims

1. A communications adapter for connection to a modem and a computer terminal between which data is transmitted, comprising a modem port for connection to the modem, a terminal port for connection to the terminal, means for storing data received at the modem port and data received at the terminal port, means for reading out to the terminal port stored data received at the modem port, means for reading out to the modem port stored data received at the terminal port, and means for controlling the flow of data between the storing means and the terminal port in dependence upon signals applied to the terminal port, the data transmission rate between the storing means and the terminal port being higher than the data transmission rate between the storing means and the modem port.

2. A communications adapter according to claim 1, wherein the terminal port comprises a pin the voltage on which indicates whether or not the storing means is ready to receive data.

3. A communications adapter according to claim 1 or 2, wherein the terminal port comprises a pin the voltage on which indicates whether or not the terminal is ready to receive data.

4. A communications adapter according to claim 1, 2 or 3. wherein data is transmitted to and from the adapter in blocks of characters.

5. A communications adapter according to any preceding claim, comprising a power supply energised by signals appearing on one or both of the ports.

6. A communications adapter substantially as hereinbefore described with reference to the accompanying drawings.