DE3925723A1 - V-24 bus interface with open collector RTS and TXD stages - enables simultaneous connection of multiple peripherals to computer with standard V-24 interface - Google Patents

Abstract

More than 1 peripheral equipment items fitted with a V24 bus interface can be connected simultaneously, i.e. in parallel to a standard V24 interface of a computer or master. The line driver outputs RTS and/or TXD have open collector output stages which have pull-up and/or pull-down resistors. The RTS and/or TXD outputs are of such high impedance that they can be supplied with signals from further interfaces connected in parallel. USE/ADVANTAGE - The new interface combines the advantages of the standard V24 interface and the IEC- or IEEE-interface, i.e. economy and simplicity of use.

Description

Bus-compatible computer interface with a 5-wire connection cable and is particularly easy to use is low installation cost.

State of the art

For connecting peripheral devices to central computers (e.g. PCs) is the well-known V24 interface or RS232 interface provide. This now widespread interface enables you to use a computer in a simple and inexpensive way to connect an external peripheral. Now several pe peripheral devices are connected to the computer, so on the computer a separate V24 for each external peripheral device to be connected Interface required.

As an alternative to this type of interface, the user can use the bus-compatible IEC or IEEE interface, which is used for networking is designed for several devices. On the central computer (PC) only a single interface is required to which all of the networked external peripheral devices can be connected in parallel. This com Fortable interface design is both ver in the handling equally complex with very high installation costs.

Task

The invention has for its object to a new interface develop the advantages of the well-known V24 interface with the Advantages of the IEC or IEEE interface combines, i. H. it should an inexpensive, easy-to-use bus-compatible interface stand.

Another object of the invention is that peripheral devices, which are equipped with the new V24 bus interface Computer with the conventional V24 interface, without modification  can be connected.

solution

The object is achieved in that the two Transmitter outputs RTS and TXD with a known V24 interface open collector outputs are provided so that they at rest was, similar to Tristate outputs, parallel to others switched transmitter outputs can be assigned signals.

The connection of the various peripheral devices and the connected computer is shown in Fig. 2. The connections between the PC and the individual devices are always made using a 1: 1 connecting line. All external peripheral devices are connected directly in parallel to the computer interface. This computer interface is a well-known standard V24 interface.

The bridging of the lines at pin 6 , pin 8 and pin 20 , which is required in connection with the new V24 bus interface, can of course also be done directly on the PC, contrary to the wiring in Fig. 2. This makes it possible, as already described, to produce a connecting line between the PC and peripheral devices with only a 5-core cable. The PC transmits its output data to all connected peripheral devices. Since only one device feels addressed, a selection logic is required. Each connected peripheral device is assigned an identification number that the central computer knows and sends out if it wants to address this device.

Embodiment

Fig. 1 shows a typical partial circuit diagram of the line drivers or receivers of the V24 bus interface.

On the far left are those from a commercially available interface module upcoming signal lines RXD, CTS, RTS and TXD shown. A galvanic isolation between the potential of the peripheral and the computer potential is via the intermediate 4 optocoupl reached. This makes it possible to address the periphe devices depending on the potential of the connection book loop into the required measurement setup.  

The transmitter and receiver lines (TXD and RXD) are each equipped with a handshake line (CTS and RTS). In order to delay the sending of data from the computer on the TXD line (pin 2 of the 25-pin sub-D connector on the PC) for a short time, if this should be necessary, the associated line is CTS (pin 5 of the 25-pin sub-D Connector) is provided. Each of the connected peripheral devices has the option of using this handshake line if required. The hardware of the individual peripheral devices, which are equipped with the new V24 bus interface, is designed so that each hand passively with a 10 kOhm resistor (R 12 ) pulls this handshake line to +12 V, ie active. If one of these peripherals requires the handshake line, this line can be pulled to negative potential via an NPN transistor. This means that the connected PC stops sending data until even the slowest peripheral device is ready to take over the data.

The RXD line (pin 3 of the 25-pin Sub-D connector) is wired in a similar way. In the normal case, ie none of the connected peripheral devices sends data, the TXD line on the individual devices is passively pulled to a negative reference potential with a 10 kOhm resistor (R 20 ). As soon as one of the devices has to send data to the PC, it pulls its TXD line to the active state via a PNP transistor. This data transfer can be interrupted shortly by the PC via its RTS line (pin 4 of the 25-pin Sub-D connector). The bridging of the lines at pin 6 , pin 8 and pin 20 can, as already mentioned, be done directly on the PC. This makes it possible to establish a connection between the PC and connected peripheral devices with just a 5-core cable. The PC transmits its output data to all connected peripheral devices simultaneously (in parallel). This requires selection logic so that only one device really feels addressed and accordingly communicates with the PC (master).

The new V24 bus interface for use in peripheral devices te is designed so that the connected computer (master) Initiative, d. H. the actual data transfer must trigger.

This was necessary to avoid simultaneous sending of data from the peripheral devices. For this reason, the relevant peripheral devices are addressed one after the other by the central computer (master). For this purpose, each connected peripheral device has been given a type number and a device address. The latter can be set individually for each device. In order to enable the devices to respond properly, two different data protocols, as shown in Fig. 3 and 4 , were required. Communication between the computer and the peripheral device takes place using ASCII characters. However, some of the tax symbols specified there are used in somewhat modified form. The master sends the ASCII character SOH (Start Of Heading) to establish the connection. With this, all connected devices consider any connection that may have been established previously and log on to this control character. The master then sends out an identification code for the type of device it wants to address. The device address is then sent. This address can contain numbers from 0 to 7. The numerical values can be set in hardware using corresponding jumpers on the peripheral device. It must be ensured that the device addresses are set differently for several devices of the same type. If a device on the bus is addressed by this addressing, it must immediately output the ETX (End Of Text) character as confirmation. Every other peripheral device that is not logged in behaves passively (ie without sending data, only listening) on the bus.

With this step, the login procedure is already finished and the Master can now exchange data with the relevant peripheral device . The confirmation remains from the addressed peripheral device off, this is an indication for the master (computer) that either the selected device does not exist, wrong configuration entered the device is not switched on or is not ready for operation.

Communication between master and peripheral device is interrupted follows by sending EOT (End Of Transmit) on the computer. This Command also causes the remote LED, which is also an indication is that the connected peripheral device executes commands has gone out (provided a corresponding control LED on the periphery device was provided). This is the keyboard of the concerned Peripheral device released again for manual input. Should the manual operation of the keyboard prevented and at the same time other peripheral devices are addressed, this is through a  Easy log-in with SOH in the corresponding devices possible. To the Switch off remote LED and accordingly the keyboard for manual on to release again, the master must log into the device and properly end communication with EOT.

For data communication between the computer (master) and the peripheral device, it is a prerequisite that a successful login has taken place. Fig. 4 shows how the command transmission frame is designed for data exchange with the peripheral device. Different commands follow depending on the selected device type. These are usually marked with the letters "a" to "z" or "A" to "Z". Then, depending on the command structure, the associated parameters follow. Finally, the master sends the control character CR (carriage return). In response, the addressed and logged in peripheral device sends the requested measured values and concludes this with the control character ACK (acknowledge) as confirmation for an error-free transaction of the data. Even if no measured values or other status signals are sent back to the computer, the control character ACK mentioned must be sent in any case as confirmation of receipt of the command. If an incorrect command, incorrect parameters, a communication error or any other transmission error has occurred, the connected device acknowledges with an error number and the final control character NAK (negative acknowledge) instead of the ACK. This completes a complete command transfer. The error messages can then be processed further in the service program of the computer.

Fig. 5 shows a layer model for the driver or operating software on the PC. The lowest level contains the interface drivers that directly address the serial interface of the computer. Drivers responsible for the correct data transmission are arranged above this.

The two lowest levels of the software mentioned are in the ver different peripherals identical. Above that are the drivers for placed the different device types. Here are also special Characteristics of the peripheral devices are taken into account. As the last and the top Level follows the user program. This can be done in an individual way and Be changed by the user.

Claims (4)

1. V 24 bus interface, characterized in that more than 1, ie at least 2 or more peripheral devices equipped with this interface are connected simultaneously, ie in parallel to a known standard V 24 interface of a computer (masters) can. 2. V 24 bus interface according to claim 1, characterized in that the line driver outputs RTS and / or TXD are provided with open collector outputs. 3. V 24 bus interface according to claim 2, characterized in that the open collector outputs with pull Up- and / or pull-down resistors are provided. 4. V 24 bus interface according to claim 1, characterized in that the RTS and / or TXD outputs are so high ohmic are executed that they are connected by parallel Interfaces can be loaded with signals.