Modem for parallel transmission
A modem for parallel transmission or parallel modem is a modem , i.e. a data transmission device in which the data is not transmitted bit-serial , but character by character. The devices were frequently in use until around 1990 and were used to transfer data to a central computer via the telephone network .
Parallel modems allow a maximum transmission speed of 40 characters per second; the standard is 20 characters / s. Serial modems according to V.90 have a data transfer rate of 56,000 bit / s; a current VDSL2 connection can theoretically reach 200 Mbit / s.
Pharmacies and booksellers have often used this technique; Apothecary modem was a slang term for parallel modems. The Telecommunications Technical central office (FTZ) issued at the time private modem is not licensed, these devices had from the German Federal Post for hire. In 1990 the monthly rent including the repair fee for a parallel modem was 27.99 DM , a serial modem with a data transfer rate of 4800 bit / s cost 62.02 DM a month.
A typical terminal device of the time was the small hole card reader . The intended applications are data collection systems (central station) that are selected by many users (outstations) and that transmit a very small amount of data. This technology has been used in Germany since at least 1969. In 2006 they still existed within the water industry in southern Germany; among other things, changes in the level of rivers or dams are transmitted.
Data transmission using a parallel modem was guaranteed to be error-free by the Deutsche Bundespost for 95% of all connections, with serial analog modems (as of 2006) there was only one bit error rate guaranteed within the scope of the technical possibilities.
standardization
The principle of parallel data transmission has been described in various recommendations . In 1968, the CCITT passed Recommendation V.30. In 1976, this became part of ITU-T Recommendation V.20. Recommendation V.20 is technically obsolete and was withdrawn in 1988. Recommendation V.19 was also passed in 1976 by the ITU-T.
This article mainly describes modems according to the V.20 standard.
Transmission principle
In the case of parallel modems, the data terminal device transfers a character completely or half of it to the transmission device . With serial modems the characters are transferred one after the other bit by bit.
The transmission principle is based on the fact that several fixed frequencies are sent at the same time and overlap. The duration of a frequency combination or the rest position is a minimum of 25 milliseconds.
One is sent out of frequency groups with four frequencies. The rest position for character separation is in the V.19 the absence of a signal, in the V.20 the sending of the highest frequency.
Connection to the telephone line
In the first device generation of an outdoor station in Germany (D20P-A02), the connection to the central station with the associated telephone was established by the operator. After a connection was established, the telephone receiver was placed in the appropriate recess of the modem. As a result, an acoustic connection was not established between the participants as with an acoustic coupler . Hanging up the phone operated a toggle switch that disconnected the phone from the line and activated the modem. The housing of these devices had the same color as a FeTAp 61 , which was standardized at the time, and at the same time provided a support surface for this telephone.
Later generations of devices could also be connected to the telephone line by means of an automatic dialing device for data connections or a data button .
Transmission data
ITU-T recommendation V.19
The V.19 uses the frequency groups of a DTMF dialing. Only 16 characters can be displayed by combining them, which is sufficient for the intended purpose (transmission of digits).
| Channel number | 1 | 2 | 3 | 4th |
|---|---|---|---|---|
| Group A | 697 Hz | 770 Hz | 852 Hz | 941 Hz |
| Group B | 1209 Hz | 1336 Hz | 1477 Hz | 1633 Hz |
ITU-T recommendation V.20
V.20 modems work in a similar way, but with different frequencies. A third frequency group (B) can be expanded.
| Channel number | 1 | 2 | 3 | 4th |
|---|---|---|---|---|
| Group A | 920 Hz | 1000 Hz | 1080 Hz | 1160 Hz |
| Group B | 1320 Hz | 1400 Hz | 1480 Hz | 1560 Hz |
| Group C | 1720 Hz | 1800 Hz | 1880 Hz | 1960 Hz |
Character set
- 15 different characters with 20 characters / sec
. Only frequency groups A and C are used. Separation of characters is the rest position. - 63 different characters with 20 characters / sec.
All three frequency groups are used. Separation of characters is the rest position. - 16 different characters with 40 characters / sec.
Frequency group B changes between B2 and B3 with each new character. Without a rest position, the characters are sent one after the other, the apparent clock signal on B enables a separation. - 256 different characters with 20 characters / sec.
Similar to before, only half a character is transmitted at a time. The frequency B3 indicates the first half, B2 the second half of the character to be transmitted.
Receive data
In principle, parallel modems work simplex . A data transmission response was not originally intended, but other options are available.
The feedback about the received data (good / bad message) can take place via a tone of 420 Hz; the maximum transmission rate is 5 bit / s. This is a simple amplitude modulation (AM) : tone on or off. A frequency-modulated (FM) return channel with 75 bit / s was standardized, but was rarely used. The sound of the AM return channel overlaps with signaling frequencies from telephone systems (see call and signal machine ); there were often misinterpretations up to and including disconnection. Simple parallel modems had a built-in loudspeaker to reproduce the feedback.
Interfaces to the data terminal equipment
Outdoor station V.19
telephone with modem for simple data transmission in the Museum for Communication Frankfurt
A modem with an interface is not described in the standards.
From 1980 there were special telephones that could be called "modems": They had either a rotary dial or a keypad to establish a connection and a second keypad to the left for manual entry of the data to be transferred. These phones were called FeTAp 7855 ; the manufacturer was the Hagenuk company .
One of the first comfort telephones from the German Federal Post Office was the " Dallas " model . With the option to switch between pulse and tone dialing, it was advertised as being suitable for participating in simple data traffic .
At that time, devices of this type were received by mail order companies, for example .
Outdoor station V.20
The interfaces between terminal equipment and transmission equipment are described in ITU-T recommendation V.24 (German version: DIN 66020-1). The electrical parameters, however, are not those of the often used V.28 , but generally simple contacts with forward and return lines (ITU-T V.31 ).
Due to the low transmission speed, normal relays were often used for control.
The 25-pin connector is the same as that of serial modems ( ISO 2110 ). Since each interface requires two contacts through the forward and return conductors, the pin assignment is different to other modems. Confusion with modems of the central station or serial technology is possible.
The abbreviations in the first column of these tables are used colloquially.
| ITU | DIN | Hinleiter pen | Pin return conductor | Description according to DIN 66020-1 | Hints | |
|---|---|---|---|---|---|---|
| Send data and common return line of an outdoor station | ||||||
| GND | 102 | E2 | 7th | Plant earth | ||
| TXD | 103 A1 | D1 | 3 | Transmission data | Frequency group A | |
| 103 A2 | 4th | |||||
| 103 A3 | 5 | |||||
| 103 B1 | 6th | Frequency group B Note 1 | ||||
| 103 B2 | 7th | |||||
| 103 B3 | 8th | |||||
| 103 C1 | 9 | Frequency group C | ||||
| 103 C2 | 10 | |||||
| 103 C3 | 11 | |||||
| Auxiliary channel or return channel received data from an outdoor station | ||||||
| SRCV | 119 AM | HD2 AM | 20th | 21st | Auxiliary channel receive data | Line signal AM, 5 fps Note 2 |
| 119 FM | HD2 FM | 2 | 12 | Line signal FM, 75 fps | ||
| Interfaces for controlling an outdoor station | ||||||
| 108/1 | S1.1 | 24 | 25th | Switch on the transmission line | Note 3 | |
| DTR | 108/2 | S1.2 | DEE ready for operation | |||
| RTS | 105 | S2 | 16 | 17th | Switch on the transmitter | Activate only transmitter or receiver, due to minimal power supply |
| 129 | HS3 | 18th | 19th | Switch on auxiliary channel receiver | ||
| Message lines of an outdoor station | ||||||
| DSR | 107 | M1 | 22nd | 23 | Operational readiness | Active when modem is connected |
| RNG | 125 | M3 | 14th | 15th | Incoming call | Active during the call voltage |
| SDCD | 122 | HM5 | 20th | 21st | Auxiliary channel received signal detection | For FM operation only Note 2 |
- Note 1: The use of frequency group B is optional.
- Note 2: The simultaneous use of an amplitude-modulated and frequency-modulated return channel is not intended. This is why the pin assignment overlaps.
- Note 3: There are two ways to start a data transfer:
- 108/1: The DEE is in the waiting state and starts the data transmission when an incoming call is received by switching on this interface. The transmission device then immediately seizes the line.
- 108/2: The DTE is ready for operation, the transmission device is waiting to be activated. An incoming call, the manual pressing of the data key of the telephone or the interface line 204 (M24) of an AWD according to V.25 switches the transmission device to the line.
Central stations
Data transmission according to V.20 has existed in Germany since at least 1969; a central station according to V.19 was introduced in 1978. The central stations of both standards have the same interface:
- The plug connection is 25-pin (according to ISO 2110).
- Electrical values are described in ITU-T V.28.
- The function corresponds to interfaces of ITU-T V.24.
- There is the same risk of confusion as with an outdoor station.
The interface 191 (Transmitted voice answer), in the DIN 66020-1 as Sent voice response referred to is a special feature. It is a data interface on which analog signals are transmitted. One application from 1979 was the acoustic feedback of inputs: Karlchen , a timetable information of the Deutsche Bundesbahn in the Frankfurt area. The user entered his travel requirements via the rotary dial of a telephone, the train stations being identified by their telephone area code. After entering the departure time (only hours) and the travel day and month, an announcement was made about travel options. Today the selection of menu items via the keypad of a telephone when calling a service center is commonly used, which is also a type of data transfer.
| ITU | DIN | pen | Description according to DIN 66020-1 | Hints | |
|---|---|---|---|---|---|
| Earth lines (return lines, earth connections) of a central station | |||||
| FG | 101 | E1 | 1 | Protective earth | |
| GND | 102 | E2 | 24 | Plant earth | |
| Send data from a central station | |||||
| SXMT | 118 | HD1 | 17th | Auxiliary channel send data | Note 1 |
| 191 | A1 | Sent voice response, galvanically isolated from GND |
Hinleiter Note 1 | ||
| 18th | Return conductor | ||||
| Receive data from a central station | |||||
| RXD | 104 A1 | D2 | 3 | Receive data | Frequency group A |
| 104 A2 | 4th | ||||
| 104 A3 | 5 | ||||
| 104 A4 | 6th | ||||
| 104 B1 | 13 | Frequency group B Note 2 | |||
| 104 B2 | 14th | ||||
| 104 B3 | 15th | ||||
| 104 B4 | 16 | ||||
| 104 C1 | 9 | Frequency group C | |||
| 104 C2 | 10 | ||||
| 104 C3 | 11 | ||||
| 104 C4 | 12 | ||||
| Control lines of a central station | |||||
| CDSL | 108/1 | S1.1 | 22nd | Switch on the transmission line | |
| DTR | 108/2 | S1.2 | DEE ready for operation | ||
| RTS | 105 | S2 | 20th | Switch on the transmitter | for voice response |
| 124 | S3 | 25th | Use all frequency groups | ||
| 130 | S9 | 19th | Send confirmation tone | 5 fps AM note 1 | |
| SRTS | 120 | HS2 | Switch on auxiliary channel transmitter | 75 fps FM note 1 | |
| Message lines of a central station | |||||
| DSR | 107 | M1 | 23 | Operational readiness | |
| RNG | 125 | M3 | 21st | Incoming call | |
| CD | 109 | M5 | 8th | Received signal detection | |
| SQD | 110 | M6 | 2 | Reception quality | one frequency per group detected |
| SCTS | 121 | HM2 | 18th | Auxiliary channel ready to send | at 75 fps FM |
| Clock lines of a central station | |||||
| 131 | T5 | 7th | Receiving-side scanning mark | character is recognized on a negative edge | |
- Note 1: The simultaneous use of answer tone or auxiliary channel as feedback is not possible.
- Note 2: The use of frequency group B is optional.
Device technology
Modem of the outdoor station
The original technique was very simple. Many parts from existing telephones were used: the mechanical switch between voice and data operation, the indicator for test purposes, the large capacitors and transformers were standard components from telephones of the time.
A parallel modem did not initially have its own power supply; it was fed by the loop current of the analog telephone connection. This direct galvanic coupling with the telephone line made these devices sensitive to interference. Overvoltages caused by lightning strikes often destroyed the modem. The V.31 interface used and the resulting isolation offer a certain level of protection against connected data terminals; these were also less frequently affected.
The originally minimal power consumption also meant technically very precise production; complex manual fine-tuning was necessary. Each transmission frequency was determined individually and then implemented using a high-precision resistor . The last parallel modems were therefore developed using digital technology ; the labor-intensive frequency adjustment was no longer necessary. However, these devices require their own power supply.
An outstation had in the German Federal Post Office , the term D20P-A ( 20 characters / sec, P arallelübertragung, A ußenstation).
Central station modem
The central station is the recipient of the short messages from many outstations. Since this modem has to process the different tolerances of the outstations, it is complex to manufacture and therefore expensive.
Central stations according to V.19 were named at the Deutsche Bundespost D10P-Z ; Devices according to V.20 D20P-Z .
measuring technology
The measurement technology for checking the functionality of a transmission path was originally just as simple as the device technology at that time. The interfaces were operated manually using switches; essential functions were controlled by relays. Some TTL integrated circuits were used. Numitron (Minitron) tubes showed as a seven-segment display how many milliseconds the central station had recognized all other frequencies after recognizing the first frequency and the received character combination was valid.
The measuring device shown here (built around 1970) can control both an external and a central station. To check a transmission path, every possible combination of characters is sent from an outstation to the central station for test purposes, either manually individually or automatically one after the other. The evaluation of the received characters at the central station is done by the user, optically through the middle LED field or at the same time acoustically through a built-in loudspeaker.
Audio samples (all possible characters are sent one after the other):
Around 1985 the first microprocessor- controlled measuring device for parallel modems of the outstation within the Deutsche Bundespost was introduced in Germany, the TFT 20 . This device was smaller and more powerful than the existing measurement technology; For example, 511-bit long test texts could be evaluated on the frequency-modulated return channel in accordance with the CCITT recommendation V.52 (now ITU-T O.153). As a counterpart to receive the test data, however, there was only one station in the data test center (DTC) of the Deutsche Bundespost in Frankfurt am Main.
Terminals
At that time, the KKL 300 small card reader from SEL was used as the data terminal equipment. It is similar in structure to a slide projector with a round magazine and was often used in pharmacies. For many drugs there were small ABDA punched cards with a corresponding coding. If necessary, this punch card was inserted into the reader, a connection was made to wholesalers by telephone and then via the parallel modem, and an order was placed. This transmission technology is obsolete today and is no longer used; however, the punch card is still in use.
literature
- Service aid of the Deutsche Bundespost, FTZ 118 D 14 (July 1979)
- Modem for parallel transmission , description, Siemens Aktiengesellschaft , order no .: A22581-J5-A314-*-18, 6/79
Web links
Individual evidence
- ↑ Pharmaceutical trade organized by telephone and data transmission (pharmacy modem). ( Memento from March 30, 2016 in the Internet Archive ) Computer Week , 1978
- ↑ New "Miniterminal MRT" from Schrem
- ↑ Order location in the pharmacy ( Memento from March 5, 2016 in the Internet Archive ) (PDF; 350 kB)
- ↑ Parallel modem interface for pharmaceutical terminals from SEL
- ↑ Official Journal of the Federal Minister for Post and Telecommunications and the German Federal Post Office , No. 59, year 1990, June 29, 1990: Price list for the rental and maintenance of telecommunications terminals that were previously made available according to the conditions of use under public law
- ^ Slides for teaching, as of April 1, 1974, Central Telecommunications Office, Section B 34, slide archive. Series 64.07, data transmission on telephone lines, DK 621.391, 9.1969 Description on the slides: DBP, FTZ / FSA Dmst, Urh: A37 / 1969
- ↑ Note on the support of central stations ( Memento of October 11, 2009 in the Internet Archive ) (PDF) company brochure from 1999, p. 5
- ↑ Configuration of parallel modems, company brochure from 1999, p. 27. ( Memento from April 7, 2005 in the Internet Archive )
- ↑ International Telegraph and Telephone Consultative Committee: Orange book Volume 8, Part 1, Verlag International Telecommunication Union, 1977 p. 64 [1]
- ↑ ITU-T Recommendation V.20: Parallel data transmission modems standardized for universal use in the general switched telephone network
- ↑ ITU-T Recommendation V.19: Modems for parallel data transmission using telephone signaling frequencies
- ↑ Compact telephone "Dallas LX", page 5 (PDF; 775 kB)
- ↑ Computerwoche 39/1978, DBP range of services for data communication: Introduction of a central station V.19 ( page no longer available , search in web archives ) Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.
- ↑ Computer-Karlchen talks to SAMT and Siemens ( Memento from September 29, 2007 in the Internet Archive )
- ↑ comparisons to: 200 bit / sec, S erial transfer
- ↑ Data analyzer helps to find DÜ errors, reference to the 511-bit test text
- ↑ ITU-T Recommendation V.52: Characteristics of distortion and error-rate measuring apparatus for data transmission
- ↑ ITU-T Recommendation O.153: Basic parameters for the measurement of error performance at bit rates below the primary rate
- ↑ Hermann Gabler (Ed.): Text and data transmission technology, Volume 6 Telecommunications technology . R. v. Decker's Verlag, G. Schenk, Heidelberg 1988, ISBN 3-7685-2887-1 ; including pictures within the data test center
- ↑ Deutsche Telekom employee newspaper, photo on page 38
- ↑ IT terminal KKL 300 punch card reader
- ↑ SEL demonstrates pharmaceutical system ( Memento from September 29, 2007 in the Internet Archive ) SEL demonstrates pharmaceutical system
- ^ ABDA punch card ( Memento from February 20, 2004 in the Internet Archive )
