Digital Address Switches

Introduction

Many components of the Märklin Motorola digital system contain a group of miniature DIP switches that are used to set their communication address. The setting of these switches follows one of two basic codes, depending on the component type. Locomotives with either a Digital or Delta decoder, as well as digital accessory decoders, all use an address coding scheme known as trinary. Digital accessory controllers such as the Keyboard, Switchboard, and Memory, have addresses based on binary coding. Within these two basic code groups, there are also subtle coding variations due to the designs of the Delta decoder, certain Digital decoders, and the Memory unit.

This article describes the electrical operation of the coding switches and explains the basis of the digital address codes themselves. Complete coding tables are also provided for each type of component in the digital system.

Locomotive & Accessory Addresses - "Trinary" Coding

Inside almost all Märklin Digital locomotives and all accessory decoders, there is a group of eight miniature coding switches for setting the address. These switches are actually divided into four pairs of switches, which are connected to the corresponding address inputs of the digital decoder IC. Each pair of switches can be set to one of three valid electrical states: Lo ("0"), Hi ("1"), or Open ("Z"). This type of circuit is referred to as a "three-state," "tri-state," or trinary digital logic circuit. The diagram below illustrates the trinary coding circuit:

As shown above, the two switches of each switch pair are wired to the two digital supply voltages of the decoder. In general, the odd-numbered switch of each pair is wired to logic Lo and the even-numbered switch is wired to logic Hi. One switch or the other can be set ON to result in logic Hi or Lo, or both can be set OFF to result in logic Open. However, both switches should never be set ON simultaneously, as this results in a short-circuit of the decoder's digital power supply.

Each of the four pairs of coding switches represents a trinary digit of the decoder address. Since there are three valid states for each digit, this gives the Digital system its total of 3⁴=81 possible decoder addresses. Each address can be converted between its decimal (base 10) and trinary (base 3) equivalent values by following these processes:

With this understanding of the coding switch operation and trinary encoding, the patterns within the address coding table below should become more apparent. The switch settings follow the process just as described, with only two exceptions: address 80 is coded as "0000" and address 00 is coded as "ZZZZ", exactly the opposite of what you would expect. Also, address 00 (all switches set OFF) can only be used in analog operation of locomotives and is not used for accessory decoders. It is also important to note that the Märklin Digital system can address a maximum of 64 accessory decoders only.

The coding table below summarizes the address switch settings for both Digital locomotives and accessory decoders:

Delta Decoders & Digital Decoders with Solder Pads:

Delta locomotives operating in the Digital system, as well as "abbreviated" Digital decoders with solder pads, use the same trinary address encoding as standard Digital decoders, although their coding switch operation differs. These decoder boards contain four individual switches or pads (instead of switch pairs), which are capable of setting the address inputs of the decoder IC to logic levels Lo ("0") or Open ("Z") only. Each switch or pad will connect its decoder input to logic Lo when set ON, or logic Open when set OFF. As a result of this, there are only 16 possible address settings for Delta and "abbreviated" Digital locomotives. The same two exceptions described previously for standard Digital decoder addresses also apply here: address 80 is coded as "0000" and address 00 ("ZZZZ") is used only for analog operation of Delta locomotives. The switch coding circuit and address table for these locomotive decoders are shown below:

Accessory Controller Addresses - "Binary" Coding

On the rear panel of all Märklin Digital accessory controllers, there is a group of miniature coding switches to set the address of the controller in the system. Each switch can be set to either of two valid electrical states: Lo ("0") or Hi ("1"). This type of circuit is referred to as a binary digital logic circuit.

All of the accessory controllers contain four switches in the switch block. The Keyboard and Switchboard use all four switches for address coding, while the Memory unit uses only the first two switches for setting its address (the remaining two switches affect its operating mode). These switch settings are read by each unit's internal microcontroller IC to "prefix" all outgoing system data with the corresponding address of the unit. Each switch will be read as logic Hi when set ON, or logic Lo when set OFF.

Each coding switch represents a binary digit of the controller address. Since there are two valid states for each digit, this results in a total of 2⁴=16 possible Keyboard/Switchboard addresses and 2²=4 Memory addresses. Each address can be converted between its decimal (base 10) and binary (base 2) equivalent values by following these processes:

With this understanding of the coding switch operation and binary encoding, the switch patterns within the Keyboard & Switchboard address coding table below should become apparent. This table also provides the correspondence between the controller number and the decoder and accessory addresses that it controls.

Memory Units:

The address coding table for Memory units is much more simple since there are only four available addresses using switches 1 and 2. The Interlocking function of the Memory can be turned on or off by setting both switches 3 and 4 ON or OFF, respectively.

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