From Wikipedia, the free encyclopedia

In electrical and electronic engineering a daisy chain is a wiring scheme in which, for example, device A is wired to device B, device B is wired to device C, device C is wired to device D, etc.^[1] Connections do not form webs (in the preceding example, device C cannot be directly connected to device A), nor do they loop back from the last device to the first. For example, A-B-C-D-E, A-B-C-D-E & C-M-N-O (branched at C) are daisy chain, whereas A-B-C-D-E-A (loop) is not a daisy chain. Daisy chains may be used for power, analog signals, digital data, or a combination thereof.

Physical arrangement

In popular usage, the term sometimes merely describes the layout of physical connections. For example, in a daisy chain of power strips (it's worth noting here that a power strip should never be used in this fashion as it can be dangerous), while the power strips are physically plugged into each other in a chain, any devices that receive power from the chain are electrically connected in parallel; not in series (see series and parallel circuits). Additionally, a daisy chain may be an arrangement in which a daisy chain wiring pattern is in use even though neither the standard nor the physical devices in use mandate that wiring pattern. In particular, tree-based systems such as USB, FireWire, and modern ethernet can be wired in a daisy chain if desired. A notable exception, thin ethernet is usually physically arranged as a daisy chain of T-connectors on network cards, but is almost never referred to as one.

Signal transmission

For analog signals, connections usually comprise a simple electrical bus and, especially in the case of a chain of many devices, may require the use of one or more repeaters or amplifiers within the chain to counteract natural attenuation. Digital signals between devices may also comprise a simple electrical bus, in which case a bus terminator may be needed on the last device in the chain. However, unlike analog signals, because digital signals are discrete, they may also be electrically regenerated, but not modified, by any device in the chain.

SCSI

SCSI is an example of a system that is electrically a bus but, in the case of external devices, is physically wired as a daisy chain. Since the network is electrically a bus, it must be terminated and this may be done either by plugging a terminator into the last device or selecting an option to make the device terminate internally.

MIDI

MIDI devices are usually designed to be wired in a daisy chain. It is normal for a device to have both an OUT port and a THRU port and often both can be used for chaining. The OUT port gives a completely regenerated signal and may add, remove, or change messages, but introduces some delay in doing so. The THRU port provides a minimally delayed (only electrical amplification and opto-isolation) copy of the signal but suffers from asymmetry due to the opto-isolator distorting the signal with each item in the chain. If the chain is long enough, it will be distorted so much that the system will become unreliable or completely non-functional. Due to the above issues with both types of chaining, a variety of MIDI devices that take one input and split it to many outputs have appeared on the market.

SPI

Some Serial Peripheral Interface Bus IC products are designed with daisy chain capability.

JTAG

All JTAG integrated circuits should support daisy chaining (TDO pin of each device is connected to TDI of next device, first TDI and last TDO pins are connected to test equipment and TMS/TCK pins are parallel-connected between all devices). JTAG daisy chaining guidelines must be met and there are some exceptions when chip manufacturers implement JTAG TAP incorrectly ^[2]

Shift registers

Some shift registers have a Serial Out pin. This pin can be connected to the data pin of another shift register, thus combining those two chips. If one shift register has all bits (but not yet latched) in place, sending another clock signal to the chip will push the last bit out the Serial Out pin and the newest bit will take the place of the original first bit. The bit that left out the Serial Out pin will now enter the 2nd shift register through the data pin. Using a common clock and latch signal, you can shift the data from one shift register to the other. The common latch signal will latch in all bits at once. This setup is called Daisy Chaining.

References