SEI Serial Encoder Interface Bus

SEI Data Sheet

US Digital's SEI bus is a simple, quick and convenient way of networking devices to an RS232 serial port. The AD2-B adapter is available to interface to a standard RS232 port (9-pin DSUB) such as those in PCs. A wall-mount power supply furnishes the power for all devices on the SEI bus.

SEI Network


Absolute Maximum Ratings

Parameter Min. Max. Units
Operating temperature 0 45 C
Humidity (non-condensing) 0 95 %
Supply voltage (PWR) 0 16 Volts


The SEI bus can support 1 to 15 devices on the SEI bus. The network configuration can be a chain, star, or a combination of both and does not require cable terminations.

The bidirectional data lines carry commands from the host computer and responses from the device(s). The format is 1 start bit, 8 data bits and 1 stop bit. The baud rate can be changed, but it always starts as 9600 baud after reset or power up. Since the data bus is bidirectional, the host must release the bus within 20 uSec after the last command byte to avoid contention with the device's response. The AD2-B adapter does this automatically and biases these lines with pull-up/down resistors to keep them in the inactive state when the bus is idle.

The busy lines are used for flow control. When a device is busy, it asserts these lines in an open collector fashion by driving busy- low and busy+ high. They need to be biased at the host with pull-up/down resistors to keep them in the unasserted state (busy+ with a resistor to ground, busy- with a resistor to +5V) when they are not driven. They are driven by the device which has been addressed as an acknowledgment of the command. They stay asserted until the command is completed. While a device drives the busy line, all other devices on the bus ignore the data flow. If a single device is on the bus, the busy lines can be ignored and pulled to the unasserted state, but it is easier to communicate with it if they are connected.

The maximum cable length from the SEI adapter to any device should be limited to 1000 feet. If the baud rate used is higher than 19.2 kbaud, the length should be reduced proportionally, ie: 200 ft at 115 kbaud. Contact us for information about longer cables at high baud rates. The power supply requirement is 7.5 volts min., which should be considered carefully when long cables are used, because of the voltage drop caused by the cable's resistance (a typical 26 AWG telephone cable is 40 Ohms per 1000 feet). For example, the SEI adapter provides 8 volts worst case. Unless power is supplied at several points along the cable, each network branch from the SEI adapter should be limited in length depending on how many devices it supports. See table below.

DC Electrical Specifications

The above is for over operating temperature range. Typical values are specified at Vcc=12V and 25C.

Parameter Min. Typ. Max. Units Notes
Supply voltage (PWR) 7.5 12 16 Volts  
Differential output voltage
|DataL - DataH|, |Busy+ - Busy-|








Load = 100 Ohms


Differential input voltage
|DataL - DataH|, |Busy+ - Busy-|










Common mode input voltage
(DataH+DataL)/2, (Busy-+Busy+)/2








Load = 100 Ohms


Common mode input voltage
(DataH+DataL)/2, (Busy-+Busy+)/2










AC Electrical Specifications

Symbol Description Min. Max. Units Note
TBH Host command to busy active 0.01 1 mSec 1
TR1 Encoder response time (1 byte command) 0.01 1 mSec -
TR2 Encoder response time (Mult. byte command) 0.01 30 mSec 2
TBL Busy release time 0.01 0.1 mSec -
TH1 Host response time 0 300 mSec 3

Single Byte Command Diagram


Multi Byte Command Diagram


Functional Pin Description

Pin Name Description
1 GND Ground, common for power, data and busy pairs.
2 Busy+

Bidirectional differential acknowledge line, active high (open source output, must be pulled down to ground). Analog version: positive analog voltage output. *A2 analog option only.
3 Busy-

Bidirectional differential acknowledge line, active low (open drain output, must be pulled up to +5 Volt). Analog version: analog signal ground, connected to GND pin. *A2 analog option only.
4 PWR Power supply input.
5 DataL Bidirectional differential data line, non-inverted, and is pulled high through a 10k-ohm resistor to pin 4 PWR.
6 DataH Bidirectional differential data line, inverted, and is pulled low through a 10k-ohm resistor to pin1 GND.

Cable Distance From AD2-B Adapter

Devices 26 AWG cable 28 AWG cable
1 1000 ft 1000 ft
2 1000 ft 640 ft
3 670 ft 420 ft
5 400 ft 250 ft
10 200 ft 125 ft
15 125 ft 80 ft

Caution: Do not use voice type telephone cables; they commonly reverse the pin-out which may damage the device. The device network requires six wires straight (pin 1 to pin 1). We offer cables (26 AWG) of any length.

General Notes

  1. The lines busy+ and busy- are differential, and they should not be terminated.
  2. The lines dataL and dataH are RS485-type differential lines. They don't need to be terminated for cables up to 1000 ft long at 19.2 kbaud (proportionally shorter at higher baud rates, i.e. 200 ft at 115 kbaud). If terminated, make sure the lines are biased such that dataL is above dataH by at least 2 volts.
  3. For implementations with long cables or several devices on the bus, the supply voltage at the host should be appropriately higher to compensate for voltage losses in the wires.
  4. A "star" bus topology is discouraged, better performance may be obtained from a "daisy-chain" bus topology.
  5. See the SEI-UPS uninterruptable power supply for the SEI bus for information on retaining power to SEI devices after a power failure.