Issue
Does I/NET Seven allow the SubLAN to go wireless?
Environment
I/NET site
Cause
Wanting to know if I/NET Seven allows the SubLAN to go wireless?
Resolution
There are no investigated or suggested configurations of the Sub LAN using wireless interfaces.
Approaching wireless at the sub-LAN layer encounters a number of interface obstacles; some of these issues would include the following:
- There are no control signals available on the Sub-LAN port that can be used for keying the radio.
- The Sub-LAN is a 2-Wire half duplex link with RS485 electrical interface.
- Some radio and fiber interfaces have used RS485 differential level to create a Request-To-Send signal for radio transmits activation.
- The 7790/98 injects a small bias (2.2K ohm pull up/down) on the 2-wire pair to force mark condition at the MicroRegulators.
- Bias creates a potential problem for the theoretical radio interface to detect transmit activity based on differential level.
- The radio would need to accommodate some flexibility regarding differential voltage level that activates the transmitter radio would need to supply bias at the remote ends.
- Small termination resistance could be applied to reduce differential voltage on inactive LAN, but radio would have to be flexible in that voltage threshold setting.
- The Protocol is Asynchronous
- The Radio transmission startup time would need to be almost instantaneous.
- There is only a fraction of a bit time between RS485 active levels and beginning of Asynchronous Start bit.
- The Radio transmitter turn-on must not distort leading edge of start bit.
- Receive to transmit turnaround time is about 1 bit time at the remote end
RS485 Electrical Interface
The Host Controller and multiplexed Sub LAN remote controllers each provide a 2-wire RS485 transceiver for communications. Our Sub LAN, as with many other systems, utilize a resistive bias on the LAN wire pair to ensure a marking data condition on the Sub LAN receivers with the Sub LAN is quiescent. This bias is achieved at the Host controller (7793 or 7798) with a 2.2K ohm pull-up to 5V on the positive leg (terminal 1) and 2.2K ohm pull-down to ground on the negative leg (terminal 2). It is a common practice with multi-point systems using asynchronous protocol needing to ensure a quiescent marking condition on the LAN. As mentioned below, the radio will need to accommodate a higher differential voltage level for considering the LAN active (higher than with 120 ohm terminated RS485 with no resistive bias). This has been a point of contention in the past with fiber interfaces used with the Sub LAN. The use of a 620 ohm termination resistor has allowed reduction of the quiescent differential voltage to allow activity detection (on fiber interface).
Who Controls Transmission
The communications is taking place between a Host controller (7793 or 7798) and up to 32 Micro-Regulator (MR) or Door Processing Unit (DPU) controllers on the RS485 twisted pair. The Host controller initiates all communications on the Sub-LAN. There are no card readers on the Sub-LAN.
Message Size
The messages vary between 1 byte and 132 bytes in both directions.
The system is normally performing what is referred to as a Quick Poll operation. The Quick Poll consists of a single Byte Command and a single byte response (if nothing has changed in the controller’s I/O image). The messages are sent with a byte structure of 8 data bits, no parity and 1 stop bit. The messages have a 1 byte CRC at the end except the one byte command/reply formats. If the one byte command results in a multi-byte response (i.e. with the newly changed data, it will have a CRC at the end).
Frequency of Communications
Commands from the host controller and replies from the MRs and DPUs are continuously being transmitted.
The Quick-Poll commands are constantly interleaved with read-all point commands, control commands and others.
Communication Cycle
As mentioned above, all of the communications is comprised of commands from the Host Controller and responses from the MR and DPU controllers. There are some acknowledgement functions being performed, but they are imbedded within the command and reply sequence.
How Many Sites
The Host controller provides one (7798) or two (7793) Sub-LAN interfaces. Each Sub-LAN interface is intended to support up to 32 MR or DPU controllers on the RS485 bus. It is assumes each Sub-LAN interface on a Host controller would be a separate radio interface (if more than one is necessary for a project).
Summary/Caution
I am told there are some radio manufacturers that have claimed suitability of their radio interface for RS485 communications. Actual testing of a proposed radio interface by the system engineer & installer with an I/Net host controller and at least 5-6 Sub-LAN controllers including MRs and DPUs in the representative environment & distance is strongly suggested before any significant commitments. Try it and see!! My exposure to radio interfaces in the past has typically been accompanied with an associated need for system revisions increasing their tolerance for communications anomalies (noise, retries, timing, error checking, etc.). Such additional accommodations should not be anticipated here. I am not aware of any radio interfaces providing accommodation for the Sub-LAN communications.
