Datum Systems PSM-500L Bedienungsanleitung PDF herunterladen (Seite 8)

Datum Systems, Inc. PSM-500 Modem Remote Control Protocol

Rev 0.93 12/12/10

B-8

The last case is not currently possible because the M5, PSM-4900 series modems do not know how to

process an M500 protocol mode byte.

A far end designated packet is not evaluated or processed by the local modem except that its source and

destination addresses are removed. When the response is returned by the far end modem, the addresses are

inserted into the response before returning it to the original calling controller.

MCC Channel Timing

A packet sent to the far end via the MCC channel may take quite a while before a response is returned to

the original caller. The timing also varies depending on the overhead rate assigned to the MCC channel. For

example, a 9 byte request with a 50 byte response and a 9600 baud MCC channel overhead would take

approximately 9 mS for local receipt, plus 9 mS to send via the MCC, plus 250 mS satellite time, then for

the response, approximately 6 mS processing time, 50 mS to send, plus 250 mS satellite return time, plus

50 mS to retransmit to the originator. That is a total of approximately 624 mS. If the MCC overhead rate

was lowered to 1200 baud then the time would increase to approximately 1237 mS! The originating

controller must be set up to accept this order of delay.

Far End Modem Control via the ESC

The M500, and the M5 series modems equipped with the multiplexer, also have an overhead channel or

Engineering Service Channel (ESC), which can be used to relay multiple types of control messages from

one station to another. Although it requires more connecting cables, this method has several additional

configurations not possible using the MCC.

The ESC is an asynchronous overhead channel capable of being formatted as either RS-232 or RS-485

physically. To use the ESC for control of one or more modems at a far end, the ESC locally is connected to

a controlling computer. The ESC at the far end of the link is looped back into the modem’s normal control

port at J6. To control a single far end modem the physical connection could be either RS-232 or RS-485,

but if RS-485 is chosen then multiple modems at a remote location can be controlled by simply daisy

chaining the RS-485 to each modem. The ESC at the two ends of the link do not have to be the same

physical protocol or even the same data rate since the ESC channel contains buffering at both ends.

Since the ESC channel is simply a link, and knows nothing about the information being carried, it can also

be used to control a single far end modem using the VT100 control mode. This mode is limited to a single

modem however since that method does not use addresses to point to a particular modem. The VT100

mode has the advantage that all of the programming required is contained within the modem. The controller

simply needs to be a dumb terminal or a PC running a terminal emulation program like Hyperterminal in a

Windows OS PC.

For information on building the cables needed to connect a computer or a modem’s control port to the ESC

port connections on the modem’s Aux Port (J4), see Appendix C, Cabling, Section 3.

More Information on Modem Monitor and Control

Please see the separate Datum Systems Application Note 18 entitled “Remote Control of Satellite

Modems”. This App Note is available on our web site. It describes several scenarios for control of local and

remote modems within a system.

Efficient Monitor and Control Methods

A new set of software for the M500 series of modems released in 2010 beginning with Version 1.19

includes a special “Unit Polling Status” binary packet, command # 0x0A, designed to make monitor and

control easier and more efficient. SnIP m500ctl programs at version 0.36 and above coupled with the

filesystem 0.6.09 and above can use that software to advantage.

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